MONTHLY AND ANNUAL TREND OF POTENTIAL EVAPOTRANSPIRATION IN WEST OF IRANAhmad RajabiAbstract Evapotranspiration is one of the main factors of hydrological cycles. Studying potential evapotranspiration changes to know the climate change effects on hydrological cycle and to estimate the irrigation networks water requirement are significant. The present study has estimated potential evapotranspiration by Penman- Monteith method according to synoptic stations climate data of west of Iran from 1961 to 2009. Evapotranspiration trend in these stations have been studied by Mann-Kendall and Spearmans Rho tests. Potential evapotranspiration has an increasing trend in all stations in the observed record period which is much more significant from 1990 on. Among all the stations, Khoramabad station has the highest increase and Sanandaj station has the lowest one. Monthly evapotranspiration changes in these stations dont follow any specific rules which mean increase in some months and decrease in other months. In almost all the stations the trend was increasing in warm months of the year that will be so significant in estimating and providing water requirement of agriculture products.
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