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Blaney Criddle & Hargreaves method of measuring Evapotranspiration 

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The estimation of potential evapotranspiration is achieved by adopting empirical approaches, such as the Thornthwaite equation, the BlaneyCriddle formula and the Hargreaves method, all having as a requirement the availability of temperature data. The data set is made up of temperature time series, obtained from gauging stations.
The potential evapotranspiration estimated for each station using the abovementioned methods is spatially integrated, in order to obtain the areal potential evapotranspiration.
The methods adopted for the spatial integration of the point estimates are the Kriging method, the method of Inverse Distance Weighting, the Spline method and the Thiessen method, using applications in a Geographic Information System (GIS) with a spatial resolution of 200x200m^{2}. 
u k (k ) (t x p) / 100 TR = kc kt) u = monthly consumptive use (inches) kt = climatic coefficient = 0.173 * t  0.314 kc = crop growth stage coefficient t = mean monthly air temperature (°F) p = monthly percentage of annual daylight hours  
Hargreaves Method of measurement of EvapotranspirationOriginally developed in 1975
ETo 0.0023(Tmax Tmin T 17.8) R ETo = grass reference ET (mm/day) Tmax = maximum daily air temperature (°C) Tmin = minimum daily air temperature (°C) Tmean = mean daily air temperature = (Tmax + Tmin) / 2 Ra = extraterrestrial radiation (mm/day) Ra (mm/day) = Ra (MJ/m2/day) / 2.45
