Study on Reference Crop Evapotranspiration in Different Years and Different Seasons in Henan Province

Based on the daily data of 17 meteorological stations from 1981 to 2010 in Henan province,reference crop evapotranspiration was calculated by Penman-Monteith equation,the temporal and spatial variations of reference crop evapotranspiration were analyzed,and the dominant factor influencing reference crop evapotranspiration was explored using multivariate regression method.The results indicated that the annual reference crop evapotranspiration had a decreasing trend,and changed from 912.4mm to 1151.2mm.The reference crop evapotranspiration increased in winter,while the reference crop evapotranspiration decreased in other seasons.The reference crop evapotranspirations in spring,summer,fall and winter were 31%,38%,19% and 11% of annual reference crop evapotranspirations respectively.There was a decreasing trend in the eastern and southern of Henan while the reference crop evapotranspirations increased first,then decreased in the western and northern of Henan from 1981 to 2010in the spatial distribution.Distance averages of the reference crop evapotranspiration were 6.6～33mm,9.4～27mm,-46～10mm respectively in the eastern and southern of Henan from 1981 to 1990,1991-2000 and 2001—2010while they were-63～19mm,22～55mm and-14～27mm respectively in the western and northern of Henan from 1981 to 1990,1991—2000 and 2001—2010.The sunshine duration was the dominant factor influencing reference crop evapotranspiration in different years and different seasons and there was a positive relation between them.The sunshine duration decreased significantly in Henan province from 1981 to 2010.