Effect of Comprehensive Management on Runoff and Sediment Reduction in Yanwachuan Watershed, Loess Tableland, China

To evaluate the effect of comprehensive management on runoff and sediment reduction in meso-scale watersheds, the Yanwachuan watershed of Loess tableland gully region in China was investigated. Variation trends of the runoff and sediment from 1981 to 2009 were analysed, and the year of abrupt change was identified using the double mass curve method, accumulative departure method, moving t-test method and Yamamoto method. Next, the effects of comprehensive management on runoff and sediment reductions were quantitatively evaluated based on the runoff/sediment coefficient method and double mass curve method, both capable of separating the effect of climate change from that of comprehensive management. Finally, the driving forces of runoff and sediment variations were discussed from both climatic and comprehensive management aspects. The results showed that the runoff and sediment in the watershed had significantly decreased since an abrupt change in 1997. During the change period (1998-2009), the erosive precipitation and precipitation erosivity had been reduced by 32.4% and 17.4%, respectively, compared with the base period (1981-1997);  the areas of forest and grass had increased; and the normalized difference vegetation index (NDVI) had increased by 20.6% with significantly enlarged vegetation coverage. The benefit value of runoff reduction and sediment reduction were 27.3% and 76.2%, respectively. The resultant runoff and sediment reductions in the watershed were the synergy of climate change and comprehensive management, with greater than 90% reduction contributed by comprehensive management. All these results supported that comprehensive management played the dominant role in runoff and sediment reductions.