Impacts Of Land Use/Land Cover Change On Watershed Hydrological Processes

Impacts of Land use and land cover change(LUCC)on hydrological process is one of the key issues of the global change and watershed hydrological research.Over a relatively short period,LUCC is the main factor that affects the hydrological processes at different scales.Population growth,climate change,institutional change and urbanization etc.would cause the change of land use/ land cover(LULC),which in turn would further affect the hydrological process(e.g.infiltration,surface runoff,evapotranspiration and lateral flow)and water balance.The Heihe River Basin,the second largest endorheic river basin,is a typical endorheic river basin in China.The upper reach of the Heihe River generates the majority of the runoff for the entire watershed while the middle reach consumes a major portion of the water.The climate condition,natural environment and geomorphologic features are different between the two reaches.There is a need to compare the LUCC impacts on hydrological process in the two reaches and reveal the variation of hydrological process in the endorheic river basin under the impacts of LUCC.Hydrological model is an effective means to evaluate the impacts of LUCC on hydrological process at watershed scale.Currently,because of the difficulties in obtaining the yearly LULC data and the problems in calculating process in hydrological models,they simulate the impact of LUCC on hydrological process one period ech time and cannot simulate the impacts of multiple periods(e.g.multiple years)of LUCC continuously.That is the models could not call LULC data year by year seamlessly.In this research,we coupled a LUCC predicting model CA-Markov with a hydrological model SWAT,and developed a new model LU-SWAT,which can call LULC data year by year.The results show that:SWAT model performance was good when simulating the monthly and yearly stream flow in the upper reach of the Heihe River.However,the model performance was not satisfactory in the middle reach of the Heihe River,especially over non-irrigation months(December to March),the stream flow then was mainly controlled by groundwater discharge.The reason for this may due to the simple linear groundwater module in SWAT model;Because of the poor performance of the SWAT when simulating the stream flow in the middle reach of the Heihe River,this study,modified the linear-reservoir groundwater module of SWAT with a new nonlinear relationship to better represent the groundwater process and applied it to the middle reach area.The result shows that the performance of the modified SWAT was greatly improved(about 3%-58%)especially in non-irrigation months;The SWAT model was sensitive to the spatial resolution of the LULC data when simulating the monthly and yearly stream flow in the study area.,The higher spatial resolution of the LULC data,the better model performance of the SWAT.For this reason,the LUCC model CA-Markov was selected in this research to be coupled with SWAT model;In both the upper and middle reaches,the performance of LU-SWAT model is increased about 2%-20% compared with the original SWAT;In the upper reach of the Heihe River,LUCC was quite different before and after 2001.Relatively,from 1990 to 2001,under the three different climatic conditions: dry,wet and normal years,surface runoff,ET and total water yield were increasing and groundwater was decreasing.While from 2002 to 2009,lateral flow,surface runoff,ET and total water yield were decreasing but groundwater was increasing.Under dry condition,the change of all the hydrological factors were obvious than the other two climatic conditions.The main LUCC that had the largest impacts on the hydrological process in the upper reach of the Heihe River were changes of forest and barren land;In the middle reach of the Heihe River,from 1990 to 2009,the main LULC were grassland to farmland and barren land to forest.Under the three different climate conditions: dry,wet and normal years,surface runoff,groundwater and total water yield were decreasing and lateral flow,and ET was increasing.Under dry condition,the change of all the hydrological factors were obvious than the other two climate conditions.The main LUCC that had the largest impact on the hydrological process in the middle reach of the Heihe River was farm land.