rapid irrigation development in the sudan has stretched to the limit its share in the nile waters. significant savings on irrigation water can be made by improved water management. accurate estimation of crops evapotranspiration, et, is a prerequisite, et was estimated by predicting first reference crop evapotranspiration. grass was selected as the reference crop. fao-penman with penman (1948) wind function for clipped grass was ranked first among the methods selected to predict grass et. using the crop coefficients of doorenbos and kassam (1979), et for cotton, groundnuts and wheat grown in the gezira scheme were then predicted. predicted and measured et remained well within $plus or minus$ 15. for air temperature higher and lower than 28.3 $degree$ c, it was found that warm-season grass et was equal to 1.0 and 0.635 et of cool-season grass, respectively. because of this temperature effect, care must be taken not to use these two grass varieties indiscriminately to estimate crop et. (author abstract)