EVALUATION OF REFERENCE EVAPOTRANSPIRATION ESTIMATION METHODS AND DEVELOPMENT OF CROP COEFFICIENT MODELS

This study is an attempt to find best alternative method to estimate reference evapotranspiration (ETo) for the Nagarjuna Sagar Reservoir Project [NSRP], command area located at Andhra Pradesh, India. When input climatic parameters are insufficient to apply standard Food and Agriculture Organization (FAO) of the United Nations Penman?Monteith (P?M) method. To identify the best alternative climatic based method that yield results closest to the P?M method, performances of four climate based methods namely Blaney?Criddle, Radiation, Modified Penman and Pan evaporation were compared with the FAO-56 Penman?Monteith method. Performances were evaluated using the statistical indices. The statistical indices used in the analysis were the standard error of estimate (SEE), raw standard error of estimate (RSEE) and the model efficiency. Study was extended to identify the ability of Artificial Neural Networks (ANNs) for estimation of ETo in comparison to climatic based methods. The networks, using varied input combinations of climatic variables have been trained using the back propagation with variable learning rate training algorithm. ANN models were performed better than the climatic based methods in all performance indices. The analyses of results of ANN model suggest that the ETo can be estimated from maximum and minimum temperature using ANN approach in NSRP area.