Analysis on Trends in Annual Reference Crop Evapotranspiration and its Impact Climatic Factors in Henan Province During 1971 to 2010

Based on the long-term climatic observational data including daily mean temperature,maximum temperature,minimum temperature,relative humidity,wind speed and sunshine duration from 111 meteorological stations in Henan province from 1971 to 2010,authors calculated the daily reference crop evapotranspirations(ET0) for each station with the aid of the revised Penman-Monteith model recommended by FAO.The temporal spatial distribution characteristics of annual ET0 and its major climatic impact factors were analyzed using the mathematical statistics.The results showed that,the Penman-Monteith model well simulated annual ET0 in Henan province during the past 40 years,with a correlation coefficient of 0.84(P0.01) between an annual ET0 and the annual 20cm pan evaporation during same period in same stations.The mean annual ET0 of the 40 years was 796.1mm(±102.2mm,n=4169),with higher values in northern regions than southern regions.Temporally,annual ET0 of Henan province declined significantly with the rate of 24.7mm·10y-1.Among the sites with significant decreasing annual ET0 during 1971 to 2010,most of them located at around 34° N northwards.Partial correlation analysis showed that,annual ET0 of either the whole Henan province or single city closely related to meteorological factors,and further analysis indicated the most significant negative correlation between ET0 and wind speed,with an exception of Jiyuan.Stepwise regression analysis suggested that the change of annual ET0 had close relationship with mean temperature,sunshine hour,wind speed and relative humidity.The effects of mean temperature,sunshine duration and wind speed on annual ET0 were positive,while that of relative humidity was negative.Wind speed reduction during the past 40 years was the main cause of decreased annual ET0;however,the overall change of annual ET0 was synthetically affected by mean temperature,sunshine hour,wind speed and relative humidity as a whole,and the contribution of each climate factor differed spatially.