CROPWAT is a model developed by the food and Agriculture Organization of the United Nations (FAO) in 1991. It can help agrometeorologists, agronomists and irrigation engineers to calculate evapotranspiration and irrigation water requirements in a standard way. In addition, CROPWAT can suggest how to improve irrigation methods, plan irrigation schedules under different water supply conditions, and evaluate the impact on crop yields under rain sampling conditions or under sufficient irrigation conditions. The basic functions of CROPWAT include: (1) reference crop evapotranspiration (2) crop evapotranspiration (3) crop irrigation water requirement (4) formulation of irrigation schedule (5) evaluation of crop yield under rain fed or under sufficient irrigation conditions.