根据野外观测资料,比较不同农耕地的产流产沙与草地,分析了退耕还草后减流减沙效应随降雨特征、坡度以及时间变化的特征。结果表明,退耕还草减流减沙主要是在大雨量时,尤其是减流效应,在小雨量降雨时很弱,甚至不减流。退耕还草减流减沙效应随坡度的增加而增大,且减沙效应增长速度大于减流效应,退耕还草应尽快退掉25°以上的坡耕地。退耕还草减流减沙在多雨和暴雨多年份以及在暴雨多的季节效应强,主要削减的是大暴雨、高含沙的地表径流。在任何情况下,退耕还草的减沙效应都明显大于减流效应。

The Loess Plateau is well known to the world for its intense soil erosion. Increasingly critical soil erosion is an important factor contributing to ecological environment degradation such as land degradation and high sediment concentration of river. Water and soil losses are considered resulting from the most important cause of mankind irrational land use, particularly vegetation degeneration and cultivating steep slope land, and the awareness of tillage erosion as a soil degradation process has become an increasingly important concern to conservation. A large body of literature indicates that returning the steep sloping field to forest and grass land, and adjusting measures to local conditions to restore vegetation is the important measure to control soil and water loss in the plateau in our country. The study of returning the cropland to forest and grass affecting on the regional soil and water resources has been the hot. Even though there were many researches on reduced runoff and sediment for forest and grass, but based on comparing forest and grass to naked land, and less clear is the efficiency of reduced runoff and sediment resulting from returning cropland to grass. So, runoff and sediment changes characteristics after returning farmland to forest and grass should be studied. According to the comparison of the runoff and sediment amount in grass and cropland in experiment plots, the efficiency of reduced runoff and sediment with rainfall, slope and time were studied. Runoff and sediment reduction after restoring forest and grass occur mainly in heavy rain, particularly reduced runoff efficiency, in light rain, is very weak or not. Based on data calculated and analyzed, the efficiency of reduced runoff and sediment are 0.44 and 0.84 as rainfall exceeding 50mm compared 0.31 and 0.7 when rainfall under 20mm. With slope gradient increasing the efficiency increase, increasing speed of reducing sediment efficiency is much more rapidly than reduction of runoff. In those larger storms years and rainy seasons the efficiency of runoff and sediment reduction is high. At any time, reducing sediment efficiency is more strong than reduction of runoff. Returning cropland to forest and grass has been identified as main measure for conserving soil and water. The study results show, returning cropland to forest and grass is necessary, and the sloping field with a slope of ＞25° should be retured to grassland and forest as soon as possible. And it is hoped that such analyses will help understand the effect on water and soil resources from returning cropland to forest and grass.