Spatial-temporal Variation Of Evapotransporation In The Central Precipitation-gradient Zone Of Loess Plateau

The distribution of water resources and ecosystem are closely related. The evapotransporation is an important basis for the hydrologic cycle and water resources distribution, which affects the global or regional climate, ecological and agricultural production. Because the spatial-temporal distribution of the earth’s surface evapotransporation is difficult to obtain accurately, it has become to be one of the key research topic.The Loess Plateau has the serious soil erosion and fragile ecological environment in the China. The climate of Loess Plateau varies from semi-humid to semi-arid and arid condition. The average annaual precipitation decreases from the southeast to the northwest, while the soils are claier in the southeast and sandier in the northwest. Vegetation varies from trees, shrubs to grass.A precipitation-gradient zone was selected in the Loess Plateau, containing typical climatic, soil and vegetative conditions. The MODIS data and the SEBAL model were used to estimate the regional annual evapotransporation(AET). The daily, monthly, and annual AET values were estimated in 2000, 2004, 2008 and 2012 using the MODIS 、 DEM and meteorological data. The growth seasonal AET values were compared with the values estimated using the improved Biome-BGC model for evaluating the accuracy of the SEBAL model. The main results are as follows:(1) Since the project of Grain for Green has been implemented, the vegetation fractional cover showed a rising trend. In 2000, the vegetation fractional cover was 54.8%, 61.3% in 2003, and reached the maximum 65.3% in 2007. After 2008, the vegetation fractional cover fluctuated at a high level.(2) The amout of actual evapotranspiration in a year presented a “single-peak” feature. The average daily evapotranspiration during the period of the day 90 th to the day 310 th, incearsed gradually from 1 mm/d to 5 mm/d, then decreased to 1 mm/d. The average month evapotranspiration incearsed from 50 mm/m in April to 300 mm/m in July, then reduced to 30 mm/m in October. The trend of annual actual evapotranspiration was: inceased from 2000 and reached maximum in 2008, then decreased in 2012.(3) The actual evapotranspiration decreased from the southeast to the northwest. The changes of sensonal actual evapotransporation in four sub-zones were: the sensonal actual evapotransporation in desert zone remained relative steady, while the values in the steppe zone and the forest-steppe ecotone increased from 2000 to 2008 and then decreased. The seasonal actual evapotransporation in the forest zone reduced from 2000 to 2012 due to the decreased potential evapotranspiration.