Evaluating Evapotranspiration by Some Mathematical Models and the National Document of Irrigation Compared to Lysimeter Data in the High Plain of Shahrekord

Reference crop evapotranspiration (ETref) is measured directly (lysimeter method) or estimated indirectly (mathematical models). In this study، daily evapotranspiration (ET) was calculated by some mathematical models for alfalfa and was compared with ET data gathered daily from a drainage lysimeter during six months. To evaluate and select the best model، ET data values were compared using statistical criteria including R2، NRMSE، MAE، MBE، and d. Results showed that the best models in the daily timeframe were: Hargreaves-Samani (HS)، Jensen-Haise (JH) and Turc. For the HS model، values of NRMSE and d were 0. 126 and 0. 930، and MAE was 0. 477 (mm. d-1)، respectively. The best models for monthly time-frame were HS، American Society of Civil Engineering-Penman-Monteith (ASCE-PM)، and Turc. Generally، HS model had the highest R2 values of 0. 985، 0. 998 and 0. 998 for daily and monthly periods، respectively، and ASCE-PM had the lowest MBE compared to lysimeter data. To estimate total ET during the alfalfa growth period in the Shahrekord plain، results showed that، models have over-estimated، except Turc and Priestley – Taylor models. ASCE-PM had the nearest ET (1161. 7 mm) to lysimeter data (1157. 6 mm). In the national document of irrigation، the alfalfa irrigation water requirement was reported at 649 mm (in the corresponding period)، which is much less than the value measured.