Payam Najafi - Assistant Professor, College of Agriculture, Islamic Azad University, Khorasgan Campus,

CropWat 4 Windows is a program that uses the FA0 (1992) Penman-Monteith method for calculating reference crop evapotranspiration (ETo). The methods supersede the older FA0 24 procedures, which are no longer recommended as they overestimate evapotranspiration. In addition, the analysis of the performance of the various calculation methods reveals the need for formulating a standard method for the computation of ETo. For this purpose, FA0 56 recommended FA 0 Penman-Montejth as a sta~rladm efiod, The aim of thjs research was estimating the accuracy of CropWat Software to calculating ETo in arid and semi-arid region of Iran. In this research, 9 locations of arid and semi-arid regions of Iran selected and the grass lysimeter data collected too. The lysimeter data is collected duration of May through November during 1988-1997. For the comparisons of the ETo lysiineter data and ETo resulted CropWat software in different point, three statistical parameters were used include mean absolute relative error (MARE), root mean square difference (RMSD) and correlation coefficient (R~)T. he results of this research show that the average of MARE, RMSD and R' are con~puted about 29%, 2.1 and 77% respectively in the study areas. In addition, the resufk of this research show that when the average wind speed was less than 1 meter per second ar wind speed was more than 1 metex pex sec~ndan d ET Q was less than CropWat has a low sensitive for estimating ETo and it's necessary to improved the results 6m these areas. Finally based on lysimeter data, four liner regressions were presented to impar the results of the CropWat software for arid and semi-arid region of Iran.

CropWat, Potential Evapotranspiration, FA0 Penman Monteith, arid region, semi-arid region, lysimeter, accuracy