黄土丘陵区不同坡度、土地利用类型与降水变化的水土流失分异

 为了确定不同坡度、土地利用类型及降水参数对水土流失的影响,通过黄土丘陵区坡耕地、草地的3个坡度(10°,15°和20°)小区连续14年的径流、侵蚀观测数据,分析不同坡度、土地利用模式和降水变化的水土流失分异。试验结果表明:1)坡耕地水土流失量随坡度的升高而增加,20°小区显著大于10°和15°小区;草坡地小区的年水土流失量也随坡度变化,不同坡度小区之间没有显著性差异。2)坡耕地与草地的水土流失量具有显著性差异,坡耕地年均径流量和侵蚀量分别为草地的1．8倍和13．9倍,与草地比较,坡耕地明显地增加水土流失风险。3)不同土地类型水土流失模数受年降水变化的影响效应不同。坡耕地水土流失量受少数强降水控制,年径流量、侵蚀量与年降水量、产流降水量之间无显著性相关;草地的年径流量、侵蚀量分别与年产流降水量、年降水量显著相关。4)坡耕地的水土流失受降水量(P)、最大30min降水强度(I30)和综合降水指数(PI30)的显著影响,但各因素的决定系数不同,影响系数次序为PI30>I30>P。草地的径流与降水变量关系与坡耕地一致,但次侵蚀量仅与I30变化的显著性影响,而与P、PI30无显著性相关。草地的水土流失量受坡度差异以及降水变化的影响较弱,表明草地能够有效地控制水土流失,对荒坡草地采取保护措施以促进植被、土壤的进展恢复。坡耕地水土流失变化与坡度、降水变量的关系更为直接,通过降低坡度、休耕和免耕等耕作模式,减少土壤扰动和增加地面植被盖度,可有效减少水土流失。

Differentiation of soil erosion by different slope, land use pattern and variation of precipitation in loess hilly region

The loess hilly region is characterized topographically by steep slopes and highly dissected terrain, with sharp ridges and deep gullies. Population growth and growing food demand led to increasing pressure on the limited agricultural land, which caused conversion of grassland into agricultural land in the steep-hill slope; there is a growing need to assess conservation effectiveness of agriculture and grassland especially in semiarid loess hilly region. The study has been carried out in the Anjiagou small watershed, Dingxi County, Gansu province. At plot scale, an experiment investigated upon soil erosion of cultivated land and grassland on gentle-hill slope (10-20°) was performed over a period of 1986-1999. The results showed that the magnitude of soil erosion of cultivated land increases with rising of the slope, and there are significant differences among three slope plots; the annual soil erosion amount from plots of grassland varies with slope too, however, there is not a significance difference between plots. A considerable variation in soil erosion among plots indicates that cultivated land differed significantly in response to slope; there is significant difference in runoff and soil sediment between the cultivated land and grassland. Grassland provided good protection against surface runoff (which was consistently less than 2% of rainfall) and soil erosion (less than 27 g·m-2 per year), cultivated land caused twice increase in runoff and 14 times increase in soil erosion. The heavy soil erosion occurred in plots of cultivated land were mainly contributed to a few storm or rainfall events, therefore, there are not direct correlation between yearly runoff, soil erosion and yearly precipitation (total precipitation and runoff-generation rainfall) , whereas, in the plots of grassland, there is a strong correlation between annual runoff, soil erosion with runoff-generation rainfall and yearly precipitation, respectively. The relative contributions to surface runoff and soil erosion among rainfall variable were compared, examined that the precipitation ( P), maximum 30 minutes rainfall intensity ( I₃₀ ) and PI₃₀ are major factors and have positive correlation with runoff and soil erosion of rainfall events. The compound index (PI₃₀) and I₃₀ was used o quantify the relation of rainfall versus runoff and erosion, which can be predicted the runoff and erosion yield by the regression equation. The results of the study led us to the following conclusion: all plots of grassland were significantly lower in water and soil loss and more efficient in soil conservation compared with cultivated land, suggesting that reclamation of grassland performed much worse in conserving soil and water resource, overall, the slope did affect soil erosion of cultivated land. Some of their primary tillage in the fall greatly increases the erosion risk on sloping fields. To reduce such risks from fall tillage, some form of residue management should be applied, and tillage should be performed along the slope contours. In particular, the grassland acts as a buffer against fluctuations in runoff and soil erosion associated with rainfall events, agriculture use of cleared land does result in increased erosion, this study has demonstrated the major effects of slope on soil erosion and has found clear evidence that the rate of soil erosion and runoff has increased with slope. Most of these changes have occurred on land which is marginally suitable for cultivation, increasing the risk of soil erosion, accelerated water loss and further land degradation. As a result of the study, it is strongly recommended to protect the grassland, and the cultivated within steep-hill slope be limited to cultivation at a plot scale, considerable protection is offered to both the soil and water resource by grassland.