The goal of imigation is operating the right amount of water at the right time to the right place. To supply the right amount of water, we have to know what is the right amount. Tne factors that determine the amount of irrigation water are evapotranspiration, crop type, rainfall percolation and land preparation. Evapotranspiration is the change of water into water vapor from the water or soil surface into the air(evaporation) and from plants into the air(transpiration). Evapotranspiration(the moth combination of evaporation and transpiration) is influenced by the weather: solar radiation