Potential Evapotranspiration estimation for Indian conditions : Improving accuracy through calibration coefficients

Abstract

The global water consumption is doubling every 20 years and projected increase in food demand will have to be met by irrigation. Appropriate scheduling of irrigation increase the irrigation water use efficiency allowing more water available for other human and environmental uses. Timing and quantum of water to be applied requires data on Actual Evapotranspiration (AET). The measurement of AET is a very difficult and time consuming task. Because of this, the concept of Potential Evapotranspiration (PET) is widely used. Direct measurement of PET across locations is cost prohibitive for a country like India and an indirect method using meteorological data is a potential alternative. Though a number of empirical formulae / approaches are available, availability of climatic data limits their application across all the locations. In the present investigation, seven methods were employed to estimate the PET and the resultant values were compared with PenmanMonteith estimated PET for 51 locations across the country. On an annual basis,Turc method resulted in more errors followed by Thornthwaite and Blaney-Criddle. During southwest monsoon period PET estimated from Open pan and Christiansen pan method resulted in more errors whilst during northeast monsoon season Hargreaves and Christiansen pan resulted in more errors. During summer, modified Penman and Hargreaves are the best methods to adopt. During winter modified Penman and PET from Open pan resulted in few errors. Hargreaves method is surprisingly resulted in more errors during winter season compared to summer. Calibration coefficients were evolved on annual and seasonal basis for different methods to reduce the errors in PET estimation in comparison to Penman-Monteith method. The efficiency of these coefficients were determined using an independent data set which showed that the errors can be minimized to a great extent by applying these coefficients. A station close by the 51 stations studied or per se climatologically analogous can employ the calibration coefficients directly. Maps indicating the spatial distribution of the coefficients across the country were presented so that any user can estimate PET for a station interspersed two PET isolines.