Trend analysis of pan evaporation in different climates of Iran

The purpose of this study was to assess changes in pan evaporation over the period 1985-2005 in a network consisting of 14 synoptic stations of Iran using parametric and non-parametric methods. All seasonal and annual series have been checked for normality with the Kolmogorov-Smirnov test. Time trends of this variable were analyzed using parametric and non-parametric techniques (Pearson correlation coefficient، least square linear regression، Mann-Kendall and rho-Spearman correlation coefficient). The results showed that based on the Mann-Kendall test، 21. 5 % of annual series، 7% of Summer and Spring series had an increasing trend. Besides، 50% of Summer series، 43% of Spring series، 14% of autumn series and 21. 5 % of annual series showed a decreasing trend. Based on Pearson correlation coefficient، 28. 5 % of annual series، 7% of Summer and Spring series had an increasing trend. Similarly، in 21. 5% of annual series، 43% of Spring series، 50% of Summer series and 14 % of annual series a decreasing trend was observed. The study indicated، 57% of summer series have significant series using Mann-Kendall، rho-Spearman correlation coefficient and Pearson correlation coefficient. In general، decreasing series were more than increasing ones. Despite of slightly increasing temperature trend previously reported in different regions of the country، results of this study showed that evaporation paradox does exist in different climates of Iran also. These reduced values of pan evaporation، as a key element in arid and semi arid climates of Iran، would have a significant effect in hydro-meteorological studies.