The following points highlight the top two methods used for the estimation of crop evapotranspiration. The methods are: 1. Experimental Method 2. Theoretical Methods.
1. Experimental Methods:
Estimation of ETo can be made using the formula:
ETo = Kpan × Epan
Where ETo is the reference crop evapotranspiration in mm/day, Kpan is a coefficient called pan coefficient and Epan is the evaporation in mm/day from the pan.
The factor Kpan varies with the position of the equipment (say, whether placed in a fallow area or a cropped area), humidity and wind speed. Generally, the details are supplied by the manufacturers of the pan. For the US Class A evaporation pan, which is also used in India, Kpan varies between 0.35 and 0.85, with an average value of 0.7.
It may be noticed that finding out ETA would involve the following expression:
ETc = Kcrop × ETo = Kc × Epan x Kpan
If instead, Kcrop × Kpan is taken as a single factor, say K, then ETA may directly be found from Epan as under:
ETc = K × Epan, where K may be called the crop factor.
The water management division of the Department of Agriculture, Government of India has published a list of factors for common crops and depending upon the stage of growth, which have to be multiplied with the evaporation values of the USWB Class A evaporation pan.
2. Theoretical Methods:
The important methods that have been proposed over, the years take into account, various climatic parameters. Of these, only the following would be discussed, as they are the most commonly used.
a. Blanney-Criddle Formula:
This formula gives an estimate of the mean monthly values of ETo, which is stated as:
ETo = P (0.46 Tmean + 8.13)
Where P is the mean daily percentage of annual day time hours and has been estimated according to latitude; Tmean is the mean monthly temperature in degrees Centigrade and may be taken as 72 × (Tmax + Tmin) for a particular month. Thus using Equation above one may evaluate ETC for each month of the growing season, from which the total water need for the full growing season of the crop may be found out.
b. Penman-Monteith Method:
This method suggests that the value of ETo may be evaluated by the following formula:
Where the variables have the following meanings:
ET = reference evapotranspiration [mm/day’],
Rn = net radiation at the crop surface [MJ m2 day-1],
G = soil heat flux density [MJ m-2 day-1],
T = mean daily air temperature at 2 m height [°C],
u2 = wind speed at 2 m height [m s-1],
es = saturation vapour pressure [kPa],
ea = actual vapour pressure [kPa],
es – es= saturation vapour pressure deficit [kPa],
Δ = slope vapour pressure curve [kPa °C-1],
g = psychrometric constant [kPa °C -1].