Abstract:
Considering the problems associated with the availability and reliability of climatic data in developing countries and the possible errors in the more sophisticated methods for estimating crop water requirements, it has become necessary to develop a computational procedure which requires least and widely available data. The method of estimating reference crop evapotranspiration (ETO) from measured temperature introduced. by professor Hargreaves of Utah State University in 1982 has given satisfactory results in many regions of the world. The study of its applicability to Sri- Lanka could do much to improve irrigation and agricultural sectors. Three stations from different climatic regions of Sri-Lanka Here selected with the intention of studying the applicability of this new method. ETG estimates from Modified Penman, Hargreaves and Pan evaporation methods were computed for seven years and the average values Here compared. The results indicated that the Hargreaves method provides, satisfactory’ results for Yala seasons in dry zone. The application of Hargreaves method can be extended even to Maha seasons In dry coastal regions. The ETO estimates from Penman method have given over predictions specially for high wind velocities. Two average years were selected for station Mahailluppallama and Batticaloa. ETO estimates from Modified Penman were computed for varying wind speeds. The results have shown that a local calibration is required for the aerodynamic term of Modified Penman. A relationship has been develop between maximum and minimum temperatures, relative humadities, wind speed and the deviation of ETO values of Hargreaves method from pan evaporation method. This indicated that for given temperatures and relative humidity, Hargreaves method provide better results only for a particular range of wind speed. Estimated crop evapotranspiration (ET crop) for soya beans from Hargreaves and Pan evaporation methods were compared with lysimeter measurements for Agricultural Research Station Mahailluppallama. The total estimated ET crop from Hargreaves method has only varied 5.6 % from the measured lysimeter values.
