Variations of Land Evapotranspiration in the Plain of the Middle Reaches of Heihe River in the Recent 35 Years

In this article,the FAO Penman-Monteith formula is used to calculate the real evapotranspiration for different underlying surfaces in the middle reaches of Heihe River,and the evapotranspiration change and its seasonal variation are analyzed for the recently 35 a(1967_2000 year).The remote sensing images(TM) of 1967,1986 and 2000 in Ganzhou,Linze,Gaotai and Minle Counties are used to analyze the land use change.Finally,the total evapotranspiration of the three underlying surfaces was calculated through real evapotranspiration and different areas of land.The result indicated that land surface evapotranspiration overall presents a decreasing tendency.The annual mean evapotranspirations of the farmland,grassland and desert are 762 mm,340 mm and 229 mm,respectively,in the recent 35 a.The evapotranspiration varies seasonally.The seasonal variations of real evapotranspiration for the three underlying surfaces follow that in the summerin springin autumnin winter.Real evapotranspiration is influenced by the seasonal variation.Simultaneously,in the four counties land is degenerating seriously.As a whole,the grassland degeneration is more obvious,the desert area increases in evidence,and the farmland area increases a little;For the four counties and for the three underlying surfaces,the total evapotranspiration was 37×108 m3 in 1967,38×108 m3 in 1986 and 39×108 m3 in 2000.