The Effect And Mechanism Of Vegetation Cover And Rainfall Change On Runoff And Sediment Yield In Du Watershed,China

Soil erosion and sediment yield are controlled by many factors,such as soil properties,topography,land cover and climate.Soil properties and topography are usually considered relatively unchanged in the short term,while climate variability and vegetation dynamics are the main drivers of watershed hydrological characteristics.In the 1980 s,a policy called the “Household Contract Responsibility System” was implemented in China's rural areas.Consequently,more land was reclaimed for farming.In the late 1999 s,the Grain-for-Green project was introduced to increase forest and grassland cover.To combat soil erosion on sloped croplands,farmland with slopes >25° was restored.A large scale(8973 km2)mountainous watershed in subtropical China was chosen as the study area.Based on the climatic and hydrological data of long-term monitoring from the hydrological station,combined with field survey and GIS and RS spatial analysis techniques.By using SSC-Q relationship,clustering analysis,lag analysis and partial least squares-structural equation model,this study revealed 1)the mechanism of how land use/vegetation cover and rainfall influences on runoff and sediment yield in the watershed scale;2)the impact of land use/vegetation cover change on runoff and sediment yield in different time scales;3)the spatiotemporal pattern of rainstorm and the effects on runoff and sediment yield in the watershed;4)the hydrological responses to rainstorm patterns and reforestation;and 5)decoupling the effects of vegetation dynamics and climate variability on watershed hydrological characteristics on a monthly scale.This study can help water resources management agencies make more appropriate decisions for ecosystems and watershed managementThe main results are as follows:(1)By analyzing and comparing the runoff,sediment concentration and sediment yield in the Du Watershed and its Sub-watershed in different periods,the results revealed the mechanism of land use change on erosion and sediment yield.The study period 1975-2009 was divided into three phases by the double cumulative curve: 1975-1984(P1),1985-1999(P2),and 2000-2009(P3).The annual runoff depth in the Du Watershed decreased and then maintained stable.The annual sediment yield decreased during the study period.The annual runoff depth of the Sub-watershed decreased and then increased,while the annual average sediment yield decreased continuously.By analyzing the annual variation of runoff and sediment concentration in different periods,we found that the annual sediment yield in the Du Watershed and its Sub-watershed is mainly due to the decrease of runoff during the P1-P2 period.During the P2-P3 period,the decrease of annual average sediment yield in the Du Watershed and its Subwatershed is caused by the decrease of sediment concentration which caused by Grainfor-Green project.(2)Land use change affects the event scale SSC-Q relationship in Du Watershed and its Sub-watershed.The sediment concentration in the Du Watershed under the same runoff is more stable than that in the Sub-watershed,but with the increase of vegetation cover,the SSC-Q relationship of the Sub-watershed also becomes stable.In the Du Watershed,the sediment concentration during P3 was significantly lower than that in the P1 and P2 periods under the high flow rate.In Du Watershed,there is no significant difference in sediment concentration during different periods in the case of low or medium flow.In the Sub-watershed,the sediment concentration during P3 is significantly lower than that during P1 and P2 periods under medium flow,while the sediment concentration during P3 is significantly lower than that during P1 and P2 periods under high flow.There are significant differences in sediment concentration during different periods under the low flow.(3)We investigate the hydrological response to different rainstorm spatial patterns and reforestation.Based on 32 years of observations,measurements of 184 rainstorm events and 125 sediment-producing events with complete hydrographs were analyzed.The K-means clustering method was used to classify the spatial patterns of rainstorm events in accordance with their event-based spatial rainfall characteristics.The 184 rainstorm events were classified into four spatial patterns,among which the spatial features differ significantly:(?)Spatial Pattern ?(SP?)includes rainstorms with a low amount of cumulative areal rainfall(27.4 mm),the highest spatial variability(0.986),and the highest frequency;(?)Spatial Pattern ?(SP?)includes rainstorms of high spatial variability(0.759)and the largest amount of local maximum daily rainfall(106.8 mm);(?)Spatial Pattern ?(SP?)includes rainstorms with a medium amount of cumulative areal rainfall(58.7 mm)and low spatial variability(0.362);and(?)Spatial Pattern ?(SP?)includes rainstorms with the largest amount of cumulative areal rainfall(117.2 mm)and the lowest spatial variability(0.313).SP?,which is characterized by widespread rainfall events and the highest rainfall intensity,produces the largest amounts of runoff and sediments.In total,the rainfall events during the four SPs accounted for approximately 30% of the rainfall but contributed 70% of the total suspended sediment yield.(4)Vegetation cover in the Du watershed was significantly improved after the implementation of the Grain-for-Green project.The average area-specific sediment yields for the four SPs were 15.4,65.5,55.8,and 286.2 t km-2 before reforestation and decreased to 6.0,59.3,43.7 and 89.9 t km-2,respectively,after reforestation.ANOVA indicated that reforestation resulted in a significant reduction in RC under SP? and SP? and a significant reduction in SSY under SP? and SP?.A hysteresis analysis suggested that the proportion of events with a clockwise loop increased from 64.9 % before reforestation to 82.1 % after reforestation and that complex loops became less common during 2000-2010 under SP?,thereby implying a reduced sediment supply.(5)We used partial least squares structural equation modeling(PLS-SEM)to decouple the effects of climate variability and vegetation on the temporal variations in hydrological characteristics on a monthly scale.Monthly hydrometeorological and vegetation cover data were collected from 2000 to 2010.Moderate Resolution Imaging Spectroradiometer(MODIS)derived vegetation indices were used to represent the vegetation status of the watershed.The results showed that vegetation dynamics and climate variability account for up to 67% of the temporal variation in the runoff,whereas the combined effects of vegetation,climate and runoff explain 62% of the variation in sediment.Climate variability both directly affects monthly hydrological characteristics and indirectly affects these characteristics through its effects on vegetation.Vegetation is negatively correlated with both runoff and sediment,and the net effect of vegetation on the sediment load(total effect =-0.20)is greater than its effect on runoff(total effect =-0.12).Our results indicate that the direct effect of vegetation on sediment(direct effect =-0.09)is smaller than the indirect effect(indirect effect =-0.11)of vegetation on sediment through its influence on runoff;thus,vegetation reduces both the sediment concentration and the sediment load mainly by reducing runoff.