Abstract

This is a review paper on existing methodologies to calculate crop evapotranspiration (ET_c). We have attempted to present all the important ET estimation procedures to date starting from the simple empirical Blaney Criddle method to the complex Shuttleworth model. The common approach to calculate ET_c is to estimate a reference crop ET rate (ET_ref) using weather variables from nearby weather station, and multiplying it by an appropriate crop coefficient (K_c). Recently, there have been attempts to calculate actual crop ET (ET_a) directly without using K_c. The latter method is still in the developmental phase. This study reviews the existing literature on ET estimation and identifies research needs in the current methods and technology. The extension of the Shuttleworth model for hourly time step and the validity of the Irmak and Mutibwaa model at field level for various crops would be a good milestone for the one step ET estimation. Furthermore, there are indications that the development of a new variable canopy surface resistance (r_c) model, which can be applicable for different crops at different climatic conditions, would be a good contribution in this field.

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