Evaluation of FAO-56 Penman-Monteith and alternative methods for estimating reference evapotranspiration using limited climatic data at Pusa

The FAO-56 Penman-Monteith method (FAO-56 PM) is standard method recognized by the Food and Agriculture Organization of the United Nations for estimating reference evapotranspiration (ETo). Unfortunately, some of climatic variables, especially relative humidity, solar radiation and wind speed are often missing which could impede the estimation of ETo with the FAO-56 PM method. To overcome the problem of availability of climatic variables, procedures to estimate ETo with missing climate data are proposed as part of the FAO methodology. Therefore, assessing the accuracy of these procedures for Pusa Observatory is important. The comparison of ETo estimates using limited data to those computed with full data set revealed that the difference between ETo obtained from full and limited data set is small. Both the Mean Bias Error (MBE) and the Root Mean Square Error (RMSE) of the comparison were less than 0.35 and 1.00 with a minimum of "0.11 and 0.29 mm day''(1), respectively, leading to small errors in the ETo estimates. R-2, divided by(2)-test and index of agreement values confirmed strong relationships among data for the year 1998 to 2006.