Effects Of Vegetation Construction On Soil And Water Conservation In The Sloping Fields And Small Watershed Of Purplish Soil Region,Sichuan

Purple soil area is a major managing area due to serious water loss and soil erosion. By improving plant construction can tremendously decreases such affects. However, the effects of planting vary hugely among different areas. Based on the daily runoff, the sediment yield and measured data of nearly 20 years in three runoff plots from 1985-2001 of ? branch in Hemingguan watershed of purple soil area, Sichuan, rainfall characteristics and regularity of the runoff and sediment yield were compared by constructing the relationship among rainfall, runoff coefficient and sediment yield before and after vegetation construction. Combining with daily rainfall, the changing trends of runoff and sediment were analyzed by Spearman's Correlation Analysis. Moreover, the effects of soil and water conservation of vegetation construction were evaluated quantitatively using the methods of Double Mass Curve and Flow Duration Curve. The results showed as followed.(1)After 1991, the average annual precipitation was about 10% less than before. But the average reduction rate of runoff coefficient reached 88% separately. After mixed afforestation, plot of runoff reduced 95% respectively (P<0.05). Meanwhile those of plot ? reduced 90% after artificial replanting and plot of ? reduced 78% after ridge protection by brick, respectively, the average reduction rate of sediment yield reached 83%. After mixed afforestation, runoff and sediment yield of plot reduced 88%, respectively (P<0.05). Meanwhile those of plot ? reduced 80% and those of plot ? reduced 81%. Effects of mixed afforestation were the most obvious on soil and water conservation. Before 1991, runoff and sediment yield appeared as order of Cultivated slope> Destruction of nature> Slope of conservation for grass, while it is characterized that Ridge protection as brick> Artificial replanting> Mixed afforestation after 1991. The difference between cultivated slope and the others was significant (P<0.05). According to the correlation analysis and regression analysis of runoff and sediment yield and rainfall index, the correlation coefficients are smaller after site preparation. And the correlation of runoff was significantly higher than that of sediment yield. It indicated that rain factors had little influence on sediment yield after site preparation whereas soil and water conservation measurement played a key role.(2) Comparing with Basic Period (1985-1990), annual runoff depth and annual sediment module reduced 34.6% and 89.9% in Evaluation Period (1991-2001). Whereas annual rainfall only reduced by 8.1%. It is illustrated that vegetation construction, as the main part of human activity, played an important role. Meanwhile, the runoff of Evaluation Period reduced 84.2% in wet period and 76.3% in normal period than that of Basic Period. Nevertheless, the runoff increased 906.6% in drought period. Vegetation construction obviously reduced flood and increase the low flow. Combined Double Mass Curve with Separation and Discrimination method, the vegetation construction's effect on reduction of runoff and sediment is 92.9% and 94.3%.Vegetation construction can control the loss of water and soil under different scales. And the best way is mixed afforestation, which can be applied to public to keep the water and soil combining with certain appropriate construction methods.