Assessing the impact of land use change on hydrology and sediment yield in the Xiangxi Catchment (China) using SWAT
This dissertation describes the application of the eco-hydrological model SWAT to a watershed in the Three Gorges Region, which is located in Hubei Province and Chongqing Municipality in Central China. The region is heavily influenced by the construction of the Three Gorges Dam on Yangtze River, which was completed in 2009. The dam was constructed for reasons of flood protection, hydropower production and navigation benefits. It exerts substantial influence on water resources in the affected sub-watersheds of Yangtze River. In the Three Gorges Region, the construction of the Three Gorges Dam induced a large-scale land use change. This can mostly be attributed to the inundation of agricultural areas, villages and towns, which necessitated the resettlement of more than 1 million people and the relocation of agriculturally used areas from the valley bottoms to steep, formerly forested slopes. The clearance of forest on steep slopes and their use for agricultural production is expected to strongly increase the risk of erosion and diffuse sediment inputs to surface waters. Soil erosion results in the removal of nutrient-rich topsoil, which leads to an irreversible degradation of soils and to undesired off-site effects in surface waters. The sediment reduces the life span of the Three Gorges Reservoir due to siltation and carries large amounts of nutrients to the water bodies. At the same time, flow velocities are reduced and the residence time of water is prolonged in the reservoir. In combination, the increasing sediment and nutrient inputs and the altered flow regime are expected to exacerbate the risk of eutrophication and thereby hamper the usability of water resources. Especially in the backwater areas of Yangtze River tributaries, an increasing occurrence of algae blooms has been observed in recent years. The current situation in the Three Gorges Region indicates a strong need for sustainable water resources management. This dissertation aimed at identifying a suitable tool for assessing the impact of land use change in the Three Gorges Region on water quantity and quality to support the development of integrated watershed management plans. Therefore, the eco-hydrological model SWAT (Soil and Water Assessment Tool) was applied to the Xiangxi Catchment in Hubei Province. The Xiangxi Catchment comprises an area of 3200 km2 as is considered to be representative of the eastern part of the Three Gorges Region. Land use in the watershed is dominated by forest. Agriculturally used areas are restricted to relatively small areas, which are characterized by severe soil erosion. SWAT was used in this dissertation to simulate water balance, streamflow and sediment yield under past, current and possible future land use conditions. Also, the most important sources of model error were identified in this study. Results indicate that the model performs very well with regard to streamflow and water balance, whereas the prediction of sediment yield is more problematic. SWAT output was analyzed at different spatial levels ranging from the entire watershed to individual Hydrologic Response Units. Generally, there is considerable uncertainty associated with the SWAT predictions in the Xiangxi Catchment, because of a low amount of data available for model parameterization, calibration and validation. Nevertheless results of the basic model calibration were considered a sufficient basis for the simulation of land use scenarios. The forested area in the Xiangxi Catchment has increased in the recent past which led to a decrease of fast flow components and sediment yield. Scenario simulations demonstrate that a further increase in forest would result in a continuation of the trends observed in the past, whereas an increase of agriculturally used areas would induce a strong increase in sediment yields. The scenario simulations indicate a high potential for conflicts between environmental protection and agricultural production, which is aggravated by conservation efforts of the Chinese government, e.g. the Sloping Land Conversion Program (SLCP). Through compensation and subsidy payments, SLCP encouraged farmers to convert sloping cropland to forest or grassland. While this is an effective means of reducing soil erosion on the afforested areas, it increases the pressure on the remaining cropland. The results of this dissertation suggest that in the future the application of selected Best Management Practices may be more effective for realizing sustainable watershed management plans than continuing to reallocate land use types within the Xiangxi Catchment. SWAT has already been used successfully for assessing the effects of Best Management Practices on water resources in a number of studies. However, the data currently available for the Xiangxi Catchment does not allow for a sufficiently detailed parameterization and calibration of land use and management. Future studies in the Xiangxi Catchment should focus on improving the data base by obtaining additional relevant input data. This can help to reduce the uncertainty in model results and facilitate the simulation of Best Management Practices. By testing the applicability of SWAT to the Xiangxi Catchment and identifying the main sources of uncertainty, this dissertation laid the groundwork for further research in the Three Gorges Region, which can help to preserve natural resources in this unique and sensitive ecosystem.
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