The Relationship Between Vegetation And Runoff, Sediment In The Past50Years In Yanhe River Watershed

In order to explain the sharp reduction of runoff and sediment discharges since2000inthe middle reach of Yellow River, and to estimate the benefits of runoff and sedimentreduction of vegetation, we took Yanhe River watershed as study area, which is thegrade branch of Yellow River, located in the loess hilly area. We analyzed the variationcharacteristics of runoff and sediment discharge during1960-2010, and their relations withprecipitation. We also studied the relationship between vegetation changes and runoff,sediment discharge, and the correlation of landscape patterns of Yanhe River watershed andsoil and water loss. We used the remote sensing images of the whole watershed during1970-2010to extract vegetation information. To estimate the runoff and sediment dischargereduction, we distinguished artificial vegetations from natural vegetation in quantity, andestimated the benefits of runoff and sediment reduction of artificial vegetation and naturalvegetation, according to Soil Conservation method. At the same time, we chose Morgan-Morgan-Finney (MMF) model of Morgan-Duzant (MD) version, and modified this modeland the parameters, according to the terrain and vegetation of Yanhe River watershed,laboratory test data. Further, we analyzed the correlations between vegetation factors andrunoff, sediment discharges, and did scenario simulation between adjacent periods. The mainconclusions are as follows:(1) We used accumulated deviation method to analyze the trend of runoff and sedimentdischarge, and found that it increased sharply before1970, and experienced fluctuationduring1970-1996, and then began to decrease after1996. And the runoff and sediment discharges had good linear relationship (y=4469.4x-5028.3, R2is0.8747). To detect thedifference of runoff between different reaches of Yanhe River, we contrasted the runoff ofYan’an hydrometric station and Ganguyi hydrometric station during1965-2010. The runofftrend of middle and lower reach was consistent, but the change was different to certain degree.As a whole, the runoff of lower reach varied more dramatically than the runoff of middlereach.(2) In the late1970s, the area of farmland in Yanhe River watershed was5128.83km2,but decreased to1232.11km2in2010. The background of landscape in the watershed hadalready turned from farmland to vegetation. And regression analysis found that there existedparabolic relationship between vegetation area and runoff and sediment discharge, while itwas linear relationship between dominance of vegetation and runoff and sediment discharge.Further, landscape analysis showed that runoff was negatively related with the number anddensity of farmland patches, while positively related with maximum patch index andaggregation index of farmland, and with the number and density of patches of cities andtowns. The sediment discharge was positively related with aggregation index of farmland andthe number and density of patches of cities and towns.(3) The estimation of the benefit of soil and water conservation of vegetation, based onSoil Conservation method, showed that, compared with1980s, the artificial vegetation in2003-2005reduced22.8million cubic meters of runoff, and5.39million tones of sedimentdischarges, while natural vegetation reduced12.1million cubic meters of runoff, and1.52million tones of sediment discharge. Among the influence of human activities on runoff andsediment discharge, artificial vegetation made46.94%contribution to runoff reduction and30.64%contribution to sediment discharge reduction, while natural vegetation made24.97%contribution to runoff reduction and8.62%contribution to sediment discharge reduction.(4) The MD version of MMF model could simulate the annual runoff well, and therelative error was between-26.41%to17.25%. As to the simulation of sediment discharge,the relative error was between-28.82%to45.13%, with the consideration of effect of siltdams and terraced fields. And the scenario simulation analysis found, between adjacentperiods, the influence of vegetation to sediment discharge varied from12%-30%, which wasbasically consistent with the results of Soil Conservation method. (5) Vegetation cover cannot completely reflect the soil and water conservation functionof vegetation. We also analyzed the correlation between other vegetation indexes and runoffand sediment discharge, including canopy interception rate, eroding force of rainfall undercanopy and deposition rate. We found that, runoff was slightly related with vegetation cover,but significantly related with canopy interception rate, eroding force of rainfall under canopyand deposition rate, which meant that the vegetation reduced the runoff in a comprehensiveway. However, the sediment discharge was related with vegetation differently, which wasclosely related with vegetation cover. It suggested that vegetation type and quantity made themain contribution to sediment discharge reduction.