Changes of Pan Evaporation in the West of Iran

Evaporation is an important component of the hydrological cycle and its change would be of great significance for water resources planning, irrigation control and agricultural production. The main purpose of this study was to investigate temporal variations in pan evaporation (E-pan) and the associated changes in maximum (T-max), mean (T-mean) and minimum (T-min) air temperatures and precipitation (P) for 12 stations in Hamedan province in western Iran for the period 1982-2003. Significant trends were identified using the Mann-Kendall test, the Sen's slope estimator and the linear regression. Analysis of the E-pan data revealed a significant increasing trend in 67% of the stations at the 95% and 99% confidence levels. To put the changes in perspective, the trend in E-pan averaged over all 12 stations was (+)160 mm per decade. Trend analysis of the meteorological variables for examination of causal mechanisms for E-pan changes showed warming trends in T-max, T-mean and T-min series in almost all the stations, which were significant over half of the total stations. On the contrary, no significant changes in precipitation were found approximately at all of the stations. Furthermore, a moderate positive correlation was observed between E-pan and T-max, T-mean and T-min, while a inverse correlation was found between E-pan and P data. The results indicated that the study area has become warmer and drier over the last 22 years, hence the evaporative demands of the atmosphere and thereby crop water requirements have increased.