Effects Of Rainfall And Confluence On Soil Erosion Process Of Croplands In The Typical Black Soil Region

It is important to prevent soil erosion in the black soil region of Northeast China, which directly threaten the food security and sustainable development of our country. This study took the erosivity dynamics(rainfall and confluence) in the water erosion process as the key point, selected eroded croplands of County Bin in Heilongjiang Province and Yushu City in Jilin Province as research sites, and a method of combining field situ experiments, laboratory rainfall simulation and confluence scour experiments was conducted to investigate soil erosion process and mechanism. Effects of erosive rainfall patterns on the soil erosion process of croplands in the black soil region were studied. The contribution and effects of rainfall and confluence on runoff and soil loss of croplands both for sheet erosion dominated and gully erosion dominated patterns were analyzed. Soil erosion models of croplands under the dual functions of rainfall and confluence both for sheet erosion dominated and gully erosion dominated patterns were established. Furthermore, the control effects of straw mulch cover measures on soil erosion of the black soil region were also evaluated. The main results were as follows:(1) The effects of rainfall intensity on runoff and soil loss of the black soil cropland, and the aggregate loss in the sediment were analyzed. Rainfall intensity had a significant influence on runoff and soil loss in the rainfall process. Runoff and soil loss at 90 mm h-1 were 11.1 and 1.75 times, 1364.9 and 3.50 times higher than those at 30 and 60 mm h-1, respectively. The soil loss in the earlier 20 min of rainfall period occupied 29.1%, 48.9% and 63.1% of total soil loss in the whole rainfall duration for 30, 60 and 90 mm h-1, respectively. The dominated aggregate loss size in the sediment at 30 mm h-1 was <0.25 mm, which occupied 90% of the total aggregate loss. However, the dominated aggregate loss size in the sediment were <0.25 mm and 2~5 mm at 90 mm h-1, which occupied 31.7% and 31.2% of the total aggregate loss, respectively.(2) The effects of erosive rainfall patterns on soil erosion process of the black soil cropland were clarified. The experiment designed three variable rainfall intensity patterns and one constant rainfall intensity pattern. Thereinto, total soil loss of the Reduced-type(Rainfall intensity varied from large to small) treatment was 1.03, 1.36 and 2.68 times higher than the Uniform type, Delay-type(Rainfall intensity varied from small to large) and Peak-type(The largest rainfall intensity located at the middle stage of the whole rainfall process) treatments, respectively. The rainfall intensity, which appeared at the initial stage in a rainfall pattern treatment had a larger contribution rate of soil loss to total soil erosion, compared with the same rainfall intensity which appeared at the middle stage or the last stage during a single event in rainfall patterns.(3) The contribution and effects of rainfall and confluence on runoff and soil loss of croplands both for sheet erosion dominated and gully erosion dominated patterns were analyzed. The results showed that the increase of rainfall intensity had greater impacts on soil loss than those of the increase of confluence intensity under the same total overland flow. When sheet erosion was dominated, average increased runoff rate and increased soil loss rate were 45.10% and 95.28% resulted from increasing the 50 mm h-1 rainfall intensity. Likewise, average increased runoff rate and increased soil loss rate were 35.61% and 48.38% resulted from increasing the 50 mm h-1 confluence intensity. Then when gully erosion was dominated, average increased runoff rate and increased soil loss rate were 50.49% and 95.50% after increasing the 50 mm h-1 rainfall intensity. Likewise, average increased runoff rate and increased soil loss rate were 43.83% and 67.95% after increasing the 50 mm h-1 confluence intensity. Whether for sheet erosion dominated or gully erosion dominated, the effects of rainfall and confluence on soil loss were larger than those on runoff; moreover, the contribution of rainfall to soil loss was greater than that of confluence to soil loss. With regard to total soil loss, the contribution rate of confluence was 50.77% and 71.15% of that of rainfall in both sheet erosion dominated and gully erosion dominated patterns, which illustrated effects of confluence on soil loss of croplands for gully erosion dominated were greater than on soil loss of croplands for sheet erosion dominated.(4) Soil erosion prediction models for sheet erosion dominated and gully erosion dominated were established. Based on soil erosion characteristics and the analysis of dynamics mechanism under the dual functions of rainfall and confluence, runoff caculations and erosion models of croplands both for sheet erosion dominated and gully erosion dominated patterns were established. The models had higher prediction precision. Our models were not only suitable for croplands in sheet erosion dominated but also for cropland in gully erosion dominated, and what is more, they can be applied to croplands of longer slope length erosion condition under the dual functions of rainfall and confluence.(5) The development process of gully erosion in the black soil region was discussed. At the early stage of the gully development, headward erosion played an important role, and with a certain degree of side-wall collapse, the length of gully increased by 105.1% compared with the initial morphology. Furthermore, soil erosion rates were larger and the fluctuations were stronger along with rainfall duration. At the middle and later stages of gully erosion, headward erosion weakened, side-wall collapse became the main form of gully development. Although there were little differences in total runoff among different gully development stages, but total soil loss, gully erosion and the ratios of gully erosion to total soil loss decreased with the increase of gully erosion development.(6) The control effects of straw mulch measures on croplands of sheet erosion and gully erosion in the typical black soil region were explored. Runoff and soil loss decreased as the increase of straw mulch amount. Under the large rainfall intensity(90 mm h-1), the lower coverage treatment(1 kg m-2) reduced the runoff and soil loss by 76.08% and 99.98%, respectively, and soil loss reduction reached up to 100% in the higher coverage treatment(4 kg m-2). The straw mulch buffer in the gully head could significantly reduce total soil loss and gully erosion. Total soil loss reduced by 67.5% and 76.7%, and gully erosion reduced by 72.3% and 69.1% in the straw mulch buffer in the gully head treatments compared with no-straw mulch treatments for the two different gully development stages, respectively.