Analysis of Long-term Water Balance in the Middle Reaches of the Yellow River Basin

To clarify the long - term water balance of the Yellow River basin, a new semi - distributed hydrological model was developed and applied to the middle reaches of the Yellow River basin. This analysis focused on the period during the past 40 years (1960 - 2000) using 128 meteorological stations data and high resolution remote sensing data. The model was based on soil - vegetation - atmosphere - transfer scheme and runoff formation model (SVAT - HYCYMODEL). To take the influence of vegetation change into consideration, an index of vegetation cover ratio (VCR) was introduced. By applying a VCR to the original hydrological model, the observed river discharge in the middle reaches of the Yellow River basin was captured by the model. According to the model simulation, it was found that the discharge from tributaries in the middle reaches of the Yellow River basin has been decreasing rapidly. Furthermore, the discharge ratio (discharge/precipitation) was also decreased significantly during the past 40 years. Consequently, we confirmed that the available water resources in the middle reaches of the Yellow River basin decreased considerably with not only precipitation decrease, but also increasing evapotranspiration ratio (evapotranspiration/precipitation) with the massive land - use change in the Loess Plateau.