1304761989-Cropwaterrequirements3

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Crop WaterRequirements


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Crop Water Requirements

It is the total amount of water required by the crop in a givenperiod of time for normal growth, under field conditions.

It includes evapotranspiration, water used by crops formetabolic growth, water lost during application of water andthe water required for special operations such as landpreparation, tillage and salt leaching etc.

it is expressed as the surface depth of water in mm, cm or

inches per unit cropped area.

CWR = Consumptive use (Cu) + conveyance losses(Wu) + water required for special operation (Ws)


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Sources of water for Crop Use

Effective Precipitation (ER): It is that part of total precipitation which is used by crop

as soil water reserve. It is the precipitation falling duringthe growing period of a crop that is available to meet the

evapotranspiration needs of the crop. It is determined as:

ER = Total rainfall (P)Runoff (R)deep percolation (PW)

Gross Irrigation requirements of crops (IRg):

It refers to the amount of water applied to the field fromthe start of land preparation to harvest of the croptogether with the water lost through distributaries andfield channels and during water application to the cropfield.


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IRg = CWR(ER + SW + GW)

Net Irrigation requirements It refers to the amount of water needed to replenish soil moisture

deficit in the crop field.

IRn = IRg x Efficiency of water application= Cu ER - conveyance losses

Soil Water Contribution for Crop Use (SW): It refers to the difference in moisture content at the time of sowing

and harvesting of the crops that may be positive or negative. It isgiven as:


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Where: SW = soil water contribution in cm Msi = moisture content at the time of sowing in the ith

layer, %

Mhi = moisture content at the time of harvesting in the ithlayer, % Asi = Apparent specific gravity of soil (The specific gravity

of a porous solid when the volume used in the calculationsis considered to exclude the permeable voids)

Di = depth of ith layer of the root zone soil, cm

Ground Water Contribution for Crop Use (GW): It refers to the water used by crops due to capillary rise in case

of shallow water tables.CWR = ER + IRg + SW + GW


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Objective of Crop WaterRequirement Study:

To decide possible cropping pattern of area

Effective use of available water

Plan and design an irrigation project

Plan water resource development in an area

Assess irrigation requirement of an area

Management of water supply from sources


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Evapotranspiration (ET) andConsumptive use (Cu)

Evapotranspiration: It is defined as the water transpired by crop plants and the

water evaporated from the soil in the crop field and interceptedprecipitation by areal parts of plants in any specified time period

Consumptive use: It is the evapotranspiration plus the water used by plants for

metabolic activities which is hardly 1 % of ET

Consumptive useis the water required by plants to fulfill the

evapotranspiration needs of crops. (FAO)

Consumptive useis the total amount of water used by the plantsin transpiration (building of plant tissues etc) and evaporation fromadjacent soils or from plant leaves in any specified time period.(S.K. GARG)


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Classification of Consumptive use Daily Consumptive use:

The amount of water consumptively used during 24-hours.

It is estimated usually to record the peak periodconsumptive use rates to formulate the cropping pattern

and to decide the water supply from sources duringdifferent periods of cropping.

Peak Period consumptive use: It is the average daily consumptive use during a few days

(6 to 10 days) of highest consumptive use in a season.

It occurs when the vegetation is abundant, temperature ishigh and crops are in flowering stage.

It is used in the planning of an irrigation system


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Classification of Consumptive use

Seasonal consumptive use: It is the amount of water consumptively used by crops

during the entire cropping season/period.

It is used to evaluate and decide the seasonal watersupply to a command area of an irrigation project.


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Important terminology on

Evapotranspiration

Potential / reference crop evapotranspiration (ETo): The highest rate of evapotranspiration (ET) by a short and

actively growing crop or vegetation with abundant foliage(leafage) completely shading the ground surface and

abundant soil water supply under a given climate.

An extensive surface of short green grass cover of uniformheight (0.12m), actively growing, completely shading theground and no water shortage resembles the reference

crop.

Actual crop evapotranspiration (ETc): It is the rate of evapotranspiration by a particular crop in a

given period under prevailing soil water and atmosphericconditions.


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Important terminology onEvapotranspiration

It refers to the evapotranspiration from a disease free cropgrowing in a large field under optimal soil conditions withadequate water and fertility and giving full potentialproduction under the given environment.

Usually calculated by multiplying the Crop Coefficient (Kc)for the period with ETrc, thus

ETcrop = Kc. ETrc


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Factors affecting Evapotranspiration

Climatic factors:Climatic factors include:

Precipitation, with greater frequency and amount of rainfall,ET becomes higher. In un-irrigated areas, water needs ofcrops are mainly met from precipitation and in irrigated areasit decides the amount of water available for irrigation.

Solar radiations, it supplies energy for ET processes. Withincreasing day length or solar radiation, ET becomes more.

Temperature, Temperature of plant and soil rises because of

more amount of solar radiation received from the sun andconsequently increases ET.

Wind speed, ET from soil surface and plants occurs at ahigher rate on a windy day. The moist air in the immediatevicinity of a moist soil or leaf surface is swept away by wind

and the dry air occupies the space.


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Relative humidity, ET varies inversely with the atmospherichumidity

Growing season: Length of growing season and the actual date of sowing

and maturing are important factors. The growing season of acrop coinciding with the hotter part of the year is expected toincrease ET. Crops grown in different seasons have different ET.

Crop characteristics: Growth habit, canopy development, leaf area index, plantdensity, duration and time of year when the growth is made, areimportant consideration to study the effect of cropcharacteristics on ET.


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Soil characteristics: Hydraulic conductivity and water holding capacity of soil

affect ET.

Cultural Factors: Irrigation frequency, method of irrigation, depth of

irrigation, fertilizer application and mulching are theimportant cultural factors affecting ET.


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Crop Coefficient

Crop coefficient: It is the ratio b/w the actual crop Evapotranspiration to the

reference crop evapotranspiration.Kc = ETc / ETo

It determined experimentally for various crops, Etc isdetermined by Lysimeter technique and ETo by USWB class Aevaporation pan.

Kc is different for different crop and for different crop growthstages.

It is mainly affected by crop type, soil type and climate of thearea.


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Crop Coefficient (Kc) Curve


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Soil-water (moisture)-plantrelationship

Water is essential to crop plants for their growth anddevelopment. It is the basic input influencing the cropproduction. Amount of water required by the crops isinfluenced by the soil type.

Soil water plant relationship is a process that requires to beregulated for maximization of yields with a given unit ofwater. An understanding of this relationship is essential inorder that water management principles are applied tovarious climate, soil and cropping regions of both rain-fed and

irrigated lands.

To understand this relationship, the concept of soilwater/moisture and field capacity is essential.


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Soil water/moisture and field

capacity Gravitational moisture:

When the water falls over the ground, a part of it getsabsorbed in this root zone, and the rest flows downwardsunder the action of gravity, and is called as gravitational

moisture.

Field Capacity: Immediately after the rain or irrigation water application,

when all the gravity water has drained down, a certainamount of water is retained on the surface of soil grainsby molecular attraction and by loose chemical bonds(adsorption). This water cannot be drained under theaction of gravity and is called the field capacity. It is themoisture content after free drainage has taken place for

sufficient period (2 to 5 days).


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Soil water/moisture and fieldcapacity

Field capacity is very important because it is the waterwhich is available in the soil for crop use

The total field capacity water is not used by the crops. The

plants can extract water from the soil till the permanentwilting point is reached.

Fields capacity is further divided into two types:

Capillary moisture: It is that moisture which is attached to the soil molecules

by surface tension against gravitational forces and whichcan be extracted by crop through capillarity.


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Hygroscopic moisture: It is that moisture which is attached to the soil molecules

by loose chemical bond and it is not available to the plantsfor use.

Permanent wilting point: It is moisture content at which plant can no longer extract

sufficient water for its growth and wilts up.

Available moisture: it is the difference in moisture content between field

capacity and permanent wilting point.


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MoistureContent(%)


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Duty of irrigation water, delta ofcrops, base period

Duty of irrigation water: It is defined as the no. of hectares (acres) of land irrigated

for full growth of a given crop by supply of 1 m3/sec (1ft3/sec) of water continuously during the entire baseperiod. It is affected by crop type, climate and season,useful rainfall, type of soil and efficiency of cultivationmethod etc.

Delta of crops: It is the total quantity of water required by the crops for

full growth as depth to which water would stand on the

irrigated area.Delta = Volume (acre-ft) / Area (acres)

Base period: It is the time between first watering of crops at the time of

its sowing and the last watering of crops before harvesting


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Relationship between Duty, Delta

and Base periodLet there be a crop of base period B days.

Now the volume of water applied to this crop during B days @ 1 m3/sec = V =(1 x 60 x 60 x 24 x B) m3 = 86400 B

By definition of duty, 1 m3 of water supplied for B days matures/irrigates Dhectares (104 m2)of land.

So total depth of water supplied to this land (Delta) = Volume/Area =86400B/104D

Therefore,

Delta = = 8.64 B / D (meters)Delta = = 864 B / D (centimeters)

Example: find the delta of a crop when its duty is 864 hectare/cumecs with

base period of 120 days.


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Methods of estimating

Evapotranspiration These methods are classified into three types: Direct methods

Lysimeter method Field experimentation method

Soil water depletion method Inflow-outflow method

Pan evaporimeter method USWB class-A pan evaporimeter

Empirical methods Blaney criddle method Penman method Modified penman method Radiation method

Penman Monteith equation


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Direct Methods

Lysimeter method: Used to measure ET and various components of water

balance

It is a container (usually 0.5m 2 m in diameter) having

an experimental soil separated from the surrounding soil inthe crop field

Lysimeter are installed in fields with a large guard areahaving the same crop as in the lysimeter

Measurements of different components for water balance

studies such as water added to lysimeter throughprecipitation and irrigation, change in soil water storageand water lost through evaporation, transpiration, runoffand deep percolation are made, The relationship is:

ET ER IRn SW


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Direct Methods

Lysimeters are so constructed that measurements of deeppercolation and surface runoff are possible or it is possibleto avoid these losses

Both weighing and non weighing type lysimeters areused for measurement of ET

For very short period (daily or hourly) estimates of ET,weighing type lysimeter is used


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Direct Methods

Field experimentation method: Field experiments with varying level of irrigation are

carried out to estimate seasonal consumptive use of

irrigated crops Measurement of water supplied to the crops through

effective rainfall and irrigation and changes in the soilmoisture reserves during the growing season are made

The water thus supplied under varying levels of irrigation

is then correlated the yields obtained The quantity of water used to produce most profitable

yield is taken as CU


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Direct Methods

Soil water depletion method: Soil water contents in different layers of root zone are

measured just before and after irrigation or rainfall and

during the period between two successive irrigations asfrequently as possible depending upon the degree ofaccuracy desired

The soil water depletion during any short period isconsidered as the consumptive use fro that period

The seasonal consumptive use is obtained by summing upsoil water depletion or losses during the different periodsof measurement in the growing season


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Direct Methods

Inflow-outflow method: Used to estimate yearly consumptive use over large area,

also called as water balance method

Change in soil water storage is considered negligible and it

is assumed that the subsurface inflow into the area issame as subsurface outflow

CU P I GW R


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Pan evaporimeter method

USWB class-A pan evaporimeter: There exist a close relationship between the rate of

consumptive use by crop and the rate of evaporation from

properly located pan evaporimeter. Pan evaporation is the combined effect of all atmospheric

factors and is independent of plant and soil factors

Crop evapotranspiration rates for various crops may beestimated from the pan evaporation rates multiplied by afactor known as crop factor (Kcrop) which varies with thestages of growth, extent of ground cover with foliage,climate and geographical locations


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USWB class-A pan evaporimeter

It is the most widely used evaporimeter for findingevaporation from the free water surface

The Class A Evaporation pan is circular, 120.7 cm in diameterand 25 cm deep. It is made of galvanized iron (22 gauge)

with a stilling pan The pan is mounted on a wooden open frame platform whichis 15 cm above ground level to facilitate the circulation of airbeneath the pan

Daily evaporation rate is given by the fall in water levelmeasured in the stilling well by hook gauge

Adjustments are made to the evaporation values if rainfalloccurs during a period of measurement

After measuring the drop in water level each time, wateris added to the pan to bring back the water level tooriginal position of pointer tip level


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The relationship between potential evapotranspirationand pan evaporation is given as:


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Empirical methods

Blaney criddle method Penman method

Modified penman method Radiation method Penman Monteith equation


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Empirical methods

Blaney criddle method: Developed a formula for estimating CU based on

temperature, daylight hours, and locally developed crop

coefficients

100

ktpCU Cu KF kf


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Empirical methods

Penman method Developed the formula using important climatic

parameters such as solar radiation, temperature, vapour

pressure and wind velocity to compute the evaporationfrom open free water surface

ET is obtained by multiplying with crop coefficient

it is quite satisfactory for both humid and arid regions undercalm weather conditions

It drawback is that it uses many climatological parameters

that are difficult to obtain

n ao

Q EE


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Empirical methods

Modified Penman method

ETo* = Refenrence crop Evapotranspiration (unadjusted)

ETo= Refenrence crop Evapotranspiration (adjusted)

C = adjustment factor to account for day and night weather effect

* . (1 ). ( ).( )o n a d ET W R W f u e e *o oET CET


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Empirical methods

Radiation method:

( . )o sET C W R

(0.25 0.50 )s An

R RN


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Penman-Monteith Equation

Where:Rn is the net radiation,G is the soil heat flux,(es - ea) represents the vapour pressure deficit of the air,

a is the mean air density at constant pressure,cp is the specific heat of the air, represents the slope of the saturation vapour pressure temperature

relationship, is the latent heat of vaporization and is psychrometric constant, andrs and ra are the (bulk) surface and aerodynamic resistances.


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The surface resistance, rs, describes the resistance of vapourflow through stomata openings, total leaf area and soil surface.

The aerodynamic resistance, ra, describes the resistance from

the vegetation upward and involves friction from air flowing overvegetative surfaces.

The latent heat of vaporization, , expresses the energyrequired to change a unit mass of water from liquid to water vapourin a constant pressure and constant temperature process. The valueof the latent heat varies as a function of temperature

The specific heat at constant pressure Cp is the amount ofenergy required to increase the temperature of a unit mass of air byone degree at constant pressure. Its value depends on thecomposition of the air, i.e., on its humidity


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The vapour pressure deficit is the difference between the saturation(es) and actual vapour pressure (ea) for a given time period.

The solar radiation received at the top of the earth's atmosphere on

a horizontal surface perpendicular to suns rays is called theextraterrestrial (solar) radiation, Ra

The net radiation, Rn, is the difference between incoming andoutgoing radiation of both short and long wavelengths. It is thebalance between the energy absorbed, reflected and emitted by theearth's surface or the difference between the incoming netshortwave (Rns) and the net outgoing longwave (Rnl) radiation

The soil heat flux, G, is the energy that is utilized in heating thesoil. it is positive when the soil is warming and negative when thesoil is cooling. The soil heat flux is small compared to Rn and mayoften be ignored

C t f Gl b l b l


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Components of Global energy balance


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