Simulating the Water Balance of the Wuding River Basin in the Loess Plateau with a Distributed Eco-hydrological Model

With an aid of the geographical information system of the hydrological, meteorological, soil and land use data, the Digital Elevation Model and NOAA-AVHRR remote sensing information, a distributed eco-hydrological model based on soil-vegetation-atmosphere transfer theory is developed to simulate the spatial and temporal variation of water balance components over the Wuding River Basin, the Loess Plateau, China from 1982 to 1991. It is found over the ten years, the maximum Normal Difference Vegetation Index (NDVI) changes year by year while the annually averaged NDVI is stable. There is not an obvious linear relationship between the annually averaged NDVI and annual precipitation, indicating that the variation of vegetation is not fully controlled by precipitation in this basin. The simulated evapotranspiration values over the Wuding River Basin and one of its sub-basins (Chabagou) agree well with the observed, the difference between the annual precipitation and runoff, with the relative errors being less than 5%. Over the simulated periods, the annual precipitation (372±53 mm yr-1) is consumed mostly by evapotranspiration (334±33 mm yr-1), of which about one third is transpiration (130±21 mm yr-1). Compared with the high yearly variation of precipitation and evapotranspiration, that of runoff is relatively stable. The seasonal variation pattern of evapotranspiration is similar to that of precipitation and net radiation with peaks in August, while that of runoff is unique with the high peak in March, indicating that freezing process should be further included in the model development. By increasing gradually from the northwestern part of the basin where the NDVI is low, the precipitation and evapotranspiration reaches the highest in the southeastern part of the basin where the NDVI is high. The spatial pattern of runoff shows the highest values in the middle part of the basin. Replacing the vegetation patterns in 1991 with each single vegetation type covering the whole of the Chabagou basin, canopy transpiration and soil evapotranspiration all change dramatically, whereas the changes of total evapotranspiration and runoff are small.