CHAPTER 4 GROUNDWATER RECHARGE ESTIMATION:...

Groundwater Recharge Estimation: Conceptual Foundation 173

CHAPTER 4

GROUNDWATER RECHARGE ESTIMATION:

CONCEPTUAL FOUNDATION

Groundwater recharge is a natural phenomenon linked to the occurrence of precipitation.

However, man-made structures such as dams, reservoirs, irrigation canals, tanks and

ponds, and return flow from irrigation, also contribute to recharge. The groundwater

recharge, therefore, represents the net addition of all such water to the water table.

In the estimation of groundwater recharge, in particular, in arid and semi arid regions, it

is essential to (i) recognize and understand the complexity of the geological formations

and the flow processes that occur therein; (ii) the high degree of variability, poor

distribution and uncertainty of precipitation and its role in generating recharge; (iii)

improvement in the data collection; and (iv) improvement in the recharge estimation

methods.

This chapter deals with establishing conceptual foundations of groundwater recharge and

estimation methods. This is done through literature review. The Literature Review is

organised in three sections: one, groundwater recharge: conceptual foundations; two,

global efforts for groundwater knowledge and practice; and, three, groundwater recharge

methods-an overview.


SECTION 1

GROUNDWATER RECHARGE: CONCEPTUAL FOUNDATIONS

Two principal types of recharge are identified (FAO, 1981; Llyod, 1986; Lerner et al.

1990; De Vries et al. 2002): [i] Direct recharge is defined as water added to the

groundwater reservoir in excess of deficits (from surface evaporation and plant

transpiration, together termed as evapotranspiration) through the unsaturated (or vadose)

zone. Two types of evapotranspiration are distinguished: potential evapotranspiration

(PET) is a measure of the ability of the atmosphere to remove water from the surface

through the processes of evaporation and transpiration assuming that there is no

limitation or control on water supply. Actual evapotranspiration or AET is the quantity of

water that is actually removed from a surface due to the processes of evaporation and

transpiration (Pidwirny, 2006). In distinguishing direct recharge, the emphasis is on

infiltration of precipitation where it falls, without surface storage; hence assumption of

vertical movement due to gravity becomes predominant. Noteworthy is the fact the

potential of precipitation. In New Mexico (USA), a precipitation recharge of only 2

mm/year over the entire area of the state can provide all water needs for domestic and

industrial users (Hendrickx et al. 1997). This fact emphasises not only the potential of

precipitation, in particular, in arid and semi arid regions, to support all human activity,

but also the need for improving accuracy of estimation of recharge. [ii] Indirect recharge

occurs when the precipitation (run off) water is added to the water table through surface

water bodies. The precipitation that gets stored in surface water bodies such as rivers,

streams, tanks and ponds, or in joints or beds of surface water courses percolates to the

water table; the water that accumulates in depressions, small irregularly shaped channels,

or drains, and similar structures that are not connected to the drainage line, also

contribute to the recharge. Put differently, two sub categories of indirect recharge are

identified: one that is associated directly with surface water courses, and two, localised

recharge resulting from accumulation in surface water bodies that are not located in the

course of well-defined channels or drainage lines.

http://www.physicalgeography.net/physgeoglos/e.html#evaporation

http://www.physicalgeography.net/physgeoglos/t.html#anchor273047

http://www.physicalgeography.net/physgeoglos/a.html#actual_evaporation

http://www.physicalgeography.net/physgeoglos/e.html#evaporation

http://www.physicalgeography.net/physgeoglos/t.html#anchor273047


In the case of the first sub category, the source comprises water bodies such as the rivers

and streams that flow intermittently. This flow is contributed in two major ways: (i)

directly by the rainfall, which is uncertain and erratic, and (ii) by the groundwater

discharge especially at medium or lower reaches of a watershed, provided there are

enough recharge areas104

at the upper reaches, usually the mountains or hills, combined

with lower extraction along the groundwater flow path. Sustained groundwater discharge

into water courses is uncommon in arid and semi arid regions because rainfall is scarce

and its high intensity easily leads to the generation of surface run off flow and

throughflow105

. The remaining rainfall replenishes the dry soil meaning that hardly any

water is left to recharge the groundwater system (Tallaksen & Lanen 2004).

In the case of the second sub category, in areas with rainfall lower than 200 mm (defined

as arid areas), higher evaporation losses due to climate factors such as higher

temperatures do not leave much water for the recharge process. However, in areas

receiving more than 200 mm and the semi arid regions, both direct and indirect recharge

become important although the recharge is generally low and highly variable (Simmers,

1997). The localised recharge structures though not connected with the (natural) drains

often contribute in a big way to the total recharge. Localised recharge implies horizontal

movement of surface and/or near-surface water and occurs in weathered hardrock or

limestone terrain, topographical depressions, minor wadis106

or arroyos, and in mountain

systems (Simmers, 1997). Some classic examples include localized recharge through

numerous depressions measuring tens to thousands of metres across as wetlands or lakes,

and referred to as „potholes‟, „sloughs107

‟, or „playas108

‟. Meyboom (1966), as quoted in

104

Recharge areas are defined as regions where water surplus (excess of precipitation over actual

evapotranspiration) flows downwards to feed the saturated groundwater system (Tallaksen & Lanen, 2004). 105

Throughflow occurs from thin saturated horizontal layers with low permeability, where percolation

exceeds the vertical hydraulic conductivity (Tallaksen & Lanen, 2004). 106

steep-walled, eroded valley; gully or streambed, same as 'arroyo'. 107

A type of swamp or shallow lake system.

108 A playa (or pan) is a dry or ephemeral lakebed, generally extending to the shore, or a remnant of, an

endorheic lake. Such flats consist of fine-grained sediments infused with alkali salts. Playas are also known

as alkali flats, sabkhas, dry lakes or mud flats. If the surface is primarily salt then they are called salt pans,

salt lakes or salt flats. Playas are typically formed in semi-arid to arid regions of the world. The largest

concentration of playa lakes in the world (nearly 22,000) is on the southern High Plains of Texas and

eastern New Mexico. While most playa lakes are very small, other examples of playa lakes include Lake

Alablab in Suguta, Kenya, and Wild Horse Lake, Oklahoma. Salar de Uyuni in Bolivia, near Potosí, is the

http://en.wikipedia.org/wiki/Ephemeral

http://en.wikipedia.org/wiki/Endorheic

http://en.wikipedia.org/wiki/Lake

http://en.wikipedia.org/wiki/Sediment

http://en.wikipedia.org/wiki/Alkali

http://en.wikipedia.org/wiki/Salt

http://en.wikipedia.org/wiki/Sabkha

http://en.wikipedia.org/wiki/Salt

http://en.wikipedia.org/wiki/Salt_pan_%28geology%29

http://en.wikipedia.org/wiki/Salt_lake

http://en.wikipedia.org/wiki/Texas

http://en.wikipedia.org/wiki/New_Mexico

http://en.wikipedia.org/w/index.php?title=Lake_Alablab&action=edit&redlink=1



http://en.wikipedia.org/wiki/Suguta

http://en.wikipedia.org/wiki/Kenya

http://en.wikipedia.org/wiki/Wild_Horse_Lake_%28Oklahoma%29

http://en.wikipedia.org/wiki/Oklahoma

http://en.wikipedia.org/wiki/Salar_de_Uyuni

http://en.wikipedia.org/wiki/Bolivia

http://en.wikipedia.org/wiki/Potos%C3%AD


Simmers (1997), one of the first to study localised recharge, has reported that a pothole

with a bottom diameter of 40 m and a rim height of 3-8 m measuring up to only 15% of

the 0.8 ha watershed contributing area in south-central Saskatchewan (Canada)

contributed 70% of the total recharge. Wood & Sanford (1995) estimated that half (4-5

mm/year) of the annual recharge (9-10 mm/year) to the Ogallala Aquifer on the southern

High Plains in the USA occurs through playa floors that cover only 6% of the area.

Localised recharge for Maheswaram watershed in India was estimated as 20% of the total

recharge (of 156.5 mm) for the year 2003 (Dewandel et al. 2007).

Localised recharge from the morphological depressions or ill-defined dry valleys in the

Botswana Precambrian area ranges from 10 mm/year through alluvial loamy sediments to

30 mm/year through coarse-grained sediments and fractured outcrops for a mean annual

rainfall of 500-550 mm/year. In more arid areas, where the average rainfall is less than

200 mm/year, the recharge is only of the order of a few mm/year with a coarse-grained

soil or fractured-rock outcrops (Issar and Passcher, 1990 quoted in De Vries, 2002).

Exposed karst areas in Saudi Arabia absorb 47% of the average annual rainfall (93

mm/year) into sinkholes and corrosionally extended joints, while in Portuguese Algarve,

the high-intensity winter precipitation facilitates 150-300 mm annual recharge (as quoted

in De Vries et al. 2002).

Often, the localised recharge is the largest in arid and semi arid areas (Lerner et al.

1990:145). However, estimation of such local recharge on an areal basis presents

difficulties [a] due to the small area of local recharge as evidenced by research studies

using chloride, and [b] the large number of small structures giving rise to dispersed nodes

of local recharge. The study villages have constructed many structures that give rise to

local recharge such as the farm bunds, farm ponds, tanks, drainage barriers, and check

dams.

largest salt flat in the world at 4,085 square miles (10,582 square km) (www.wikipedia.org accessed 6

March 2009).

http://www.wikipedia.org/


FLOW PROCESSES THROUGH UNSATURATED ZONE

When rainfall occurs, water infiltrates into the ground, and under adequate supply

conditions, ultimately adds to the water table. During infiltration, the flow through the

unsaturated zone or vadose zone is predominantly vertical due to gravity; when a zone of

saturation is reached, the water flow and its direction are controlled by the hydraulic

gradient (Saksena, 1989).

Initially, under unsaturated conditions, the water molecules tend to hold tightly to the soil

particles by (adsorption); this water is called hygroscopic water. As the water molecules

are added, it forms a film around the soil particles due to forces of surface tension. This

water is called capillary water. The film gets thicker with further addition of water till the

point when the force of gravity becomes more than the adhesive force109

. At this stage,

the water flows downwards due to gravity. When the soil moisture reduces in the soil,

greater force is required to release the interstitial water. This is called soil moisture

tension110

.

109

Adhesion is the force of attraction between solid surfaces for water molecules (Michael, 1987). 110

Soil moisture tension is a measure of the tenacity with which water is retained in the soil and shows the

force per unit area (measured in atmospheres) that must be exerted to remove water from soil. 1 atmosphere

= 1036 cm of water or 76.39 cm of mercury (Michael, 1987).


Gunn (1983) has conceptualised the flow as shown in the Figure 4.1 (Simmers, 1997).

Soil

WZ

WZ

Conduit flow dominated aquifer

1. Overland flow; 2. Through flow; 3. Subcutaneous flow; 4. Film flow;

5. Fracture flow; 6. Vadose seepage. WZ: Weathered zone

: lithological discontinuities

Figure 4.1: Types of water flow (after Gunn, 1983).

As can be seen from the Figure 4.1, there is flow over the surface (1) and a flow in the

top layer of the soil zone (2). A subcutaneous flow (3) takes place in the weathered zone.

All these three flows lead to formation of a film flow or shaft flow (4) where the water

moves as thin films along walls of vertical shafts. Where joints and fractures are

encountered by the subcutaneous flow, then macropore flow takes place. Pores larger

than larger than 3 mm in diameter are categorised as macropores. In macropores, the

effects of capillarity are no longer felt and the flow process is dominated by viscous

forces and gravity (Simmers, 1997). Openings of spheroidal weathering, horizontal and

vertical joints, columnar joints (such as in basalts), fractures, root channels and cracks

come under the category of macropores. When such macropores or similar structures or

heterogeneities are present, the water tends to take these paths due to lower hygroscopic

or adhesive force compared to gravity. Macropore flow or vadose flow (5) is defined as

vertically moving water; significant water flows through the macropores in view of the

high porosity. Gravity being prominent, the flow occurs rapidly. These structures result in

1

6 6 5

2

3

4


quick build up of water levels due to floods or rains, within hours or days (Beven &

Germann, 1982; Hendry, 1983; Simmers, 1990), as a function of the distance between the

source such as a river, stream, tank or a pond, and the point of observation such as a well.

Also, some water movement takes place through vadose seepage as a vertical flow

through small, tight joints and fractures, or as intergranular flow. The contribution of a

type of flow to recharge to water table depends upon the local hydrogeological

conditions. When surface water bodies are present, film flow or shaft flow becomes more

significant than fracture flow. Further, fractures and joints respond quicker than fractures

filled with soil (Gunn, 1983).

The significance of preferential flow is seen in the fact that in some cases, the

contribution of preferential flow is as high as 90% of the estimated total recharge (De

Vries, 2002). In the arid and semi arid zones, the flow through unsaturated formations

occurs as (i) a wetted front, and (ii) along preferential pathways. The wetted front is

nothing but the foremost layer which is moist. When water infiltrates the soil zone,

wetted front moves vertically downwards due to gravity. Two conditions could be

visualised here: if there is no addition of water at the surface, then the soil moisture from

the soil layer in contact with the atmosphere starts drying up due to evapotranspiration

(drying up of bare land and transpiration by plants). There would be no pushing down of

soil moisture layer. However, there could be some upward (capillary rise) and downward

fluxes. If the soil surface is connected with any water source, then the water body exerts

downward (hydraulic) pressure so that soil moisture moves as a wetted front, or as a

piston flow as proposed by Zimmerman et al (1967) and Munich (1968a, b) as quoted in

Lerner (1990). The piston flow assumes that soil moisture moves downwards in discrete

layers, and any addition of a fresh layer pushes down an equal amount of water

immediately below. This process of pushing the soil moisture down continues till the last

such layer in the unsaturated zone is added to the saturated regime or the water table. The

movement of water through the soil is generally slow and depends upon a number of

factors, one of the major ones being the size of the soil particle.


Even in relatively homogeneous formations, preferential pathways could have an

irregular distribution; often, these are connected by horizontal flow over lithological

discontinuities (see Figure 4.1) that are typical for deposits in intermittent stream beds

such as layered sands, silts and clays (Simmers, 1997).

Another preferential pathway flow, termed fingered flow, is said to occur when an

unstable wetting front advances as fingers in the unsaturated zone. In sands, for example,

the wetting front is more or less horizontal and hence does not constitute a fingered flow.

In sharp contrast to saturated fracture flow where water tends to move rapidly along

fractures, under unsaturated conditions, air filled fractures prohibit water flow and force it

to move through interconnected pores in the matrix (Simmers, 1997). Although the

phenomenon of fingered flow is yet to be understood completely, research by a number

of scientists over the past two decades has indicated conditions under which fingered

flow is generated (Simmers, 1997). They are: infiltration of ponded water as a wetting

front following air in the interstitial pore space; redistribution of water in the soil profile;

water-repellent soil; increase in water content with depth; continuous non-ponding

infiltration, and when coarse textured soil layers are overlain by less permeable layers.

Fingered flow in the top soil is often linked to hydrophobic top layers.

Further, it is also important to understand the transport of vapour occurring within the

soil zone due to temperature gradient, for example, between seasons. The temperature

fluctuation causes an imbalance between the percolation below the (shallow) root zone

and actual recharge, and or between actual recharge and total discharge flux (De Vries,

2002). There is both an upward movement of vapour during winter and a downward

transport during summer. However, the low fluxes and long relaxation time are often not

in a steady state, and therefore, contain residual components of hydraulic head and

hydraulic gradient from palaeoclimatic conditions that give rise to phenomenon like

oases in large depressions (De Vries, 2002). In other words, there is an imbalance

between recharge and discharge.


GROUNDWATER MOVEMENT AND SOIL CHARACTERISTICS

Generally, in situ weathering of pre-existing rocks results in soil particles; sometimes,

they are also deposited in bulk due to transportation by weathering agents such as wind or

water. Table 4.1 gives the classification by United States Department of Agriculture

(USDA) which is universally adopted111

:

Table 4.1: Particle diameter Fraction Particle diameter (mm)

Gravel > 2

Coarse sand 1-2

Fine sand 0.1-0.25

Silt 0.002-0.05

Clay <0.0002

Clay is sometimes further subdivided into coarse clay with diameter in the range of 0.002

to 0.0002 mm, and colloidal clay of less than 0.0002 mm in diameter. Colloidal refers to

particle diameter which is intermediate between particle size that is visible under an

optical microscope and invisible molecules.

Sand and silt particles are usually found to be spherical or cubical in shape. At the time of

in situ formation, they have sharp edges, but when transported these edges become

rounded. They also become rounded due to working out of the soil, exposure to

weathering agents such as water and wind, although remaining at the same place. The

process of rounding takes longer time in situ than when transportation is involved.

Binding of the particles to each other defines what is known as the soil structure.

The space in between the particles, referred to as pore space, plays a key role in

movement of water through the soil zone. Rounded particles have more pore space than

sharp edged particles and hence become important in soil moisture retention or studying

recharge processes. Large pores induce aeration and infiltration, medium-sized pores

facilitate capillary conductivity, and small pores induce greater water holding capacity

(Michael, 1983).

111

The classification by the International Soil Science Society broadly agrees with the values recommended

by USDA.


The soil texture is determined by its composition in terms of silt, sand and clay. Silty clay

refers to a soil in which clay characteristics are predominant but also has silt particles in

substantial measure. A loam soil has equal proportions of silt, sand and clay while silty

clay loam is more or less like silty clay but with significant sand content. Generally,

sandy soils are classified as coarse-textured, loam soils are medium-textured and clay

soils as fine-textured (Michael, 1983).

The soil textural classification chart representing various combinations of the silt-sand-

clay was developed by USDA112

. Water flow through the soil zone takes place as a

function of the textural combination and molecular arrangement. For example, clay,

which is at one extreme with reference to the size of particles and complexity of

arrangement, contains 40% of clay particles and 45% of sand or silt. Because of their

plate-like shape, clay particles have large surface area; most clay surfaces are negatively

charged enabling holding of more water and minerals compared to sandy soils.

Montmorillonite mineral in clay absorbs a lot of water and swells; deep cracks appear

during long spells of absence of water. On the contrary, presence of clay mineral

kaolinite in soil does not result in swelling or shrinking on water availability due to

strong chemical bond between silica and illite micelles.

Loam soil displays properties intermediate between those of sand and clay due to its more

or less equal composition of sand, silt and clay. While clay also holds significant water,

loam is considered better by farmers because it is easier to plough and work with than

clay soil.

Presence of organic matter, mulching etc. improves textural properties from the point of

view of water holding.

112

This has been used in developing SPAW – a Soil-Plant-Air-Water computer model that simulates the

daily hydrologic water budgets of agricultural landscapes by two connected routines, one for farm fields

and a second for impoundments such as wetland ponds, lagoons or reservoirs. Climate, soil and vegetation

data files for field and pond projects are selected from those prepared and stored with a system of

interactive screens. For more details see Saxton, K.E, Pullman, Patrick H. Willey and Dr. Walter J. Rawls,

2006.


Clay soils also referred to as black soils are grouped into three categories depending upon

the thickness: shallow black soils less than 30 cm; medium black soils of 30 to 100 cm

and deep black soils of more than 100 cm. Deep black soils are also referred to as black

cotton soils as cotton thrives very well in this soil.

Soil moisture constants

Soil comprises of three phases: the solid phase comprises mineral and organic matter, and

various chemical compounds, the liquid phase constitutes the soil moisture and the

gaseous phase is called the soil air. Figure 4.2 gives an approximate proportionate

volume and mass relationships of the three soil phases (Michael, 1983).

Air

Water

Solids

Figure 4.2: Phases of soil

The volume composition of the solid-liquid-gas phases varies depending upon the soil

system. For example, silt loam soil contains 50% solids, 30% water and 20% air. The

moisture in the soil is constantly under pressure; the pressure gradients and vapour

pressure differences cause it to move. This is one aspect that contributes to inaccuracy of

recharge estimates.

Gradually, the saturation capacity of a soil is achieved when all the pores of the soil are

filled up with water; the water at saturation capacity has zero tension and hence results in

a free water surface. As the soil moisture saturation occurs and the drainage due to

gravity becomes negligible, the „field capacity (FC)‟ is said to have been reached. In this

state, the large soil pores are filed with air, the micro pores are filled with water, and


further drainage is slow. Where fractures are present, the water movement through

fractures is rapid under saturated conditions; however, in unsaturated conditions, the air

in the fractures inhibits water movement; in which case, water tends to move through the

interstitial pore spaces.

When sufficient moisture supply is available to the soil, the actual evapotranspiration

equals the potential evapotranspiration rate (Tallaksen & Lanen, 2004). Vegetation in dry

soils tends to reduce transpiration rate to cope with available moisture, which gets

reflected as lower actual evapotranspiration compared to the potential evapotranspiration.

If such a phenomenon occurs in water abundant soils, then it implies inhibition of water

intake by the vegetation due to lack of oxygen. Thus, soil moisture storage is the key

factor controlling actual evapotranspiration (Tallaksen & Lanen, 2004).

When the moisture supply is not adequate to replenish the soil moisture loss, especially

combined with high potential evapotranspiration, this situation results in reduction in

actual evapotranspiration, a lower groundwater recharge, or both (Tallaksen & Lanen,

2004).

AET/PE

0 AP FC CP WP

High Soil moisture storage Low

Figure 4.3: Relationship between the ratio of the AET, and the PET, PE, and the soil

moisture storage. AP: Excess water point, FC: field capacity, CP: critical point and WP:

wilting point (after Tallaksen & Lanen, 2004).


Permanent wilting point (also referred to as permanent wilting percentage, WP) is the soil

moisture content at which plants can no longer obtain enough moisture to meet with their

transpiration requirements. There is no doubt some (hygroscopic) water around soil

particles, but because the soil moisture tension is quite high, of the order of 7 to 32

atmospheres, the plant cannot extricate this water and therefore starts wilting (Michael,

1983). In a humid atmosphere, for example, the plants would be able to access this water,

therefore showing a different wilting point. The soil type, soil texture, plant species and

the quantity of soluble salts are the determining conditions for the wilting point, in

addition to the climatic conditions.

Figure 4.3 gives the relationship between the soil and climate parameters. Apart from the

FC and WP, another important constant called the critical point (CP) lies somewhere

between the field capacity and the permanent wilting point. Fine-textured soils such as

clay have a wide range in between field capacity and the permanent wilting point

compared to coarse-textured soils such as sand which release most of their water within a

narrow range due to the presence of large pores or macropores.

It is in this context that rooting depth113

becomes an important consideration. Cotton, for

example, is a deep rooted crop whose roots may go up to 120 cm while groundnut has

rooting depth of 90 cm (Michael, 1983).

How does the plant extract moisture for its survival and growth? An understanding of this

helps to provide proper soil moisture inputs (FC, CP and WP) to the NUT_MONTH

program for recharge estimation. Figure 4.4 describes the moisture extraction pattern

from different layers of the soil zone.

113

This term, rooting depth, is used in CRU meteo-NUT_MONTH program which is used to mean the

effective root zone; effective root zone is the depth from which the roots of a mature plant extract water for

replenishment by irrigation. This is not equivalent to the root depth as often the tap roots extend much

beyond the effective root depth. Field capacity, wilting point etc. are other parameters that are given as

input to the program for estimating potential groundwater recharge.


25% Moisture used: 40% of total

Effective 25% 30%

Root

Zone 25% 20%

25% 10%

Figure 4.4: shows an average moisture extraction pattern by plants from different layers

under adequate moisture supply conditions (after Michael, 1983).

The effective root zone is divided into four equal parts. Studies have established that the

moisture extracted by plant in general can be considered as 40%, 30%, 20% and 10%

from the four parts beginning from the top layer (Michael, 1983).

GROUNDWATER RECHARGE: UNDERSTANDING THE NEED AND THE

PROCESS

(a) Need for Groundwater Recharge Estimates in arid and semi arid regions and

related aspects

Two basic reasons could be stated on the need for proper estimates of natural recharge in

arid and semi arid regions: [a] One-third of the land surface of the world has arid and

semi arid conditions. These regions support significant populations across rural, peri-

urban and urban areas. In many south Asian and African countries such as China, India,

Mexico and Yemen, a vast majority of population in such areas is engaged in agriculture

and animal husbandry as primary livelihood occupations for which groundwater provides

a crucial support. Not only is there no systematic monitoring of groundwater occurrence

and draft, but management of such resources has for long remained in private informal

channels, with public agencies playing only an indirect role (Shah et al. 2001)114

. The

114

A recent World Bank memorandum on water management in Yemen noted: „the problem of

groundwater mining represents a fundamental threat to the well-being of the Yemeni people. In the

highland plains, for example, abstraction is estimated to exceed recharge by 400 per cent‟ [Briscoe 1999].

Yemen is probably the only country where groundwater abstraction exceeds the recharge for the country as

a whole (ibid). Another country, Mexico, too has its aquifers amongst the most overdeveloped. India,


critical water situation is due to (i) disconnect between development and research and (ii)

due to historical legacies, because of which there have been serious restrictions on the

internal mechanisms for the growth of knowledge or the ability to locate or generate new

knowledge on water systems related to the social and environmental dimensions

(Bandyopadhyay, 2006). It is estimated that declining water levels in various parts of

India have potential to reduce India‟s harvest by 25% or more (Seckler et al. 1998). [b] In

the semi arid and arid regions, surface water supplies are always subject to climatic

uncertainty, unreliability and poor spatial and temporal distribution. In addition, the

evaporation losses from surface water bodies are generally high; the annual potential

evapotranspiration is higher than the precipitation except during the monsoon months.

According to a survey that compared the decadal change on source of irrigation, canal

irrigation was the main driver of irrigated agriculture during 1970-73; by 1990-93,

groundwater had become the primary force behind irrigation sustaining almost 60% of

India‟s irrigated area (Roy et al. 2002).

Nearly 40% of India is classified as arid and semi arid with a rainfall of 500-1000 mm

(Kittu, 1995). Hard rocks, which cover 58% of India (Athavale, 2003) possess negligible

primary porosity; the yield however depends upon the secondary porosity and

permeability. The specific yield, which is a measure of the water yield in volume per cent

too is consequently quite low-of the order of 1-3% for basalts and 2-4% for granites

(Athavale, 2003). Seventy percent of the farmers comprise small land holders who cannot

cultivate their entire one hectare of their holding; 67% of the cultivated area is rain

dependent contributing 44% of the food grains and supporting 40% of the human and

65% of the livestock population (Venkateswarlu, 2005). Groundwater extracted through

shallow wells plays a crucial role in this livelihood scenario, which occurs generally

under unconfined conditions (De Vries and Simmers, 2002), for which there is intense

competition leading to „pumping race‟ which necessitates a socio-technical perspective

for designing sustainable and equitable groundwater management systems (Knegt and

Vincent, 2001).

according to David Seckler, IWMI‟s director general, a quarter of India‟s harvest may well be at risk from

groundwater depletion (Shah et al. 2001).


In addition to supporting livelihood occupations, groundwater in such areas also caters to

industrial, urban and other demands and hence comprises the key to overall economic

development. The Special Economic Zones115

being created in India as a tool for

economic development allowing huge chunks of agricultural and non agricultural lands

for industrial, commercial, corporate, entertainment and other non-agriculture purposes

are hampering natural groundwater recharge in a big way due to change of land use

(Mudrakartha & Rajput, 2009). Since agrarian occupations in arid and semi arid regions

survive on natural recharge, which is often small but critical, this policy has immediate

adverse impact on groundwater in the form of enhancing water scarcity and mining

conditions, water quality deterioration and sometimes land subsidence.

In spite of its critical role, water resources planning in India suffers from inefficiency, a

key reason being lack of adequate and accurate hydrological information. Often, the data

suffers from uncertainty, unreliability and inadequacy (Moench, S, Mudrakartha, MS

Rathore et al. 2003). In India, the rainfall has high inter-annual variability, poor

distribution, highly varying number of rainy days, extreme temperature conditions and

high evaporation losses. Surface water sources which are directly dependent upon rainfall

are therefore unreliable. Groundwater is the other complementary source which often

becomes the main source of water in many parts of India. With heavy shift towards

groundwater dependence during the past two decades, groundwater has become scarce in

many parts of India, as indicated by the growing number of overexploited and critical

blocks. Estimation of natural recharge, the process of which happens only after a

dynamic threshold value of rainfall (Scanlon, 2006), becomes extremely important to

sustain these livelihoods connecting the primary livelihoods.

Groundwater recharge process is determined by the interaction of climate (temperature,

sunshine hours, wind speed), geology, morphology, soil condition, water table and

115

These zones are created as part of the Special Economic Zone Act of 1995 and provide huge fiscal

benefits and incentives. The total land area required for acquisition for the proposed SEZs is 1781 sq. km or

178,100 hectares. This land, as of now, constitutes 0.061% of the total land area and 0.112% of the total

agricultural land in India. However, since it is only a couple of years of the Act, the land exclusion is going

to increase. At the current level of land exclusion, at least 200,000 households or around 1 million

population in India is affected (Mudrakartha, 2009).


vegetation (De Vries, 2002; Lanen, 1998). The multiple parameters above as also state

variables such as water table elevations and concentration of specific contaminants in

water are handled by different departments in India, who often lack in cooperation,

coordination and commitment. The density and distribution of weather stations in India is

insufficient in view of the high degree of the climatic variations and hence the climate

data including rainfall are not adequately representative. Data on the water levels is

collected four times in a year (January, May, August and November) by the Central

Ground Water Board (on 15,332 wells) and twice (pre and post monsoon on 30,000

wells) by most of the states, except such as Tamil Nadu which collects almost on a

monthly basis. Almost 90% of these well structures are open wells. This is considered

grossly insufficient to represent the fluctuations within a year due to uncertain and erratic

rainfall pattern and diverse hydrogeology; further, it is not clear how far this water level

would be representative of the groundwater potential estimated given the complex aquifer

systems.

A related point here is that the block (or taluka in some states) is the lowest geographic

level for groundwater recharge estimation in India; a block (or taluka) comprises many

villages, and is a flexible unit in the sense that there is variation in the geographic area as

well as climatic factors. Often, several tens of villages are included to define a block (or

taluka). Therefore, groundwater recharge is computed at taluka level making several

assumptions and broad figures are given out. Although the CGWB uses block or taluka

for the purpose of groundwater assessment, the government of India and the state

governments use this data as a tool for controlling groundwater extraction by imposing

ban on new electrical connections or bank loans for drilling wells. In addition to the

geographic aspects, there is also the aspect of averaging out recharge. Gee & Hillel

(1988) discuss this „fallacy of averaging‟ and point out that most of the recharge in arid

and semi aid regions is episodic, i.e., occurs during short time periods on often restricted

portions of the target area (Simmers, 2002). The arid and semi arid regions of India very

much display this characteristic of a few short high intensity spells which comprises

anywhere from 50 to 80% of the total annual rainfall (Pisharoty, 1990).


Bandyopadhyay (2006) states that the Indian government had adopted a “closed”

approach-that was committed more to institutional hierarchy than a professional inquiry.

The result was that there had been serious restrictions on internal mechanisms for the

growth of knowledge or the ability to locate or generate new knowledge on water systems

related to social and environmental dimensions. Serious research by the minority

researchers suffered from cognitive stagnation. The National Commission for Integrated

Water Resource Development Plan (GoI, 1999) in India too was unable to get all the

necessary hydrological data and observed that “….the secrecy maintained about water

resources data for some basins is not only highly detrimental but is also counter-

productive. Hydrological data of all the basins should be made available to the public on

demand…”

(b) The process of Groundwater Recharge

The process of groundwater recharge is determined by the interaction of climate

(temperature, sunshine hours, wind speed, vapour pressure), geology, morphology, soil

condition, water table and vegetation (De Vries, 2002; Lanen, 1998; Tallaksen & Lanen,

2004). During the process of groundwater recharge (Figure 4.5), the soil gets fully

saturated before further percolation takes place to the groundwater storage (Michael,

1983; Tallaksen & Lanen, 2004). The groundwater flow processes and the rate of

recharge are dependent upon the soil type116

, soil texture (composition of sand, silt and

clay), soil structure (arrangement of individual soil particles), and soil profile (vertical

section through the soil mass that reveals significant variations in the soil texture and soil

structure). Vegetation plays a critical role in the recharge process because the soil has to

meet the moisture demands by the plants, which in effect implies interruption of the

recharge beyond soil zone. Vegetation cover affects actual evapotranspiration (AET)

through the net precipitation, potential evapotranspiration (PET)117

and the rooting depth

116

The soils of India are broadly divided into four major groups: (i) black soils, (ii) alluvial soils, (iii) red

soils, and (iv) lateritic soils. The other soil groups are forest and hill soils, desert soils, saline and alkaline

soils, peat and marshy soils (Michael, 1987). 117

The potential evapotranspiration is also a function of the crop type, crop density and crop length (that is

stage of growth as the canopy varies). Further, the rooting depth which is also the function of crop stage

determines the availability of soil moisture to the crop.


(Tallaksen & Lanen, 2004). The evapotranspiration is usually computed on the basis of

the climatological data relating to temperature, humidity, wind, length of day and

sunshine hours. A complication in this context is the difficulty in obtaining the data on

the climate parameters and the problems with adequacy and accuracy.

Rainfall

Evapo(transpi)ration

Ground level

Plant root zone

Unsaturated zone

Pot. recharge

Water table

Saturated zone

Figure 4.5: Key elements in potential groundwater recharge.

Estimates of recharge are quite difficult to make in arid and semi arid regions not only

due to the variety of climate variables that influence the process of recharge but also

owing to the difficulty and expense in measurements. Nevertheless, many studies are

being made all over the world to understand the diversity of problems associated,

improve measurements and accuracy of estimates (Mudrakartha 1988, 1989).

From the point of view of water table, we can distinguish two scenarios: shallow water

table and deep water table. This distinction is made with respect to the rooting depth in

the context of groundwater recharge. If the water table is closer to the surface (that is, to

the rooting depth), then capillary rise occurs to replenish soil moisture deficiency, leading

to higher values of actual evapotranspiration. The amount and extent of capillary rise is

also a function of the thickness of the rooting depth, water table depth, and the

unsaturated hydraulic conductivity (Tallaksen & Lanen, 2004). Capillary rise cannot help

in case of deeper water tables.


In the case of shallow water table, again, the flow processes in the root zone and the

subsoil (weathered zone) could be treated as one, but in deep water table situations, they

can be treated as independent because the flux through the lower boundary of the root

zone (precipitation excess) always has a downward direction and is not affected by the

conditions in the subsoil. This implies that capillary rise does not happen here (Lanen,

1996), that is, in deep water table situations.

Vegetation plays an important role in groundwater recharge. The amount of water used

by vegetation depends upon the type of vegetation and rooting depth. Grasses have

rooting depths of not more than 3.5 m while shrubs and trees go up to 5 m. Lysimetry

studies on two adjacent vegetation sites in USA have brought forth interesting findings

(Gee et al. 1994, quoted in Simmers, 1997). The site without vegetation (bare site) has

stored additional 150 mm water than in the surrounding vegetated sites during the winter

precipitation. Post winter, the entire stored soil moisture was used up by the vegetated

sites, while the bare site lost minimal. However, upon natural re-vegetation, the bare site

lost almost the entire stored soil moisture within four months. Studies (quoted in De

Vries, 2002) indicated that removal of indigenous vegetation in large parts of the semi

arid areas of south-eastern Australia more than hundred years ago has resulted in

increased recharge to groundwater due to changes in the hydraulic properties of the

topsoil which also resulted in enhanced runoff, accumulation of water in depressions, and

in increased recharge. Further, poor vegetation cover on a permeable soil or a fractured

porous bedrock near the surface, together with high-intensity rainfall, create favourable

conditions for recharge (De Vries, 2002).

Studies employing isotope techniques in arid and semi arid areas have indicated that

plants and trees suck moisture from depths as high as 50 m (Coudrain-Ribstein et al.

1998; Adar et al. 1995; De Vries 2000). The root zone tends to store soil moisture in

significant quantities, and would release water for downward movement depending upon

the hydraulic pressure or pressure heads it is subjected to at the inflow end, with a time

delay, though. In short, vegetation and land use change influence recharge in a significant

manner.


Rushton (1988) introduced the term potential direct recharge to imply the amount of

precipitation water that passes beyond the soil zone to reach the water table. Put

differently, it is all the precipitation water that is not up-taken by the plant roots (capillary

rise is included in the root water uptake). The potential recharge at the same place could

vary depending upon the water table. In areas with shallow (near surface) water table, the

potential recharge, that is the excess of precipitation over evapotranspiration losses,

cannot contribute to the shallow water table aquifer as it is already saturated. This results

in „losses‟ in the form of runoff, evaporation, evapotranspiration, perched water table, or

local groundwater systems. Sometimes, this is also referred to as „rejected recharge‟ as it

would have contributed to the aquifer if adequate storage space would have been

available in the aquifer (Karanth, 2004). However, if the water table is deeper, then there

would be larger scope for recharge and hence the potential direct recharge could be

higher.

Actual recharge is part of the potential recharge that reaches the water table, and is not

lost by interflow to nearby surface water bodies. During modelling studies, when

generating (future) scenarios, potential recharge is estimated rather than actual recharge,

since it is assumed that water tables decline further with use; also because, „actual‟ for a

future scenario is incompatible.

Scanlon et al. (2006) have synthesized recharge estimates for arid and semi arid regions

globally to evaluate recharge rates, controls and processes and to assess the impacts of

climate variability and land use/land cover changes on recharge. The study showed that

average recharge estimates over large areas range from 0.2 to 35 mm/year, representing

0.1-5% of long-term average annual precipitation. Where focused recharge conditions

were obtaining such as from ephemeral rivers and lakes, and preferential flow existed

through fractures and joints, then the recharge rates ranged very high, to around 720

m/year (Scanlon et al. 2006). The Chloride Mass Balance technique118

was found to be

widely used in the recharge estimates. Land use change brought about by replacement of

118

In the chloride mass balance (CMB) method, measurements of chloride in pore water are used to

estimate the recharge rate when both precipitation and chloride inputs are known (Gee et al. 2004).


savannah by crops in Niger (Africa), has resulted in increase in recharge by an order of

magnitude even during severe droughts. Whereas in irrigated areas, the recharge varies

from 10-485 mm/year representing 1-25% or irrigation plus precipitation.

Localised recharge from the morphological depressions or ill-defined dry valleys in the

Botswana Precambrian area ranges from 10 mm/year through alluvial loamy sediments to

30 mm/year through coarse-grained sediments and fractured outcrops for a mean annual

rainfall of 500-550 mm/year. In more arid areas, where the average rainfall is less than

200 mm/year, the recharge is only of the order of a few mm/ year with a coarse-grained

soil or fractured-rock outcrops (Issar and Passcher, 1990 quoted in De Vries, 2002).

Exposed karst areas in Saudi Arabia absorb 47% of the average annual rainfall (93

mm/year) into sinkholes and corrosionally extended joints, while in Portuguese Algarve,

the high-intensity winter precipitation facilitates 150-300 mm annual recharge (as quoted

in De Vries, 2002).

Seckler (1996) recognises that putting barriers across rivers in the form of check dams,

for example, results in raising of water tables to the plant root zone providing not only

irrigation with less evaporation but also providing considerable storage. He believes that

closed basins can be reopened by adopting the optimisation theory which is obtaining a

“local optimum” position in a sub optical portion of the whole system. Martin-Rosales et

al. (2007) report about the usefulness of engineering structures such as check dams on the

groundwater recharge. A study on a network of 107 check dams, including 64 check

dams located on permeable substrates, showed that the structures helped conserve the

critical flood waters due to intense rainfall events occurring in a semi arid area of south

eastern Spain. Modelling studies indicated that the infiltration was of the order of 3-50%

of the runoff.

GROUNDWATER RECHARGE STUDIES IN BLACK COTTON SOILS OF

BASALTS

A study by Hodnett and Bell (1981) on the Indian black cotton soils has revealed the

following important findings. The Deccan black cotton soils comprise the major


agricultural soil type across the 500,000 sq. kms. of the Deccan Trap basalts of India.

They are dark coloured, silty, swelling clays which have developed on a light olive brown

silty clay parent material (“yellow clay”) derived from in situ weathering of basalts

(Mudrakartha, 1987). Two types of soil and basalt rock conditions are encountered: one,

a soil zone of about 2 m overlying weathered basalt; two, a soil zone followed by yellow

clay occurs till around 10 m depth, sometimes deeper, overlying weathered basalt.

Black cotton soils show marked swelling and shrinking properties and an extensive

pattern of cracks, of up to 75 mm width and 6 m depth in grassy and shrub areas, during

the dry season. Where cultivation is practised, the cracks are generally between 1.5 and

2.0 m. Generally devoid of rock pieces, black cotton soils often contain limestone

concretions, called “kankar” in local (Indian) parlance (Mudrakartha, 1987). The water

table generally is between 2 and 11 m below ground level at the end of the dry season.

During monsoon, it rises to 0.2 m. The study reports that the study area (near Bhopal in

Madhya Pradesh, in Betwa river basin), displayed extremely uniform water holding

capacities; the depletion of its stored water in dry season varied around 230 mm,

depending upon the crop type, and occurrence of winter rainfall. Depletion below 2.5 mm

was found to be insignificant in the cropped areas, often restricted to 1.5 m. Grassy and

shrub areas had showed high depletion, of around 500 mm, typically.

Deep drainage or recharge from the upper 2.5 mm of the soil was found to be only 30

mm (or less) during dry season, from September to May. This was occurring within two

weeks of the monsoon. Subsequently, the total drainage for the remainder of the dry

season was 10 m at most.

Evidence from the other study area Dhaturi, south of Bhopal, structured black cotton soil

clay forms an upper aquifer system which is virtually isolated from the more conductive

weathered basalt aquifer by the presence of “yellow clay” layer. Recharge in this area

was found to be negligible during dry season, and less than 1 mm/day during monsoon

mainly due to the poor hydraulic conductivity of yellow clay layer. The „losses‟ here

occur due to surface runoff, interflow and evapo-transpiration.


Another experiment conducted in India during late seventies employed the water level

and specific yield formula to study the rainfall-recharge relationship. Analysing the

hydrographs of 15 observations wells for the years 1978-80 in the Hooghly district of the

Indian state of West Bengal, Bhattacharjee (1982) reports that good correlation was

found between the seasonal rainfall and recharge in the study area (as reflected by the

annual incremental value for rise of water level). The study area comprised clay, silt and

sand of different grades, occasionally mixed with gravel. The long term average rainfall

varied from 1430-1735 mm; the rainfall during each of the years at each of the stations

was not less than 900 mm and varied around a small band. The area can be considered a

high rainfall area with permeable top soil. The annual increment to groundwater reserve

was reflected by the average summer and average winter water levels, which was

considered inclusive of all types of recharge-direct recharge from rainfall, indirect

recharge from large water bodies such as rivers, streams and tanks, seepage from unlined

irrigation canals, and return flow from irrigation. The specific yield was determined from

pumping tests. The study also determined that no recharge takes place below a rainfall of

580 mm.

Similar studies were done on shale and dolomite areas and a linear relationship with

adjusted slopes and threshold values was found to exist in South Africa in studies by

Bredenkamp (1990) as reported in Lerner (1990).

Athavale et al. (1983) conducted recharge studies in the rainfall shadow region of the

north-south trending Sahyadri mountains, almost in a parallel direction to the West coast

of India. While the western side has a monsoonal rainfall of 3000-4000 mm, the larger

plateau area on the eastern side of the continental divide has an average annual rainfall of

500-700 mm. Dug wells tapping the phreatic aquifers are traditionally the main source of

groundwater, both for agriculture and domestic purpose, in this plateau region. There has

been rapid increase in exploitation of groundwater from the sixties.

The recharge studies were conducted on two representative basins, namely Kukadi and

Godavari-Purna basins. The lava flows comprise compact, massive or vesicular, basalt,


with or without amygdales, with a thickness varying from 5-50 m. The top zone is made

of black cotton soils derived in situ. The thickness of the soil and weathered zone varies

between 1.5 m and 9.5 m (with an average of 5 m) for Kukadi basin, and 1.5 and 39 m

(with an average of 14 m) for the Godavari-Purna basin. The rainfall for the study year

1980-81 was 612 mm, and (pan) evaporation 2226 mm for Kukadi, and 652 mm and

1710 mm for Godavari-Purna respectively. The recharge values obtained from 19 sites

using the tritium injection method for the Kukadi basin varied from ranged from 135 to -

8 mm. While the high value of 135 mm was found near the river, three sites gave

marginally negative values of 8, 6 and 4 mm. Here, the recharge is either zero, or the net

movement of water in the unsaturated zone is vertically upward. This was attributed to

several factors such as the local soil composition, evapotranspiration rate or the location

of sites in a groundwater discharge zone; hence the flow may have a vertical component

directed upwards. The mean of all 19 recharge values was found to be 46 mm or 7.5% of

the rainfall during 1980-81. The recharge values obtained in the case of the Godavari-

Purna basin for 24 sites were found to vary from 208 to -28 mm and the mean value was

found to be 56 mm or 8.6% of the rainfall in 1980. The authors have shown that the

recharge to the phreatic aquifer can be calculated by multiplying the specific yield with

the maximum water level change. Results from 7 out of 11 sites were within reasonable

agreement (Athavale, 1983).

Limaye (1986) has described some of his observations and findings based on his

experience in a report, as quoted in Lerner et al. (1990). Basalts show highly varying

rates of recharge depending upon the local hydrogeological conditions. In high rainfall

areas in excess of 2000 mm, the rate of recharge is found to be quite low-as small as 40

mm. A major portion of the rainfall runs off as surface flow. In undulating areas, part of

the recharge becomes runoff when encountered by the bedrock outcrops. In low rainfall

areas, a rainfall of about 250-300 mm is observed to contribute to recharge as high as 100

mm. This quantum of recharge is found to be possible when the topography is gentler,

and a thick weathered material is available with adequate porosity and permeability. On

the contrary, he states that in some areas, presence of a thick black soil precluded any

recharge from rainfall. He concludes that basalts have low primary porosity; the


secondary porosity created due to weathering is often the major source of groundwater

storage. Deeper confined to semi confined zones may occur underlying the older basalt

flows where a weathered zone and or a fractured zone may be present. While the recharge

has to pass through the soil and the weathered zone and fractured zones, where present, to

reach deeper layers, the hydraulic connectivity with surface water bodies becomes the

major source of supply. Sometimes, there could be connectivity with the overlying

weathered/fractured zone.

At a practical level, it is also important to understand how one could enhance the

productivity of water which is also linked to recharge. Seckler (1996) argues that the key

to improving productivity of water lies in [a] understanding and managing evapo-

transpiration and [b] water savings. This is because of the popular notion that evaporative

“losses” and drainage “losses” result in inefficiency of water use. Less than one percent

of the water consumed by crops is used for fluids in the plant: the rest is used to control

the heat of the plant. While the huge percentage used up in controlling the heat of the

plant may be considered “wasteful” in one sense, it is also equally essential for growth

and crop yields. Further, stage of crop, crop yields and actual evapo-transpiration are

highly correlated although the exact relationship is still not established (Seckler, 1996).

Therefore, the choice of crop depending upon climatic considerations, such as in hot

season would result in large savings in consumptive use of water. The rate of evaporation

is determined by the potential evapo-transpiration (PET) which is a function of climate

variables such as temperature, sunshine hours, vapour pressure and humidity. While PET

can be measured by the rate of evaporation from pan evaporimeter, the actual

evapotranspiration (AET) could be determined by multiplying the crop coefficients

determined for each crop in different climatic conditions119 (AET =crop coefficients x

PET).

On the water savings, Seckler (1996) recommends use of the concept of wet and dry

savings as is done in USA. The water that is saved by using say sprinklers as against

flood irrigation only leads to “redistribution” of water within the basin. If the farmers in

119

For more details see Seckler (1996).


the entire whole basin adopt sprinklers, the maximum efficiency that can be achieved

would be equal to the efficiency of the sprinkler itself. Therefore, the technology and

users also determine the “availability” (or access) of water within the basin. This is

termed as wet savings which is also referred to as “paper” savings. The conclusion is that

for real efficiency gains [a] crops selection should be appropriate with the climatic

conditions-in other words, increasing the output per unit of evaporated water [b] reducing

water losses to sinks, and reducing pollution water.

SECTION 2

GLOBAL EFFORTS FOR GROUNDWATER KNOWLEDGE AND PRACTICE

During mid-eighties, UNESCO, under its International Hydrological Programme

(IHP)120

, has sought to compile and disseminate knowledge and practices on water

resources management in arid and semi arid areas. For this purpose, an international

workshop was held in 1987 and a manual of practice produced (Lerner et al. 1990)121

.

The workshop gave a tremendous impetus to research across many countries on methods

of estimation of groundwater recharge, and recharge experimentation in various climatic,

geological, and hydrological conditions. There has been significant development of

physical and mathematical models to simulate flow conditions and understand flow

mechanisms. The follow up publication by Simmers (1997)122

captures the research

progress made in the past decade, and is considered a compendium to the volume 8. The

book covers diverse elements of groundwater recharge such as the recharge processes and

physics of water flow in a variety of geological environments. It also describes findings

120

In India, the IHP operates through an Indian National Committee on Hydrology (INCOH). INCOH is an

apex body with its Secretariat at National Institute of Hydrology Roorkee with the responsibility of

coordinating various activities concerning hydrology in the country. In order to carry out specific activities

in various fields of hydrology and water resources, INCOH consists of a main body and three sub-

committees. The INCOH has its members drawn from central and state government agencies as well as

experts from academic and research organizations. The Ministry of Water Resources (MoWR) provides

funds to INCOH to carry out various hydrological activities

http://www.unesco.org/water/ihp/nat_reports/pdf_18th/india_nat_report_2008_en.pdf accessed 19 January

2009. 121 David N. Lerner, Arie S. Isaar and Ian Simmers (1990). Groundwater Recharge: A Guide to

understanding and Estimating Natural Recharge, vol. 8, International Association of Hydrogeologists. 122

Simmers (1997). Recharge of Phreatic Aquifers in (Semi-) Arid Areas, vol. 19, International

Association of Hydrogeologists.

http://www.unesco.org/water/ihp/nat_reports/pdf_18th/india_nat_report_2008_en.pdf


by various practitioners and researchers based on field experimentation and modelling. A

special volume of the Hydrogeology Journal devoted to recharge includes papers on

recharge processes and methods of estimating recharge, including remote sensing,

ground-based and modelling approaches, artificial and urban recharge, and case studies in

(semi-) arid regions (Scanlon and Cook, 2002).

Another international effort is the setting up of the International Centre for Groundwater

Resources Assessment (IGRAC) early 2000 established under the Global Groundwater

Monitoring System supported by IGWCO, GARS and UNESCO. The need for a global

network was felt by the IGRAC and a GTN-H (Global Terrestrial Network for

Hydrology) supported by the joint efforts of the WMO Hydrology and Water Resources

(HWR) Department, the Global Climate Observing System (GCOS) and the Global

Terrestrial Observing System (GTOS) was set up. The GTN-H is a global hydrological

“network of networks” for climate that is building on existing networks and data centres

and producing value-added products through enhanced communications and shared

development.

Global Groundwater Monitoring System (GGMS) is a system that aims at providing

spatially aggregated global information on groundwater variables. The Global

Groundwater Monitoring System includes monitoring variables on aquifers using

aggregation procedures for groundwater variables world-wide. According to these

procedures, the aggregation of point measurements is necessary to ensure

representativeness of information in all studies at all scales, and the complex process of

aggregation needs to be carried out by people network of experts on regional

hydrogeology, measurement practice, water use, etc.

The need for a GTN-GW is motivated by the following main factors:

Generally perceived poor access to geo-referenced information on groundwater at

a global scale: even if the information does exist, it is often so difficult to access

it; in fact the main objective of the first workshop on “Global Monitoring of

http://igrac.nitg.tno.nl/index.html

http://igrac.nitg.tno.nl/index.html

http://gtn-h.unh.edu/PHP/index.php

http://gtn-h.unh.edu/PHP/index.php

http://igrac.nitg.tno.nl/docs/WorkshopGM.pdf

http://igrac.nitg.tno.nl/docs/WorkshopGM.pdf


Groundwater Resources” (18-19 October 2007 in Utrecht, the Netherlands) is

assessing the state of relevant information resources, which can contribute to the

analysis and planning of groundwater at the global, regional and even national

levels thanks to the Global Groundwater Monitoring System

Inadequacy of groundwater data acquisition in many countries: many

groundwater systems have not been explored and assessed sufficiently, while

variations in time of the groundwater conditions – essential information for

adequate management – are monitored only occasionally.

IGRAC pursues these objectives by the development of a Global Groundwater

Information System (GGIS)123, by activities related to Guidelines and Protocols for

groundwater data acquisition (G&P) and by participation in strategic global and regional

projects with a strong groundwater component (WHYMAP, WWAP, UNESCO working

groups, ISARM, etc.). The guidelines and protocols placed on the internet in the public

domain and inform the user about the purpose, content, type, nature and scope of the

guideline or protocol. It also guides on the type and variety of data that needs to be

collected, and the standards to reduce inaccuracies. Cooperation with UNESCO, WMO,

IAH124

, national groundwater organizations and many other partners all over the world is

among the key mechanisms established to achieve IGRAC‟s goals125

.

123

GGIS database of IGRAC (see: www.igrac.nl), intends to bring together all information relevant for

groundwater, draws heavily on data from these and other global or regional databases. GGIS not only

builds a global database on groundwater-related attributes, but it also provides possibility for online

visualization. One of the GGIS views is „country-oriented‟, like the AQUASTAT and GEO databases, and

contains 77 standardized attributes. In addition to these administratively defined spatial boundaries, IGRAC

has developed a system of Global Groundwater Regions, in order to organize data according to more

physically based units. For this second view, 43 uniform attributes have been defined. 124

The International Association of Hydrogeologists‟ World Wide Groundwater Organisation was

established formally in 1956 as an international forum on the management of groundwater for the benefit of

mankind and the environment. The scientific and educational organisation aims to promote research into

and understanding of the proper management and protection of groundwater for the common good

throughout the world. IAH has over 3500 members in 135 countries and works with agencies in the UN

system, especially UNESCO, FAO, IAEA and the World Bank, with other water-related NGOs, and is a

member of the World Water Council. The IAH publishes Hydrogeology Journal, newsletters, and

occasional books and publications (www.iah.org). 125

www.fao.org/GTOS/doc/2008-meetTOPC11/ecv/ECV-T03-groundwater-report-v05.doc accessed 19

January 2009.

http://igrac.nitg.tno.nl/ggis_start.html



http://www.fao.org/GTOS/doc/2008-meetTOPC11/ecv/ECV-T03-groundwater-report-v05.doc


IGRAC has defined three main fields of activity (www.igrac.nl):

a. The first one is the development of a Global Groundwater Information System

(GGIS) for various categories of stakeholders. The System is envisaged as an

interactive and transparent portal to groundwater-related information and

knowledge. The GGIS will be publicly accessible through the Internet.

b. The second field of activity is the development and promotion of guidelines and

protocols for the assessment of groundwater resources. This activity aims to

stimulate the proper acquisition of sufficient and comparable groundwater data

world-wide. It pays special attention to monitoring of time-dependent

groundwater data.

c. Finally, IGRAC uses any opportunity to participate in or contribute to global and

regional projects in need of groundwater-related inputs.

Dealing with groundwater, IGRAC is part of a family of global information centres and

global observing systems aiming to improve our understanding and management of the

earth's freshwater resources. The UN World Water Assessment Programme has identified

IGRAC as a pillar of their groundwater assessment programme. IGRAC strives for an

active role in other global programmes as well, such as the implementation of the action

plan of the World Summit for Sustainable Development.

The monitoring variables identified include: groundwater level, groundwater abstraction,

salinity and other indicators of water quality, storage coefficients, well head elevation,

screen depth, and local aquifer characteristics including aquifer type, thickness and

whether measurements are for confined/unconfined units. All these variables will serve to

monitor groundwater resources as well as to support related inquiries126

.

It is interesting to note that the Government of India has recognised the need for a

National Spatial Data Infrastructure (NSDI) as back as early 2001. Although a clear

126

www.fao.org/GTOS/doc/2008-meetTOPC11/ecv/ECV-T03-groundwater-report-v05.doc accessed 19

January 2009)

http://www.igrac.nl/

http://www.fao.org/GTOS/doc/2008-meetTOPC11/ecv/ECV-T03-groundwater-report-v05.doc


consensus on the concept of SDI has not been arrived at, various definitions tend to

include the policy and organisation, interoperability and sharing, and discovery, access,

and use of spatial data. The SDI also emphasises on the need to understand the nature of

the socio-technical networks that constitute SDIs, including data, databases, ICTs,

standards, people, institutional histories and practices, and applications. The Task Force

set up for the purpose in October 2000 has come out with a blueprint NSDI: Strategy and

Action Plan. After quite a delay, the NSDI portal (India Geoportal-

URL:www.nsdiindia.org.in) was launched by the Department of Science and Technology

(DST) in December 2008127

, where only a limited spatial metadata of India is available.

The vision of NSDI is to build infrastructure for the availability of, and access to

organised spatial data use of the infrastructure at community, local, state, regional and

national levels for sustained economic growth (Indian Space Research Organisation,

2001. All the major government departments such as the Survey of India, Ministry of

Earth Sciences, DST, Central Ground Water Board, India Meteorological Department,

Planning Commission, National Bureau of Soil Survey and Land Use Planning, Census

of India, National Informatics Centre are collaborative members on the NSDI128

. Though

delayed, the NSDI shows promise.

SECTION 3

GROUNDWATER RECHARGE ESTIMATION METHODS-AN OVERVIEW129

Selection of method(s) of natural recharge estimation is incumbent on the purpose. The

purposes may vary from groundwater assessment and monitoring for management

decisions130

, augmenting groundwater recharge, to identifying sites for disposal of

hazardous waste materials. Spatial variability in recharge at local and intermediate scales

127

When the portal was accessed on 1 May 2009, the domain name was announced to have been expired on

25.4.09. 128

Singh, P.K., (2009). Spatial Data Infrastructure in India: Status, Governance Challenges, and Strategies

for Effective Functioning. Working paper 210. Institute of Rural Management Anand (IRMA), Anand,

India. 129

This section draws, inter alios, from Scanlon (2002). 130 The CGWB in India assesses the groundwater status, talukawise, in order to take policy decisions on

whether to allow further extraction; it also plans artificial recharge schemes based on this information.


is critical for water-resource assessment; more so for waste disposal sites, because

preferential flow allows contaminants to migrate rapidly through the unsaturated zone to

underlying aquifers. Locating areas of low recharge is important for radioactive and

hazardous-waste disposal sites (Tyler et al. 1996; Scanlon et al. 1997 as quoted in

Scanlon et al. 2002). Time scales of recharge estimates are important: decadal or multiple

years of recharge are helpful in water resource planning while contaminant transport

requires recharge information of duration varying from days to thousands of years,

depending on contaminants‟ (nature, type and decay characteristics) consideration

(Scanlon et al. 2002). In arid and semi arid regions, however, local recharge estimates are

important in view of the criticality of groundwater need especially for irrigation and

drinking water. In hard rock areas, often, the aquifers are unconfined and shallow; local

recharge measurements help in taking decisions on crop selection.

The recharge measurement methods could be broadly categorised into the following:

Direct Measurement methods;

Darcian approaches;

Environmental tracers; and

Water balance methods.

Direct measurement methods

Lysimetry is the only method that facilitates direct measurement of precipitation recharge

flux. The key to using lysimeters is to ensure “natural conditions of soil” so that the

realistic recharge measurements are made. Lysimeter is a technique that would measure

water flows through disturbed or undisturbed soils; blocks or cylinders of diameters as

large as up to 10 m are constructed in situ which are hydrologically isolated, the isolation

aimed at causing minimal change in soil natural, existing conditions. Lysimeters are also

employed in arid and semi-arid climates although they are considered to be best

applicable in humid conditions. Since they are point measurements, the larger the area

and density of measurements, higher is the cost. Thus, they prove expensive in studies

regional in nature. It is essential to be aware of the limitations when using the lysimeters


for recharge measurements. Two lysimeters placed 50 m apart have produced recharge

rates of 159 and 114 mm/year (Kitching et al. 1977)-a difference which is significant in

arid and semi arid areas where much lower recharge rates are frequently encountered. In

addition, the variation in soil conditions and climatic conditions is also quite

deterministic.

Darcian approaches

Darcy‟s law describes the flow of fluid in a porous medium and is given by the relation

(Michael, 1983; Lerner et al. 1990):

Q = KA(h1−h2)/L = KiA

where:

Q is the rate of flow of liquid through a cross-sectional area,

K is a permeability coefficient (describes the porosity of the underground formation),

A is the cross sectional area,

h1 is the height of the inlet head,

h2 is the height of the outlet head,

i is the hydraulic gradient, and

L is the path length of the flow.

The flow in unsaturated zones is usually vertical due to gravity. The water in fully or

partially saturated zones has a total potential comprising gravity and the pressure

component. The flow in an unsaturated formation differs from that of a saturated flow in

terms of the sensitivity of hydraulic conductivity. Hydraulic conductivity in unsaturated

formations varies with the moisture content, and these two vary with the hydraulic

pressure. These principles are used in estimation of precipitation recharge in unsaturated

formations. In an unsaturated flow, the pressure arises from the capillary forces that hold

water in the interstitial pore spaces. This capillary force is always negative, below the

atmospheric pressure. In transient or non steady flow conditions, water is either taken in

or released from storage as per changes in the moisture status of the formation.


Tracer Methods

Tracers used in groundwater flow studies comprise both chemicals and isotopes.

Combined with water balance models, tracers provide valuable information on the flow

processes in the vadose zone. Three types of tracers exist: artificial, historical and natural.

Artificial tracers such as dyes are applied artificially in to the soil zone; when (rain) water

starts flowing, the tracer also moves dissolved in water. Such tracers can be applied in

quantities to make measureable measurements. Tritium is an isotope that is commonly

used in flow measurements. Historical tracers are used in a fashion similar to the artificial

tracers except that the tracers in this case are pre-existing due to historical events such as

nuclear testing, disaster events, farming practice or industrial pollution. Environmental

tracers are co-existing in nature in the landscape and their spatial pattern or overall mass

balance is measured to trace the groundwater flow paths. Chloride is the most commonly

used environmental tracer using the chloride mass balance; others used for flow studies

include nitrate and stable isotopes of water. The selection of tracer depends upon the

situation. A description of the use of chloride and tritium is given below to explain the

process.

The principle of use of environmental chloride is:

Recharge in mm=Chloride in Rainfall (mg/l)/Chloride in groundwater (mg/l) x Rainfall

in mm

Put differently, mass of Chloride into the system (precipitation/rainfall and dry fallout)

times the Chloride concentration in Rainfall is balanced by the mass out of the system

(drainage, D) times the Chloride concentration in drainage water in the unsaturated zone

if surface runoff is assumed to be zero (Scanlon, 2002). This method is found to work

well in conditions of low drainage because chloride concentrations are sensitive to small

changes in drainage. Hence, even low rates of recharge, less than 1 mm/year can be

measured while on the upper end, about 300 mm/year is reported. This method provides

point estimates, for time scales ranging from decades to thousands of years (Scanlon,

2000). A critical assumption is that there are no chloride sources or sinks in the path of

recharge process.


Like Chloride, tritium isotope is also used for point determination of natural recharge

rates. A number of such measurements help arrive at an average natural recharge rate.

Average natural recharge rate x effective infiltration area = total recharge. The tritium is

injected into the soil zone just when the monsoon is due. When the rainfall occurs, the

tritiated water percolates down through the soil. Recharge can be calculated at the end of

one hydrologic cycle by making measurements on the tritium activity (tracer

displacement), soil moisture concentration and wet bulk density at the same spot. NGRI

has used tritium injection method in many of their experiments such as in the Kukadi and

Godavari-Purna river basins. The recharge values obtained were respectively 7.5% and

8.6% of the rainfall of 612 mm and 652 mm during 1980-81 for potential evaporation of

2226 mm and 1710 mm respectively. These values represent the minimum recharge

values because they do not include the contribution from the stream and lake beds (which

comprises indirect recharge). Recharge estimates were also computed by two methods,

namely, the tritium injection method and the WL & SY method in the Godavari-Purna

basin. A reasonable agreement was obtained between the two methods in seven out of 11

sites (Lerner et al. 1990).

Water balance methods

There are four water balance methods in vogue, namely, soil moisture budget, water table

fluctuation method and the regression method.

[i] Soil moisture balance

Developed initially in the 1940s by Thornthwaite, and revised later, the soil moisture

balance is essentially a book keeping procedure which estimates the difference between

inflow and outflow as the change in the soil moisture storage (Kommadath, 2000). The

method assumes that precipitation recharge occurs only when the soil is fully saturated,

and begins to release water downwards. In practice, depending upon the type of soil,

downward movement of water happens even before complete saturation of the soil zone

(Simmers, 1997). First applied in mid-fifties to temperate regions, this method was also

applied by some to semi arid regions where clear distinction of dry and wet periods

exists.


The soil moisture balance for any time interval can be expressed as (Karanth, 1987):

P = AET + I + R+ δSm

Where P=Rainfall; AET=Actual Evapotranspiration; δSm=change in soil moisture

storage; I=Infiltration; and, R=surface runoff. The soil moisture budgeting helps compute

moisture available for runoff and infiltration after meeting the evapotranspiration losses.

Generally, movement of moisture beyond the root zone would take place only after the

field capacity is achieved; even during the process of field saturation, evapotranspiration

continues to happen. In sum, both field capacity and the wilting point determine the

moisture movement beyond the root zone, and to the groundwater table. If rainfall in a

given month is less than potential evapotranspiration, then AET is equal to rainfall; what

we have then is a water deficit condition. If the rainfall is greater than the PET, then AET

is equal to PET, and the balance of rainfall going to saturate the soil to field capacity.

Thus, excess of rainfall over PET results in a water surplus condition, and a continued

water surplus condition results in recharge to the groundwater table. Under saturated

conditions, a soil provides for runoff as well as evapotranspiration. In the arid and semi

arid areas, runoff occurs for very small time duration, and, occasionally; hence can be

ignored, and the entire surplus can be treated as groundwater recharge (Karanth, 1987).

Excess of soil moisture at the end of the (hydrological) year is carried over to the

succeeding year.

The soil moisture method assumes that the ratio, AET/PET has a linear relationship with

the available soil moisture. The ratio becomes zero when the soil moisture availability

reaches the wilting point. The ET at this point is nil. At field capacity, the AET and PET

are equal or the AET/PET ratio is equal to 1.

A major difficulty in this method is the measurement of evapotranspiration. The

commonly employed methods to estimate actual evapotranspiration include the Penman

formula (1948, 1949), and the Penman-Monteith (1965, 1981) for humid climates. Actual

evapotranspiration is generally estimated through potential evapotranspiration or

reference crop evapotranspiration (Lerner et al. 1990). However, both these methods


involve several parameters and conditionalities which bring in inaccuracies when

extended for application to larger areas. For example, data from a climate station in a

large, well irrigated area of crops cannot be applied to semi arid areas with natural

vegetation (Lerner et al. 1990). Put differently, the method requires huge data to estimate

recharge with data repeated for areas with different precipitation, evapotranspiration, crop

type and soil type. Moreover, storage of moisture in the saturated zone and the rates of

infiltration along the various possible routes to the aquifer, form important and uncertain

factors (Kommadath, 2000).

Further, the rooting depth also brings in uncertainty because the groundwater movement

within and beyond soil zone is also influenced by the rooting depth. Rooting depth varies

between crops, stages of crops, trees and other vegetation. Therefore, calibration becomes

very important which is time consuming.

[ii] Water Level Fluctuation & Specific Yield method

This is one of the popular, indirect methods widely used in India for computing

groundwater recharge because it is simple and direct, and the method makes use of

readily available data on rainfall, runoff and water levels. These data account for some

inflows and outflows in a groundwater system such as extraction through wells and

recharge through injection. The recharge values obtained represent average values over

space because the water levels are also spatially represented. The volume of water stored

beneath a water table is considered equal to the recharge after allowing for inflows and

outflows such as pumping wells and aquifer throughflow (Lerner et al. 1990). CGWB,

the apex government body dealing with groundwater policy for India, employs the

following equation to compute natural recharge values:

NR = (A x δh x Sy) + DW – (Rs + Rigw + Ris) x (NF + Rs + Ris) (1)

where NR = monsoon recharge (MCM/yr); A = Area suitable for groundwater recharge

(sq. Km.); δh= change in storage of groundwater between pre and post monsoon (m); Sy


= specific yield of formation (%); DW = Groundwater draft during monsoon (MCM/yr);

Rs = Recharge from canal seepage and tank seepage during monsoon (MCM/yr); Rigw =

Recharge from recycled groundwater in monsoon (MCM/yr); Ris = Recharge from

recycled surface water in monsoon (MCM/yr); and NF = Normalisation Factors =

Average – Long term RF/Avg-short term rainfall.

Generally, the water level reaches a minimum (that is, the lowest) level before monsoon

and a maximum (the shallowest) post monsoon. The difference between the maximum

and the minimum levels gives the water level fluctuation for the year. The water level

fluctuation is assumed to be representative of the net inflows and outflows of the area

under consideration. However, this is not true as some subsurface flows as discussed

below are not accounted for. In arid and semi-arid regions, there are no sources of water

other than the rainfall, except for the regeneration (return) flows from agriculture,

seepages from canals in limited areas and from tanks mostly during monsoon and a little

later.

The specific yield, a property of unconfined aquifer, is defined as the volume percent of

groundwater yielded by a saturated rock formation through gravity drainage. As

discussed earlier, some moisture is retained by the formation, which is termed specific

retention. Specific retention is the quantity of water that a unit volume of aquifer retains

when subjected to gravity drainage. Specific yield and specific retention determine the

porosity of the formation. Over many decades, the Central Ground Water Board has

conducted pumping tests on wells in various geological formations and determined

specific yields. These are generally determined to lie in a range; for basalt formation, it is

determined to be 1-3% (Sinha & Sharma, 1988; Athavale, 2003).

In order to arrive at recharge values for India using this method, the Central Ground

Water Board collects data on water levels from the 15,000 hydrographic network of their

well stations and 30,000 observation wells monitored by the state groundwater boards

(Athavale, 2003). Using the standard specific yield values determined from pumping tests

in the concerned areas, and substituting values for other facts in equation (1) determined


from either experiments or from assumptions, the recharge values are computed.

[iii] The Regression Method

Efforts to develop rainfall-recharge equation were attempted early seventies using daily

rainfall and climatic data (Bredenkamp, 1990 in Lerner et al. 1990). Soon the monthly

and annual data were used as inputs with encouraging results. The experiments on

dolomite, shale, and some other formations showed a linear relationship between average

annual rainfall and average annual recharge. These recharge values agreed closely with

values arrived at by using other methods such as water balance, water balance finite

element model, and finite element simulation (Bredenkamp, 1990, Table 19.5). A

regression line was fitted among the data plots between these two parameters and

regression equation derived for each type of formation. The intercept of the regression

line on the x-axis indicated a rainfall value below (400 mm) which there was no recharge

produced.

Experiments in granites and gneisses (Houston, 1990 in Lerner et al. 1990) indicated

similar results. Recharge values computed varied due to the uneven distribution of

rainfall and intensity, both in space and in time. In terms of space, areas with rainfall

above 700 mm were generally found to receive significant recharge than those with

rainfall below 700 mm. Below 400 mm-the point of intersection of the regression line on

x-axis, the recharge was uncertain and unlikely. Importantly, the experiment indicated

clearly that on a temporal basis, rainfall determined recharge, which often correlated with

the short heavy spells within a year, and a few years with heavy rainfall. It was observed

that 2-5% of rainfall for granites and 0.5-1% of rainfall for gneisses normally transformed

into recharge (Lerner et al. 1990). While such insights are available from the studies,

broadly there is no linear relationship that holds between rainfall and recharge. This is

because of various factors. For example, a low intensity of rainfall may not be able to

produce any recharge while a moderate to high intensity of rainfall may produce

recharge, depending upon the soil characteristics and depth of water level. Similarly, the

monthly and annual recharge estimations may miss out on a few days of recharge, of


whatever magnitude, produced due to high intense rainfalls of a few days. Thus, the time

steps used in recharge estimation methods, especially in modelling, becomes important.

In India, one of the earliest natural groundwater recharge estimates were made by

Chaturvedi (1973), who derived an empirical relationship between rainfall and recharge

(when rainfall exceeded 400 mm):

R = 2.0 (P - 15)0.4

Where

R = net recharge due to precipitation during the year, in inches; and

P = annual precipitation, in inches.

This was modified by UP Irrigation Research Institute, Roorkee, as R = 1.35 (P-14)0.5

.

Here, there is a lower limit of rainfall of 14 inches below which there was nil recharge.

The percentage of rainfall recharged commenced from zero at P = 14 inches, increases

upto 18% at P = 28 inches, and again decreases. The lower limit of rainfall in the formula

may account for the soil moisture deficit, interception losses and potential evaporation.

These factors being site specific, one generalised formula may not be applicable to all the

alluvial areas (Kommadath, 2000).

Among other methods, Krishna Rao (1970) proposed an empirical relationship to

determine the groundwater recharge in limited climatologically homogenous areas

(Kommadath, 2000):

R = K (P - X).

The following relation is stated to hold good for different parts of Karnataka;

R = 0.20 (P - 400) for areas with P between 400 and 600mm;

R = 0.25 (P - 400) for areas with P between 600 and 1000mm;

R = 0.35 (P - 600) for areas with P above 2000mm;


where R & P are expressed in millimetres, K and X are constants.

The National Geophysical Research Institute conducted a large number of natural

recharge measurements covering four rock types, namely, granites, basalts, sedimentary

and alluvium, across 36 basins distributed all over India (Athavale, 2003). The mean

natural recharge on x-axis was plotted against the seasonal rainfall for the four rock types

and regression equations developed for each rock type.

The y-axis intercept of the least square fit denotes rainfall beyond which the surface run

off is generated. In other words, rainfall till this point is absorbed by the soil zone as

moisture. Therefore, the natural recharge RE (mm) is computed using the Regression

equation (Athavale, 2003):

RE = 0.174 (Rainfall in mm)-62. (2)

The next step is to calculate the volume of natural recharge by multiplying the natural

recharge obtained from the above equation with the Area. (3)

[iv] Surface water budget

Rivers often constitute a major source of groundwater. Although the rivers in arid and

semi arid zones flow only seasonally, they aid groundwater recharge to the aquifer

system depending upon its hydraulic connectivity with the aquifer. In particular, when the

aquifer is shallow and unconfined, the recharge benefits are quickly realised. However,

beyond the monsoon season, in particular, in areas with unimodal rainfall pattern, the arid

and semi arid regions are generally characterized by losing surface-water bodies, because

surface-water and groundwater systems are often separated by thick unsaturated sections.

Therefore, surface-water bodies often form localized recharge sources in such settings.

Recharge can be estimated using surface water data in gaining and losing surface-water

bodies (Scanlon, 2002). However, in practice, this method is difficult to apply as it


involves determination of infiltration losses which are small numbers (and evaporation

losses from surface bodies which are significant and comparable often to rainfall in

quantity) compared with the precipitation and river flows which are large numbers

(Simmers, 1997). However, if measurements can be made of stream flows, the method

can be used.

Recharge measurements are also difficult when a low conductivity layer lies below the

river course resulting in perched water. This perched water helps in developing and

continuing the aquifer-stream interconnectivity.

CHALLENGES IN RECHARGE ESTIMATION131

The following are some of the challenges faced in the arid and semi-arid regions:

The primary challenge faced in recharge estimation in particular in arid and semi arid

regions relates to inadequacy and inaccuracy of data; further the data is of small term in

nature (Lerner et al. 1990). Another basic difficulty is in identifying the probable

recharge mechanisms because of highly varying hydrogeology and climatological factors.

In order to identify the probable recharge mechanisms, one needs a lot of a wide variety

of data on the study area, which would be both time consuming and expensive. In

addition, recharge is a non-linear phenomenon with respect to time and is highly varying

across space. This results in complications in collection of data as well as scarcity

(Sophocleous, 2004).

Further, in the arid and semi arid regions of India, the annual potential evapotranspiration

is higher than the annual rainfall, the two components in the soil water balance equation.

The rainfall and evapotranspiration are big numbers compared to recharge which is a

small number. Therefore, even a small error in either of the rainfall or evapotranspiration

components results in a large error in the small recharge number.

131

Some of the points in this section are drawn from Simmers (1997).


The localised recharge which occurs as a horizontal flow into local topographical

depressions is a function of the surface runoff and is difficult to measure on two counts.

Firstly, the depressions are too widespread to practically carry out measurements unless

under some focused research programme. Secondly, the seasonal streams which are also

considered „local‟ are generally not gauged.

The current understanding of the flow processes through deep vadose zone, though

limited (Simmers, 1997), is evolving.

(v) Groundwater Modelling to Enhance Accuracy of Groundwater Recharge Estimates

The process of groundwater recharge is determined by the interaction of climate

(temperature, sunshine hours, wind speed, vapour pressure), geology, morphology, soil

condition, water table and vegetation (De Vries, 2002; Lanen, 1998; Tallaksen & Lanen,

2004). During the process of groundwater recharge, the soil gets fully saturated before

further percolation takes place to the groundwater storage (Michael, 1983, Tallaksen &

Lanen, 2004). The groundwater flow processes and the rate of recharge are dependent

upon the soil type132

, soil texture (composition of sand, silt and clay), soil structure

(arrangement of individual soil particles), and soil profile (vertical section through the

soil mass that reveals significant variations in the soil texture and soil structure).

Vegetation plays a critical role in the recharge process because the soil has to meet the

moisture demands by the plants, which in effect implies interruption of the recharge

beyond soil zone. In the arid and semi arid regions, the surface evaporation, and

transpiration (the process in which the water vapour leaves the plant body into the

atmosphere) are together termed as evapo-transpiration133

. Evapo-transpiration therefore

includes all the water that is consumed by the plants and that which evaporates from the

bare land and water surfaces in the area occupied by the crop (Michael, 1983). The

evapo-transpiration is usually computed on the basis of the climatological data relating to

132

The soils of India are broadly divided into four major groups: (i) black soils, (ii) alluvial soils, (iii) red

soils, and (iv) lateritic soils. The other soil groups are forest and hill soils, desert soils, saline and alkaline

soils, peat and marshy soils (Michael, 1987). 133

Evapotranspiration is the quantity of water transpired by plants during their growth, or retained in the

plant tissue, plus the moisture evaporated from the surface of the soil and the vegetation (Michael, 1987).


temperature, humidity, wind, length of day and sunshine hours. A complication in this

context is the difficulty in obtaining the data on the climate parameters and the problems

with adequacy and accuracy.

During the past three decades, many countries including India have focused on enhancing

their understanding of the complexity of groundwater flow mechanisms in arid and semi

arid regions. As part of this process, they have also set up systems to collect in-country

data in a bid to improve research efforts. At the global level, certain efforts have been

made to collate, compile and place data in the public domain. One such effort is that

made by the University of East Anglia which has collected, compiled and placed climate

data after necessary rectification in a public access domain. This data is available for the

entire land mass of the globe and is easily accessible by anyone for the period 1901-2002.

Platforms such as the International Hydrological Program (IHP)134

are being promoted

not only to improve data availability and accuracy, but also disseminate research findings

while identifying critical research gaps. International Hydrological Program (IHP),

International Centre Groundwater Resources Assessment (IGRAC) and GTN-H (Global

Terrestrial Network for Hydrology) are some of the international efforts focusing on the

ways, methods and means of improving data collection and analysis at individual country

level, including the institutional aspects.

The past two decades in particular have seen groundwater modelling emerging as a

decision making tool for not only understanding the complex groundwater flow

mechanisms and contaminant transport but also for estimating groundwater recharge

(Chiang et al. 1998). Models are numerical tools that help in understanding physical

systems and aiding decision and actions. In spite of severe limitations due to inability of

the models to precisely mimic the physical “real” situations, groundwater modelling

gives reasonably good insights into the flow processes and helps in visualising flow

patterns in various complex geological situations.

134

IHP is UNESCO's international scientific cooperative programme in water research, water resources

management, education and capacity-building, and the only broadly-based science programme of the UN

system in this area. The themes are: Global changes and water resources, Integrated watershed and aquifer

dynamics, Land-habitat hydrology, Water and society, Water education and training (www.unesco.org).

http://typo38.unesco.org/en/themes/global-changes-and-water-resources.html

http://typo38.unesco.org/en/themes/integrated-watershed-and-aquifer-dynamics.html



http://typo38.unesco.org/en/themes/land-habitat-hydrology.html

http://typo38.unesco.org/en/themes/ihp-water-society.html

http://typo38.unesco.org/en/themes/water-education-and-training.html

http://www.unesco.org/


In addition to the Water Level & Specific Yield methods, the CRU_NUT MONTH

method has been used to estimate groundwater recharge of the study areas. This section

describes the procedure that has been used for the use of CRU_NUT MONTH method.

For the purpose of extracting the climate parameters for the study villages from the CRU

TS 2.1 database developed by University of East Anglia, the CRU meteo program has

been used. The climate parameters obtained are used as input, along with inputs for soil

moisture data on field capacity, critical point and wilting point (or wilting percentage),

into another soil water balance program called NUT_MONTH, to compute groundwater

recharge values for the study villages.

ESTIMATION OF RECHARGE USING A SOIL WATER BALANCE MODEL

Brief description of the approach (CRU data, CRU_TS21_READ_METEO,

NUT_MONTH)

This section describes an important effort to construct a global database relating to

climate parameters by Dr. TD Mitchell and Michael Jones (formerly) of the University of

East Anglia drawing from various sources following New et al. (2000) and Mitchell et al.

(2004). The climate variables to which the sources contributed are temperature (tmp),

diurnal temperature (dtr), precipitation (pre), vapour pressure (vap), cloud cover (cld),

sunshine duration (spc), and wet days (wet). The dtr includes information from individual

records of daily temperature minima (tmn) and maxima (tmx). These data are sourced

from individual countries collected from their own weather observation stations. The set

of grids extend from 1901 to 2002, cover the global land surface (excluding Antarctica)

at a 0.5 deg. resolution, and provide best estimates of month-by-month variations in nine

climate variables. This dataset is labelled CRU TS 2.1 and is publicly available

(http://www.cru.uea.ac.uk/).

One important aspect in the construction and posting of the data has been the issue of

handling inhomogeneities. The Global Historical Climatological Network (GHCN)


method of homogenization has been used, as it is designed for the automatic treatment of

large datasets with global coverage, and has already been applied to a well-established

dataset (Peterson and Easterling, 1994; Easterling and Peterson, 1995). The method uses

neighbouring stations to construct a reference series against which a candidate series may

be compared. Neighbouring stations are selected by a correlation method. If the

correlation is performed on absolute values, then a candidate station with a discontinuity

may be better correlated with an inhomogeneous neighbour than with one without the

discontinuity. Therefore, series of first differences are correlated, to limit the effect of any

discontinuity to a single value (Mitchell & Jones, 2005). The database and the grids

subsequently constructed from it are designed to depict the month-to-month variations in

climate experienced at the Earth‟s surface, rather than to detect changes in climate

resulting from greenhouse gas emissions. Since any inhomogeneities were corrected so as

to make the record consistent with its final values, near-real-time observations may be

appended without introducing inhomogeneities.

Data from manual weathered stations is preferred because the satellites came into picture

only during seventies; further, satellites measure conditions through the depth of the

atmosphere rather than at the surface (Mitchell & Jones, 2005).

Mitchell & Jones, (2005) describe in complete detail how the data had been expanded,

improved and updated in order to construct a set of climate grids (CRU TS 2.1). The

paper also discusses limitations and difficulties.

The sources with longer series all show a steady increase in the number of stations

available during the 20th century, a peak around 1980, and a rapid decline to the present,

that is, 2002. This method of detecting inhomogeneities has its weaknesses. One

weakness is that it is designed to detect abrupt rather than gradual inhomogeneities,

although gradual inhomogeneities will also be detected unless they are widespread. This

method also has none of the advantages of a manual method; an automated method is

essential to handle such large quantities of data. However, the method should be


sufficient for a database designed to provide best estimates of inter-annual variations

rather than detection of long-term trends.

The data is often collected based on a hydrological year or a water year. A hydrological

year comprises a 12-month period selected in such a way that the overall changes in

storage are minimal (Tallaksen & Lanen, 2004). Different countries adopt different

starting points based on local climate conditions. For example, the Netherlands adopts

dry period as the starting month of the hydrological year coinciding with April while

Denmark adopts June; USA, Spain and South Africa adopt a wet (winter) season

beginning October while Germany adopts the month of November. Sometimes,

researchers adopt a different starting point based on the objective of the study. In India,

the hydrological year begins from June to May of the following year. The advent of

monsoon is considered as the starting point. The CRU database is organised to coincide

from January to December.

A program CRU_TS21_READ_METEO in fortran compiled by the authors (Mitchell &

Jones) helps one to extract climate data for coordinates (latitude and longitude) of a land

area on the globe. For this, we need to provide the location coordinates in an input file,

CRU_input (Figure 4.6) which is a text file. The program can accept values for latitude -

90 deg. to +90 deg. and longitude -180 deg. to +180 deg. We also need to specify an

output file and the duration of years for computation. This program is written to carry out

data extraction for the years 1901-2002. One can extract data either for the whole range

or for any duration within this range by specifying in the input file appropriately.


Figure 4.6: Input file sample for CRU_TS_READ_METEO programme for extracting

global database

In order to retrieve the meteorological data for the specified years and for the specified

locations, the program uses large meteo files, each having a size of 410 MB, and whose

names can be seen in the Figure 4.6 above as “cru_ts_2_10.1901-2002.ext” where the

.ext represents the climate parameters, namely, month-wise precipitation, number of

precipitation stations, average and minimum temperature, water vapour pressure, and

number of vapour pressure stations.

By running the program CRU_TS_READ_METEO.exe, an output file for each grid,

described below, is generated as two sets of data: the first set of data contains columns of

data against each year, monthwise, namely, precipitation, temperature (average),

temperature minimum, vapour pressure, and evaporation; the second set of data mentions

number of precipitation stations, month-wise, against each year. It can be easily observed

from the second set of data that there had been a steady increase in the number of weather

stations, with the highest number around the year 1980; subsequently there is a rapid


decline as one goes towards 2002. In addition, the file also contains explanations of

terms, and names of input and output files for the specified year range.

We need to identify the subcell which contains the location coordinates of the subject

village. This identified file becomes an input file to the NUT_MONTH program that

computes precipitation Recharge.

The main input file for the NUT_MONTH program, also in fortran, is the nut_month_inp

file (Figure 4.7). The input file would specify all the input values, directly and through

other files. The output file from CRU corresponding to the sub cell of the study area

would comprise one input file; the second one would be the soil characteristics being

input as water_down_under_soils. The other input values provided through the

nut_month_inp file include specification of the water table condition (shallow or deep

water table) and the rooting depth in cm. The program has options to compute

groundwater recharge by opting either for one or all the four types of evapotranspiration

methods, viz., Langguth & Voigt (Grunwasser 2/2006), Thornthwaite_Malmstrom

(Dingman-2002), WASOMOD-M (Widen_Nilsson et al. 2007) and WASMOD-m

(Widen-Nilsson et al. crude twet). One can choose the method based upon the climatic

conditions. This output file from the CRU_METEO becomes an input file for the second

part of the model which is called NUT MONTH.


Figure 4.7: Main input file for NUT_MONTH

As mentioned above, the second input file water_down_under_soils (Figure 4.8)

comprises data about the soil characteristics. This file called water_down_under_soils is a

text file that contains soil number, soil type, number of layers in the soil zone, and the

thickness (in mm) and FC, CP and WP in percentage for each of the specified number of

sublayers. One could give three sets of input values at a time, and the program can be run

quickly by specifying the soil number each time for which the computation should be

made.

Figure 4.8: Input file Water_down_under_soils.txt for soil characteristics


The output file gives data on soil moisture content for the layer thickness and layer values

of FC, CP and WP. The recharge values are also computed month-wise for all the years

specified in a table which also gives precipitation, actual and potential

evapotranspiration. The year-wise values are also given.

The CRU and NUT_MONTH methods are very useful as „in the absence of actual

hydrological data such as observations of river flow data at a number of points along a

river and its tributaries; long period basic meteorological data like rainfall, temperature,

humidity, etc., can be used for estimating water potential of a region or a basin and its

variation in space and time by using suitable technique‟ (Kulkarni, 2003 as quoted in

Ramesh and M. G. Yadava, 2005).

Steps adopted for running the CRU Model

The following steps are adopted for running the CRU Model:

Identify the location coordinates of the areas for which climate variables are sought to be

extracted; this can be obtained from a toposheet, published by the Survey of India for

India.

The coordinate referencing in CRU program is made in the form of degrees. Therefore,

convert the coordinates into degrees for easier working as shown in the Table 4.2 for the

study villages.

Table 4.2: Study Villages and Coordinates

S.

no.

Village Latitude

(in deg., min.,

sec)

Longitude

(in deg., min.,

sec)

Cell

coordinates

(in degrees)

Reference

node on CRU

global

database

a. Ambaredi

22_21-71_70

70031

‟30

” E 21

058‟0”N 70.5

0-71.0

0E,

21.50-22.0

0N

70.750E,

21.750N

b. Jalsikka

23_22-71_70

7100

‟0

” E 22

030‟0”N 71.0

0E,

22.50-23.0

0N

70.750E,

22.750N

c. Vithalpar

23_22-71_70

70059

‟0

” E 22

071‟0”N 70.5

0-71.0

0E,

22.50-23.0

0N

70.750E,

22.750N

d. Haripar-Kerala-

Bella (located

close to each other)

23_22-71_70

70045

‟0

” E 22

032‟0”N 70.5

0-71.0

0E,

22.50-23.0

0N

70.750E,

22.750N


In the CRU input file, provide the location coordinates along with village name as shown

in the Figure 4.6 (CRU_input file), and the years for which the climate data is desired.

Running the program CRU_TS21_READ_METEO by double clicking would use the

CRU_input file and generate an output file that contains data organised in four files

corresponding to four subcells. Consider an area represented by latitude and longitude of

one degree by one degree; the CRU program divides each such area into a cell of 0.5 deg.

by 0.5 deg. Thus we have four subcells in one degree by one degree. The data is

organised around the mid-point of each subcell; the outputs from CRU are also generated

around the mid-values of these four sub cells.

We need to identify the subcell that represents the study area since the resolution of the

CRU program is 0.5 deg by 0.5 deg. The last column of the Table 4.2 represents the

subcells for the study villages in Rajkot district. Figure 1.1 shows the location of the

villages on the district map. As already mentioned elsewhere, the output file of CRU

contains month-wise precipitation and number of stations corresponding to the years

specified, for the four sub cells.

The data of the sub cell that comprises the location of the study area forms an input file

for the NUT_MONTH program that computes precipitation recharge.

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