Abstract

Evapotranspiration and evaporation measurements are vital parameters for many agricultural activities such as water resource management and environmental studies. There are several models which can determine pan coefficient Kp, using wind speed, relative humidity and fetch length conditions. This work analyzed three exiting pan models to estimate Kp values for the study area. The, Kpan estimated by the models were statistically compared with the Penman-Monteith – FAO estimates. Monthly mean reference crop evapotranspiration (ET0) was calculated according to the pan-ETo model. The monthly mean value of Kp, determined by PMF-56 standard model for the study area was approximately 0.73. Similarly, the monthly mean ETo was 10.85 mm/month. The results showed that estimated pan coefficients by Raghuwanshi-Wallender and Snyder models were not statistically accepted to be used in the pan-ETo conversion method in this study. However, Orang model was found to be best for estimating the pan coefficient with an average value of 0.67, which was close to pan coefficient value estimated by the PMF-56 model. The reference evapotranspiration estimated using pan coefficient obtained from Orang model showed highest coefficient of determination (R2) = 0.80 and agreement Index (D)= 0.94,lowest Mean Absolute Error (MAE) = 0.81, Root Mean Square Error (RMSE), = 0.08, and coefficient of efficiency (E) =85%. The results from the analysis showed that for estimation of ETo, the most appropriate pan coefficient is calculated using the Orang model for the study area, which give almost nearer estimates