Evaluation of evapotranspiration estimation methods and development of crop-coefficients for potato crop in a sub-humid region

The study was carried out at the experimental farm of the Agricultural and Food Engineering Department, Indian Institute of Technology, Kharagpur, India, having a sub-humid climate. A weighing type lysimeter of 75 cm diameter and 75 cm height was installed at the center of a 10 m×10 m experimental plot and daily reference crop evapotranspiration (ET0) was measured with an electronic datalogger connected to the lysimeter. Grass was used as the reference crop for observing the actual ET. A total of 10 climatological methods were selected for estimating reference crop evapotranspiration on a daily basis. Some of these methods are based on combination theory and others are empirical methods based primarily on solar radiation, temperature and relative humidity. All the methods were originally developed for a well watered reference crop, either alfalfa or grass. An attempt was made in the current study to develop regional relationships between the evapotranspiration measured by the lysimeter and that estimated by the climatological methods, such as Penman, FAO–Penman, FAO–Corrected–Penman, 1982-Kimberley–Penman, Penman–Monteith, Turc–Radiation, Priestley–Taylor, FAO–Radiation, Hargreaves and FAO–Blaney–Criddle. Performance of the climatological methods in estimating the ET0 values as compared to the lysimeter-measured values was evaluated on the basis of root mean square error (RMSE). Almost all combination methods performed quite well. Radiation methods also gave good results but the ET0 values estimated by these methods did not match closely with the measured ET0 values unlike those estimated by the combination methods. The Penman–Monteith equation gave the best result followed by 1982-Kimberly–Penman, FAO–Penman, Turc–Radiation and FAO–Blaney–Criddle. The RMSE in all the cases varied between 0.08 and 0.756.