Temporal trend analysis of reference crop evapotranspiration (Case study: Hamedan station)

The main aim of this study is the trend analysis of the reference crop evapotranspiration (ET0) at Hamadan (Nodjeh) station. For this purpose the information of this station was used in the period of 1951-2005. The amount of ET0 was estimated using the Penman-Monteith (PM) approach. For trend analysis two methods which are a) fitting a straight line on time series and calculating the slope of trend line using the least square method, and 2) Mann-Kendall test and calculating the slope of trend line by Sen's estimator method were used. Stepwise linear multiple regression analysis was also used for the purpose of sensitivity analysis of ET0 to meteorological parameters. Results show that lowest and highest (7.2 mm/day) ET0 occurred on January and July, respectively. The slope of trend line was positive for all months (except January and February). In the warm six months of the year slope of trend line was statistically significant at 1% level. The largest positive slope of ET0 trend line observed at July, which is statistically significant at 1% level. Results of sensitivity analysis showed that in the hot months of the year the wind speed and maximum air temperature are two main important factors which influence the ET0.