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There are many methods of estimating evapotranspiration.
Some of these are briefly described in the following:
1. Soil Moisture Sampling:
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A large number of samples are taken from various depths in the roots zone and the soil moisture depletion studies are made.
2. Tanks and Lysimeter Experiments:
Keeping an account of water added and soil moisture changes by weighing the tanks.
3. Inflow-Outflow Method (by Water Balance):
For large areas where yearly inflow into the area, annual precipitation, yearly outflow from the area and the change in ground water level are evaluated.
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4. Field Experimental Plots:
Small amounts of water are applied at each irrigation to avoid deep percolation.
5. Installation of Colorado Tanks:
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The tanks are 90 × 90 × 90 cm made of 1.6 mm mild steel sheets with 2.5 × 2.5 cm angle stiffeners welded to the edges and rims. The tanks, with bottoms for studies of evapotranspiration and without bottoms to include percolation, with crop and without crop inside, are placed in position (sunk) in a level field with the rim 15 cm above ground level and 7.5 cm standing depth of water inside as shown in Fig. 2.7 (UNDP). After lowering the tanks inside the ground to the desired level the repacking of the soil excavated has to be done layer by layer in the same order and well compacted.
After installation of tanks and preparation of fields for planning, the water level changes inside the tanks are measured accurately by means of a point gauge. The inflow to and outflow from the fields are measured over V-notches installed and the ground water levels are noted in buried piezometers. The separated values of percolation, evaporation and transpiration are obtained and basin-wise decrements are worked out for accurate water balance studies.
The daily and seasonal consumptive use rates are arrived at and the total water requirements calculated for ready adoption in the field by working out suitable irrigation schedules. Thus, the evapotranspiration data are obtained by climatological factors with correlation to soil properties and crop factors.
(i) Lowry-Johnson Equation Used by USBR:
The constants in this equation are based on estimates of consumptive use by the inflow-outflow method for a number of irrigated valleys. The constants can also be determined from the consumptive use data in the immediate region of the project and then adjusted for temperature differences.
(ii) Penman Equation:
Estimation of evaporation is made on the basis of data for wind velocity, vapour pressures for saturated vapour and air and the crop evapotranspiration determined by multiplying the evaporation values by empirical constants depending on latitude and length of day light. This method has been used throughout the world and found to give good results in humid areas. However, its use has been limited because of its complexity and the fact that it utilises several parameters which are not available in the published weather data.
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(iii) Thornthwaite Equation:
*Ept = cta …(2.5)
Where E pt = potential evapotranspiration, cm/month; t = mean monthly temperature, °C; and c, a = constants to be selected for the climatological conditions of the area latitude and the month of the year for specific crops.
(iv) P/E Index Method:
Based on studies of weather and crop data from the irrigated areas in the western US, Munson found that the following P/E (precipitation-evaporation ratios) holds adequately for normal plant growth.
The sum of the 12 monthly P/E ratios is the P/E index equal to 47. For a given month, the corresponding value of the P/E ratio and the average monthly temperature t in °F can be substituted in the modified Thornthwaite formula –
P = 0.014 (t – 10) (P/E)0.9 …(2.6)
To determine P the required monthly precipitation or actually the monthly consumptive use. It is necessary to exercise judgment in the use of this method.
(v) Blaney-Criddle Equation:
The consumptive use is given by the empirical formula:
u = kf and U = KF = ∑kf; f = tp/100
U = ∑ktp/100
Where t = mean monthly temperature, °F; p = monthly percentage of day time hours of the year; k = monthly consumptive use coefficient determined from experimental data (see Table 2.3); u = monthly consumptive use; U = seasonal consumptive use or evapotranspiration; f= monthly consumptive use factor; F = ∑f, for the growing season; and K = seasonal consumptive use coefficient for the crop.
In metric units, u = kp ((45.7t + 813)/100) mm …(2.7 a)
Where t = mean monthly temperature, °C and f= p(0.46t + 8.13).
Phelan proposed an empirical modification of the Blaney-Criddle formula-
k = kt kc …(2.7 b)
Where kt = climatic coefficient related to the mean temperature t as kt = 0.0173t – 0.314 and kc = coefficient reflecting growth stage of crop which has to be determined experimentally.
The Blaney-Criddle formula does not take into consideration such factors as humidity, wind velocity, elevation etc.
The monthly use for rice in the Cauvery delta in Tanjore has been worked out by UNDP as follows.
The Cauvery delta lies between latitudes 9°50′ and 11° 50′ north and the maximum hours of sunshine for 10°N latitude are:
The monthly sunshine hours expressed as a percentage of yearly sunshine hours are:
(vi) Evaporation Index Method:
Analysis of data on consumptive use indicates a high degree of correlation between pan evaporation values and consumptive use. The relationship between the evapotranspiration Et and pan evaporation Ep is usually expressed as-
Et = kEp …(2.8)
Where k is the Et/Ep ratio. The values of k for different crops at 5% increments of the crop growing season are presented by Hargreaves and has recently introduced several modifying coefficients into his pan evaporation formula. The values of k at 10% increments of the stage of growth for the principal Indian crops are given in Table 2.4. Since these coefficients are average values, care must be taken in their use.
In recent years greater emphasis is being given for determining pan evaporation by using climatological factors. Christiansen and others have developed the pan evaporation equation from the climatological factors as-
Ep = 0.459 Rct cw ch cs ce …(2.9)
Where Ep = computed pan evaporation (equivalent to class A pan evaporation); R = extraterrestrial radiation and ct, cw, ch, cs, and ce = coefficients for temperature, wind velocity, relative humidity, % of possible sunshine and elevation, respectively.
Hargreaves has developed a similar formula for estimating pan evaporation.
The monthly consumptive use for rice is worked out by Blaney-Criddle method as shown in Table 2.5 (UNDP):
Seasonal consumptive use for the paddy crop kuruvai from June 16 to October 15 (growing season) is worked out as follows:
Example: For the rice growing area in the Cauvery delta the following data are available:
Calculate the field irrigation requirement for the kuruvai rice crop assuming an irrigation efficiency of 70%.
Solution:
Growing season = June 16-Oct. 15 = 122 days.
From Table 2.6, Seasonal consumptive use U = 85.78 cm
Total depth of irrigation water for the growing season d = 67.79 cm
From the above discussion it follows that the main factors governing consumptive use or evapotranspiration are the mean temperatures and daylight hours. Other climatic factors affecting evapotranspiration are humidity and wind velocity. The type of crop, its stage of growth and the moisture available in the root zone, also influence the evapotranspiration.
1Peff —effective rainfall; taken from rainfall data of a number of preceding years, which are available in 75-80% of the years. Mean values would be available only in 50% of the years and should not be taken.
2NIR—net irrigation requirement.
3FIR —field irrigation requirement; FIR = NIR/ηi, ηi = irrigation efficiency = 70% or 0.7.
4U— determined from pan evaporation data is very near to that determined by Blaney-Criddle method (U = 88.7 cm) by UNDP.