Improvement And Application Of SWAT Hydrologic Model In Mountainous Upper Heihe River Basin

As a principle water source to the Heihe river, the upstream mountainous area of the Heihe river basin(HRB) is an important water resources generation area of the Hexi Corridor of Gansu. The amount and quality of water from the upper HRB have serious economic and environmental impacts on the mid-and lower HRB. The mountainous area of HRB is in semi-arid and cold area in high altitude that is complex in hydrographic and hydrogeological conditions. The total runoff is comprised of surface streamflow, groundwater and snowmelt, where snowmelt is a dominant water supply in Springs.The SWAT model is a continuous deterministic hydro-model. Watershed average slope length (ASL) is related to the calculation of lateral flow and routing timing, which eventually impacts simulation of the total surface runoff when the AVSWAT model was applied to simulate monthly streamflow (1990-2009) of the Yingluoxia outlet in the upper HRB. The simulation shows that ASLs calculated by the AVSWAT model are not consistent with the actual cases in the upper HRB. As a result, it leads to an inaccurate simulation. To address this problem, a new approach has been adopted to correct ASL of each sub-basin. A regression equation, which reflects the relationship between average slope and average length, was established by measuring average slope of each sub-basin. Using the corrected slope lengths and the default parameter set of SWAT together with meteorological and hydrologic data from1990to2009, Nash-Sutcliffe coefficient (NSE) of monthly runoff simulation is obviously improved. Sensitivity analysis and an examination of CN2parameter were also conducted to prove the applicability of this correcting method.In cold and arid regions, there is less precipitation and snowmelt runoff is the important water source in Springs. However, SWAT uses a relatively simple temperature-day approach to calculate snowmelt. As a result, SWAT underestimates snowmelt in the upper HRB and consequently influences overall simulation accuracy. The FASST model is a surface process model with explicit physical mechanism, including a snowmelt runoff component that makes use of mass and energy balance equations. Existing applications of FASST show good simulations of snowmelt in terrain-complex mountainous watershed. This thesis proposes a coupling approach that FASST is integrated to SWAT to improve the simulation of snowmelt by additional considerations of the effects of vegetation, soil temperature, topographic conditions and snow phase changes. A set of validations, by comparing streamflow, snowmelt runoff and surface runoff, have been tested for the original SWAT and the coupled. The results confirm the applicability of the SWAT-FASST coupled approach and it can significantly improve the simulations in the upper HRB.