| Guizhou Province is the ecological protective barrier of the upper reaches of the Yangtze River and the Pearl River, it plays an important role in ecological and environmental protection. However, serious soil erosion and rocky desertification issues caused currently in this region worsen the original fragile ecosystem and environment, which not only restricted the local economic and social sustainable development, but also seriously hampered it plays the function of ecological protection.In order to predict and control soil erosion and rocky desertification effieectively, it is urgent to systematically study the laws and mechanism of soil erosion in this area, and to ascertain the properties of the main influence factors of soil erosion and their interaction mechanism.Topographic factors (mainly refers to slope gradient and slope length) are the main influence factors of soil erosion. Since their impact on runoff and sediment yields is related to many factors, they are more complicated than other factors, and has strong regional difference. In this study, on the basis of analyzing the soil erosion environment in karst areas in Guizhou Province, using the data obtained from field runoff plot experimental observation, we comprehensively analyzed the impacts of the slope gradient and slope length on runoff and sediment yields of slope land in this area through methods of mathematical statistical analysis, model simulation, etc. Moreover, estimation formulas of slope gradient value and slope length value were established, respectively. The main conclusions are as follows:(1) The relationship between the runoff and sediment yields of slope land and slope gradient and slope length factors was analyzed. In the range of5°～15°, the runoff and sediment yields of slope land increased along with the increase of slope gradient, which is similar to the law of the runoff and sediment yields changed with slope gradient in non-karst areas. Because of the impact of the exposed rocks in the runoff plots, the runoff and sediment yields of20°runoff plot was abnormally low, the runoff and sediment yield of25°runoff plot also declined slightly. The drop difference between the two runoff plots was resulted from the difference of the shape, location, areas of the exposed rocks. With the increase of the slope lengths, the runoff of slope land increased, then decreased when the slope length reached15mm. There was no regularity between the sediment yields and the slope lengths, it mainly because of the heavy rains during the experimental observations which caused the sediment yields abnormally high and imbalance. After eliminating the sediment data obtained from the heavy rains, the sediment yields increased gradually with the increase of the slope lengths.(2) Under the condition of different concentrated rainfall intensity, runoff and sediment yields changed along with the slope gradients showed as follows:When I30was10mm/h or less, the runoff climbed up with the increase of the slope gradients, then declined when the slope gradient reached20°, and the sediment yields had a linear relationship with the slope gradients. When I30was between10mm/h and30mm/h, the runoff increased overall with the increase of slope gradients, and the sediment yields had a linear relationship with the slope gradients. When I30was30mm/h or more, the runoff and sediment yield increased and decreased fluctuately with the increase of the slope gradients. The runoff and sediment yields changed along with the slope lengths showed as follows:When I30was10mm/h or less, the runoff and sediment yields increased and decreased irregularly. When I30was between10mm/h and30mm/h or more than30mm/h, the runoff increased with the increase of the slope lengths, then decreased when the slope length reached15mm, the sediment yields had a linear relationship with the slope lengths, but it increased and decreased alternatively in a certain range.(3) With the increase of the slope gradients, the degree of correlation between runoff and the three representative factors of rainfall characteristics (P, I30, I60) decreased. When the slope gradient reached20°, the runoff almost had no correlation with P, I30and I60.Under different slope gradients, the sediment yields had significant correlation with P, I30and I60at the0.01level, and the degree of correlation between sediment yield and three representative factors of rainfall characteristics was I60＞I30＞P. Under the condition of different slope lengths, runoff and sediment yields had significant correlation with the three representative factors of rainfall characteristics (P, I30, I60) at the0.01level. And the degree of correlation between runoff and the three representative factors of rainfall characteristics was P＞I60＞I30.The degree of correlation between the sediment yields and three representative factors of rainfall characteristics was I60＞I30＞P.There was a good power function relationship between runoff and sediment yields and precipitation and rainfall intensity under different slope gradients and slope lengths except20°and25°runoff plots. There was a good linear relationship between the sediment yields and EI30, PI30under different slope gradients and slope lengths. And the EI30’s modeling effect is better than PI30, but the modeling effects of runoff plots of20°and25°was relatively poor.(4) Analysis of all erosive rainfall event observed from the experiment showed that the correlative relationship between sediment and runoff yields was not significant. But the sediment and runoffyields presented a good linear relationship under different slope gradients and slope lengths after eliminating the sediment and runoff data obtained from the two heavy rain events.(5) Based on USLE and its modified model RUSLE, the estimation formulas for slope gradients and slope lengths were deduced as S=7.5942sinθ0.8248and L=(λ/22.13)0.4064in karst areas in Guizhou Province. And the LS value lookup table within the slope gradient range of5°～30°and the slope length range of5m～50m was established. |
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