The impact of land use change in the Xiangxi Catchment (China) on water balance and sediment transport

The construction of the Three Gorges Dam on the Yangtze River led to the resettlement of more than one million people including the relocation of agriculturally used areas from the valley bottoms to steep, formerly forested slopes. This is expected to induce an increase in surface runoff, soil erosion and sediment inputs to rivers and can seriously impair the quality of surface waters in the Three Gorges Region. The aim of this study was to simulate past and future land use changes in the Xiangxi Catchment with the eco-hydrologic model Soil and Water Assessment Tool (SWAT) (Version SWAT2009, Revision 530) in order to quantify impacts on hydrology and sediment transport. The Xiangxi Catchment covers an area of 3,200 km(2) and is located approximately 40 km upstream of the Three Gorges Dam. In spite of the resettlement of people and the relocation of agricultural areas, according to the land use maps used in this study, the forested area in the watershed has increased by 3.6 % between 1987 and 2007. Also, large areas of cropland were replaced with orange orchards, which are assumed to provide better soil protection than crops. Accordingly, simulation results demonstrate that surface runoff, soil erosion, streamflow and sediment loads have decreased by 5.9, 47.7, 0.7 and 41.9 %, respectively. Scenario simulations indicate that any additional increase in forested area in the Xiangxi Catchment would further reduce surface runoff and sediment yields, but at the same time would increase the pressure on remaining cropland. Positive effects of afforestation could be outweighed by the negative effects of a further intensification of land use in other parts of the watershed. Economic growth and increasing population pressure are likely to lead to a demand for additional farmlands. Simulation results have shown that when only areas with slopes < 25A degrees are brought into cultivation surface runoff and sediment yields could increase considerably. The results of this study indicate a strong need for sustainable development and management of land in the Xiangxi Catchment to find a balance between the demands of environmental protection and agricultural production.