Water shortage in many parts of Iran has laid part of the suitable lands unused,or under dry farming with low yield. Even in irrigated farming, yields are generally below the potential level which is partly due to the water shortage.
Determination of actual evapotranspiration of crops is essential for proper irrigation planning.Direct measurements of evapotranspiration has been carried out for some crops at limited number of research stations in Iran. On the other hand ,measurements of actual
evapotranspiration of all crops under various climatological conditions of Iran would need large volume of research work and long periods of time.
In this experiment ,the actual evapotransp:Lration of corn,watermelon and bean is determined under field conditions by measuring changes of soil water over time,taking into account the deep percolation of water from the root zone.The irrigation regime was the usual irrigation practice in Karaj.The estimated evapotranspirations using modified Penman and Blaney Criddle formulas were compared with the measured quantities.
The results indicate that crop evapotranspiration estimated by Penman and Blaney—Criddle formulas are different from the actual
measured values under the conditions of this study. On the other hand, the calculated monthly and seasonal values were relatively close to measured quantities.It is suggested that before using the empirical formulas, crop coefficients must be determined under optimum irrigation. Otherwise, these equations can only provide estimates of evapotranspiration over long periods of time.
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