Comparative investigation on the decreased runoff between the water source and destination regions in the middle route of China's South-to-North Water Diversion Project

Investigation on decreased runoff and driving forces in the water source and water destination catchments is vital to the water resources protection and water supply security assurance. Danjiangkou Reservoir Basin (DRB) and Miyun Reservoir Basin (MRB) are the water source and water destination regions for the middle route of South-to-North Water Diversion Project (MSNWDP) in China. Variations in runoff and driving forces behind these variations during 1960-2005 in the DRB and the MRB were comparatively analyzed and investigated by the climate elasticity method based on the Budyko hypothesis. The results showed that the runoff change process was diagnosed as the same three sub-periods (1960-1979, 1980-1989, and 1990-2005) by the cumulative anomaly analysis. The variations in the runoff exhibited large fluctuations with an overall decreasing trend of -2.5 mm/year in the DRB and -1.1 mm/year in the MRB. The variations in average annual runoff among the three sub-periods had significant differences in magnitude and direction. The runoff in the DRB increased by 81.6 mm during 1980-1989, and decreased by 67.1 mm during 1990-2005. However, the decreased runoff in the MRB during 1980-1989 and 1990-2005 were 37.3 and 31.7 mm, respectively. The specific spatio-temporal differences of climate factors (precipitation and potential evapotranspiration) and catchment characteristics jointly resulted in the differences of the runoff change. The climate elasticity method revealed that the leading factors to runoff changes in the three sub-periods had significant differences. The climate factors were the leading factors before 1990, however, the human activities became more intensive to be the leading factors after 1999. The relative contributions of climate factors and catchment characteristics to runoff change varied from (88.5%, 8.2%) during 1980-1989 to (30.9%, 73.9%) during 1990-2005 in the DRB. However, in the MRB, the relative contributions were from (51.0%, 48.2%) during 1980-1989 to (-8.0%, 110.0%) during 1990-2005. An effective way to reduce risk is to take the dynamic or elastic water resources management and the differentiated protection measures. In addition, green water management, the water demand management and saving water both in the water source regions and in the water destination regions of the MSNWDP are recommended and persistently advocated to reduce water consumption.