Impacts On Watershed-scale Runoff And Sediment Yield Resulting From Climate And Change-driven Vegetation Cover Change

This study investigates the relations between climate change and both runoff and sediment yield in watersheds and provides a scientific basis for water resources planning and design as well as watershed-scale soil and water conservation.The impact of climate change on runoff and sediment yield in a watershed does not occur in isolation,but is a synergistic process in which climate and vegetation jointly influence runoff and sediment yield.Previous studies have seldom addressed this synergistic effect.Taking the Zhenjiangguan Watershed as a case study,three downscaled General Circulation Models(GCMs)with two emission scenarios were used to generate possible climatic conditions for three future periods of near-term(2020-2039),medium-term(2050-2069)and long-term(2080-2099).Differences in scenarios were compared with the base period 1980-1999.Then,a Normalized Difference Vegetation Index climate factor regression model was established to analyze changes to vegetation cover under the climate change scenarios.Finally,a Soil and Water Assessment Tool model was built to simulate the response of runoff and sediment yield in the three future periods under two different scenarios:only changes in climate and synergistic changes in climate and vegetation cover.The main contents and conclusions are as follows:(1)Based on the collection and processing of basic data,a hydrological model called SWAT for the Zhenjiangguan Watershed was established.The model were calibrated and validated using observed runoff and sediment data from 1980 to 2005.The results show that the statistical evaluation coefficient of runoff simulation got to "very good" level and the sediment simulation got to "good" level.This indicates that the model has high reliability after being calibrated and validated,and can be used to study the response of runoff and sediment to climate and vegetation changes in the Zhenjiangguan Watershed.(2)Based on the climate projections of the three GCM models under the two SRES scenarios(A1B and B1)provided by IPCC-AR4,the original output of GCM was downscaled by using the bias correction method.Then,the SWAT model after calibration and validation is used to predict the spatiotemporal dynamics of runoff and sediment yield under climate change scenarios.The results show that compared with the base period(1980-1999),the temperature and precipitation under future scenarios will increase significantly in the Zhenjiangguan Watershed.The runoff and sediment yield are significantly affected by climate change.The range of average annual runoff change in A1B scenario for the three future periods is between 15%-3 8%,and the change in sediment yield is between 4%-32%.The range of average annual runoff change in the B1 scenario is between 16%-28%,and the change in sediment yield is between 5%-17%.The change in average annual runoff and sediment yield of each sub-watershed under different scenarios and periods show significant spatial heterogeneity.(3)Based on historical remote sensing MODIS NDVI and meteorological data(temperature and precipitation),a binary linear regression model was constructed,and the vegetation coverage information under different scenarios was updated in combination with the climatic projections from GCMs.Finally,it was coupled to the SWAT model to simulate the runoff and sediment yield in the watershed.The results show that climate factors are the main driving force for the changes in vegetation cover in the study area.Climate-driven changes in vegetation cover have an impact on runoff and sediment yield,and the differences caused by them account for 5.8%-12.9%of the total change.The runoff and sediment yield in the watershed are negatively correlated with vegetation cover,and the increase in vegetation cover can effectively reduce the runoff and sediment yield.To some extent,climate-driven vegetation cover changes will inhibit the effect of climate change on runoff and sediment yield in the study area.This investigation considers the synergistic effects of climate and vegetation cover change,which further clarifies the extent to which climate change impacts water and ecological resources.