Compound erosion effect of snowmelt, wind, and rainfall on sloping farmlands of Chinese typical Mollisol region.

Research on the processes and mechanisms of compound soil erosion by multiple forces can provide scientific guidance for precisely controlling cropland soil erosion. Based on the seasonal alternation of freezing-thawing, snowmelt, wind, and rainfall erosion forces on sloping farmlands under natural conditions from November to next October of each year, we used a set of indoor simulation experiments of multi-force superimpositions to analyze the compound soil erosion processes of snowmelt (1 and 2 L·min-1), wind (12 m·s-1), and rainfall (100 mm·h-1). We further discussed the erosion effects of multi-force superimpositions. The results showed that, under single snowmelt erosion, an increase in snowmelt flow had a greater effect on sloping snowmelt erosion intensity than that of sloping runoff rate. When sloping snowmelt flow increased from 1 L·min-1 to 2 L·min-1, sloping runoff rate and erosion intensity increased by 2.7 and 4.0 times, respectively. Under snowmelt-wind superimposition erosion, previous sloping snowmelt erosion inhibited late wind erosion occurrence. As sloping snowmelt flow increased from 1 L·min-1 to 2 L·min-1, the inhibiting action subsequently increased and wind erosion intensity caused by previous snowmelt reduced by more than 50%. Both wind erosion and snowmelt-wind superimposed erosion intensified late rainfall erosion. The early wind erosion increased rainfall erosion by 24.5%. The snowmelt-wind superimposed effect increased the later slope rainfall erosion by 132.8% and 465.4% under 1 and 2 L·min-1 snowmelt runoff rates, respectively. The compound soil erosion amount driven by multiple force superimposition was not the sum of the corresponding erosion amount caused by single erosion force, with promoting or inhibiting effects of erosion force superimposition. The erosion effect of snowmelt-wind superposition was negative, but that of wind-rainfall superposition and snowmelt-wind-rainfall superpositions were positive.

[Effects of "Grain for Green" program and coal mining on sediment production in a typical small watershed of Yushenfu Mining Region, Northwest China].

Soil erosion in the ecologically fragile zone is severely affected by the intensified coal mining activities in the Yushenfu Mining Region of the Loess Plateau. Soil erosion controlling projects such as the "Grain for Green" program are continuously implemented by the government, which has effectively mitigated or even controlled soil erosion. Coal mining results in new environmental problems, which has changed the erosion characteristics and sediment transport in watersheds. In this study, we identified the sediment source trapped behind the check dam using a composite fingerprinting approach and clarified the impact of "Grain for Green" program and coal mining on variation of the amount and sources of sediment during two decades, in a representative check-dam which controlled the watershed of Laoyeman. The two decades were divided into two stages, i.e. an earlier decade (1990-1999) and a later decade (2000-2010) with respect to the implement of the "Grain for Green" program in the flood couplets sequence established during the running of the dam. The annual average thickness of flood couplets during 2000-2010 decreased from (42.7±29.9) cm to (16.6±13.7) cm during 1990-1999, and the annual sediment deposition per unit area (square meter) at the sampling site reduced from 579.9 kg·m-2 to 245.8 kg·m-2. The main source of sediment was gully wall (averaged 79.2%±10.1%) during the study period. However, compared with sediment sources during 1990-1999, the contribution of sediment from slope during 2000-2010, which was greatly affected by the "Grain for Green" program, showed an increasing trend (average annual contribution increased by 7.4%). According to the coaling mining and the variation of rainfall erosivity, our results indicated that under the combined effects of the "Grain for Green" program and decreases of rainfall erosivity, the intensity of sediment production in the watershed was substantially decreased. Coal mining had important impacts on sediment contribution from the slope, and obviously affected soil erosion controlling effects of the "Grain for Green" program. It is obvious that coal mining has increased the risk of erosion and sediment production on the slope, and that soil erosion control on the slope should focus on areas affected by coal mining.