摘要
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 (Epan) and the associated changes in maximum (Tmax), mean (Tmean) and minimum (Tmin) 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 Epan 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 Epan averaged over all 12 stations was ( )160 mm per decade. Trend analysis of the meteorological variables for examination of causal mechanisms for Epan changes showed warming trends in Tmax, Tmean and Tmin 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 Epan and Tmax, Tmean and Tmin, while a inverse correlation was found between Epan 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.