MEASUREMENT OF EVAPOTRANSPIRATION

        The measurement of Potential evapotranspiration is done by a instrument called Lysimeter. It is a large container having crops inside it.The evapotranspiration is calculated by   measuring these crops water loss and gains. Natural conditions are simulated in these containers as closely as possible. The operator measures water added, water retained by the soil, and water lost through evapotranspiration and deep percolation. Weighings can be made with scales or by floating the lysimeters in water. Growth of roots in lysimeters confined to the dimensions of lysimeters, the disturbed soil in the lysimeters and other departures from natural conditions limit the accuracy of lysimeter measurements of potential evapotranspiration.
Lysimeter
Potential evapotranspiration from a cropped surface can be estimated by the two processes described below. 
1.Correlating potential evapotranspiration with water loss from evaporation devices
2.Estimating the potential evapotranspiration based on various climatic parameters.
Correlation of potential evapotranspiration is baesed on the assumption  that the climatic conditions affecting crop water loss (Det) and evaporation from a free  surface of water (Ep) are the same. Potential evapotranspiration (Det)can be correlated to the pan evaporation Ep as below,
                                Det = KEp
where, K is called the crop factor for that period. K depends on the crop and its stage of growth. The main limitations of this method are the differences  in physical features of evaporation surfaces compared with those of a crop surface.
In the absence of pan evaporation data, the consumptive use is generally computed as follows:
(i) Compute the seasonal (or monthly) distribution of potential evapotranspiration, which is defined as the evapotranspiration rate of a well-watered reference crop which completely shades the soil surface. It is thus an indication of the climatic evaporation demand of a vigorously growing crop. Usually, grass and alfalfa (a plant with leaves like that of clover and purple flowers used as food for horses and cattle) are taken as reference crops.
(ii) Adjust the potential evapotranspiration for the type of crop and the stage of crop growth. Factors such as soil moisture depletion are ignored so that the estimated values of the consumptive use are conservative values to be used for design purposes.
Thus, evapotranspiration of a crop can be estimated by multiplying potential evapotranspiration by a factor known as crop coefficient.
Potential evapotranspiration can be computed by one of the several methods available for the purpose. These methods range in sophistication from simple temperature correlation (such as the Blaney-Criddle formula) to equations (such as Penman’s equation) which account for radiation energy as well. Blaney-Criddle formula for the consumptive use has been used extensively and is expressed as                                   u = kf
in which, u = consumptive use of crop in mm,
k = empirical crop consumptive use coefficient, and
f = consumptive use factor.
The quantities u, k and f are determined for the same period (annual, irrigation season,growing season or monthly). The consumptive use factor f is expressed as                                       f =p/100(1.8t + 32)
in which, t = mean temperature in °C for the chosen period, and
p = percentage of daylight hours of the year occurring during the period.

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