Monthly pan coefficients for estimation of reference evapotranspiration in a semi-arid environment

In the present study, monthly pan coefficients (K _pan ) was estimated using standard FAO Penman-Monteith method (PM) and performance of different pan evaporation empirical models were evaluated for the semi-arid climate at Hisar, India. The daily data of maximum and minimum temperature, relative humidity, wind speed and sunshine hours of forty years (1984–2023) were used to calculate reference evapotranspiration (ETo). The monthly K _pan were calculated by dividing ETo with class A pan evaporation. A comparison was made among monthly ETo obtained with standard PM method and six other empirical models viz. Cuenca (CU), Allen and Pruitt (AP), Snyder (SN), Orang (OR), Wahed and Snyder (WS) and Modified Snyder (MS) along with annual fix K _pan value of 0.75 (FIX). The model performance was evaluated using mean absolute error, root mean square error, correlation coefficient, Nash–Sutcliffe model efficiency and ratio of performance to deviation. The results shows that no single model performed best among all performance criteria but considering average of their rankings in different statistical parameters, the performance of the models for estimation of ETo was found of the order as PM > OR > MS > AP > FIX > SN > WS > CU. In absence of meteorological parameters essential for calculation of PM ETo, Wahed and Snyder or Modified Snyder model can be used for estimation of ETo for similar type of climate.