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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.
Groundwater Recharge Estimation: Conceptual Foundation 174
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.
Groundwater Recharge Estimation: Conceptual Foundation 175
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
Groundwater Recharge Estimation: Conceptual Foundation 176
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).
Groundwater Recharge Estimation: Conceptual Foundation 177
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).
Groundwater Recharge Estimation: Conceptual Foundation 178
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
Groundwater Recharge Estimation: Conceptual Foundation 179
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.
Groundwater Recharge Estimation: Conceptual Foundation 180
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 Recharge Estimation: Conceptual Foundation 181
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.
Groundwater Recharge Estimation: Conceptual Foundation 182
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.
Groundwater Recharge Estimation: Conceptual Foundation 183
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
Groundwater Recharge Estimation: Conceptual Foundation 184
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).
Groundwater Recharge Estimation: Conceptual Foundation 185
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.
Groundwater Recharge Estimation: Conceptual Foundation 186
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,
Groundwater Recharge Estimation: Conceptual Foundation 187
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).
Groundwater Recharge Estimation: Conceptual Foundation 188
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).
Groundwater Recharge Estimation: Conceptual Foundation 189
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).
Groundwater Recharge Estimation: Conceptual Foundation 190
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.
Groundwater Recharge Estimation: Conceptual Foundation 191
(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.
Groundwater Recharge Estimation: Conceptual Foundation 192
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.
Groundwater Recharge Estimation: Conceptual Foundation 193
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).
Groundwater Recharge Estimation: Conceptual Foundation 194
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
Groundwater Recharge Estimation: Conceptual Foundation 195
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.
Groundwater Recharge Estimation: Conceptual Foundation 196
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,
Groundwater Recharge Estimation: Conceptual Foundation 197
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
Groundwater Recharge Estimation: Conceptual Foundation 198
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).
Groundwater Recharge Estimation: Conceptual Foundation 199
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.
Groundwater Recharge Estimation: Conceptual Foundation 200
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
Groundwater Recharge Estimation: Conceptual Foundation 201
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.
Groundwater Recharge Estimation: Conceptual Foundation 202
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)
Groundwater Recharge Estimation: Conceptual Foundation 203
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.
Groundwater Recharge Estimation: Conceptual Foundation 204
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
Groundwater Recharge Estimation: Conceptual Foundation 205
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.
Groundwater Recharge Estimation: Conceptual Foundation 206
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.
Groundwater Recharge Estimation: Conceptual Foundation 207
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.
Groundwater Recharge Estimation: Conceptual Foundation 208
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
Groundwater Recharge Estimation: Conceptual Foundation 209
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
Groundwater Recharge Estimation: Conceptual Foundation 210
= 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
Groundwater Recharge Estimation: Conceptual Foundation 211
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
Groundwater Recharge Estimation: Conceptual Foundation 212
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;
Groundwater Recharge Estimation: Conceptual Foundation 213
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
Groundwater Recharge Estimation: Conceptual Foundation 214
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).
Groundwater Recharge Estimation: Conceptual Foundation 215
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).
Groundwater Recharge Estimation: Conceptual Foundation 216
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).
Groundwater Recharge Estimation: Conceptual Foundation 217
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)
Groundwater Recharge Estimation: Conceptual Foundation 218
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
Groundwater Recharge Estimation: Conceptual Foundation 219
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.
Groundwater Recharge Estimation: Conceptual Foundation 220
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
Groundwater Recharge Estimation: Conceptual Foundation 221
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.
Groundwater Recharge Estimation: Conceptual Foundation 222
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
Groundwater Recharge Estimation: Conceptual Foundation 223
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
Groundwater Recharge Estimation: Conceptual Foundation 224
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.