Simulating temporal and spatial variation of evapotranspiration over the Lushi basin

A process-based distributed model is established to simulate the temporal and spatial variation of evapotranspiration over the Lushi basin in China. The model components include the parameterization of land surface resistances, energy transfer, soil water movement, interception and surface runoff. What is distinguished in our model is the scheme for evapotranspiration partition by introducing a working variable related to the water vapor deficit at the reference height. The basin was divided into 6227 grids with resolution of 30×30 arc seconds. A digital elevation model was used to correct meteorological variables for altitude, and the inverse distance square method was adopted for spatial interpolation of precipitation over the basin. The remotely sensed normalized difference vegetation index (NDVI) in 1996 and the land use/cover data, both with the resolution of 30×30 arc seconds, were used to retrieve the distribution of vegetation leaf area index (LAI). The evapotranspiration simulated by using 1996 NDVI-derived LAI is close to that by using phenologically derived LAI. The differences of annual amounts of evapotranspiration from 1984 to 1997 simulated by using LAI derived by 1992, 1995 and 1996 monthly composite NDVI are less than 30 mm yr−1 (or 5%), in which 1995 represents drought and 1992 wet. The simulated annual evapotranspiration (ETa) plus soil storage change from 1984 to 1997 is in good agreement with precipitation minus discharge at the outlet. The spatial pattern of annual evapotranspiration and its components in 1996 clearly corresponded to the precipitation and LAI patterns over the basin. The highest monthly evapotranspiration (ETm) occurred in the period of July to August with 104 mm month−1. The value of ETm for irrigated farmland is higher than that for other vegetation types all the year and the values of ETm for grass and arid farmland are similar and about 10 mm month−1 higher than those for forest covers in summer. The predicted annual evapotranspiration in 1996 over the Lushi basin is 628 mm with the transpiration 327 mm, soil evaporation 256 mm and canopy interception 45 mm, respectively.