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The effects of plausible climate warming on potential evapotranspiration and crop water requirements are investigated and discussed in this paper. There are numerous methods for estimating potential, evapotranspiration in the literature. However, we choose to use two acceptable methods for an arid climate: the '"Penman method" adopted by the Food and Agriculture Organisation of the United. Nations (FAO); and for the purpose of comparison the Blaney-Criddle method. Furthermore the Blaney-Criddle method is suggested for areas where only air temperature data are available.

The amount of water needed for agriculture with and without climate change is estimated send discussed. A computer program, based on Penman's method, was used with the necessary data provided by separate computer programs. To compare results of different models, two climate scenarios were used in these analyses. The results show that the effects of climate change on evapotranspiration will increase from north to south, due a rise in temperature resulting from the increase in CO2 content, and other greenhouse gases in the Earth's atmosphere.