| The Loess Plateau,one of the most severe soil erosion areas in China,discharged large amounts of sediment to the Yellow River,threatening in ecological environment and people’s safety.In order to control soil erosion,the Loess Plateru has implementation of the“grain for green project”since 1999.The large-scale vegetation restoration has greatly increased the surface vegetation coverage,which has greatly affected the process of runoff and sediment yield in watershed.Exploring the effect of vegetation restoration on runoff and sediment yield can provide an important reference for the integrated management of the watershed.In this study,the two watersheds—Yangjiagou（artificial afforestation）and Dongzhuanggou（natural restoration of vegetation）were selected as research area.Based on long-term hydrological data,this study applied a variety of mathematical statistical methods to investigate the rainfall,runoff,and sediment load variations.GeoWEPP model was used to simulate sediment yield under different rainfall patterns,comparing the spatial distribution of sediment yield under different rainfall patterns.This study clarified the regulation of vegetation restoration on runoff,sediment load and the relationship between runoff and sediment,and discriminated the effects of different vegetation restoration modes and different stages of governance on watershed runoff and sediment yield.The specific conclusions are as follows:（1）Average annual precipitation was 513.2 mm during 1981-2014 in study area,and exhibited insignificantly increasing trend at a rate of 1.76 mm/a（P>0.05）.Annual precipitation distribution was extremely uneven,mainly concentrated in 5-9 months.Affected by precipitation,annual distribution of runoff and sediment load were also extremely uneven.Runoff and sediment load were concentrated during 7-8 months in Yangjiagou,and were concentrated during 7-9 months in Dongzhuanggou.Monthly runoff and sediment load in Yangjiagou were significantly lower than those in Dongzhuanggou,especially in July when the peak appeared,which was about 30%of that in Dongzhuanggou.（2）Average annual runoff and sediment load were 3460 m3and 1040 t,and exhibited insignificantly decreasing trend at a rate of 75.11 m3/a和44.38 t/a during 1981-2004（P1）in Yangjiagou watershed（P>0.05）.During 2005-2014（P2）,annual runoff and sediment load in Yangjiagou decreased by 39.9%and 48.1%compared with P1.Annual runoff and sediment load exhibited significantly decreasing trend at a rate of 0.48×104 m3/a和0.19×104 t/a in Yangjiagou and at a rate of 0.15×104 m3/a和0.39×104 t/a in Dongzhuanggou watershed（P<0.05）.Compared with the natural restoration of vegetation,artificial afforestation had obvious effects on runoff and sediment reduction.Averages runoff and sediment load in Yangjiagou decreased by 66.2%and 66.9%compared with Dongzhuanggou during P2.（3）A total of 107 rainfall events were classified into three patterns during 2005-2014.Pattern A was characterized by medium rainfall amount,medium duration,medium rainfall intensity,medium frequency.Pattern B was characterized by low rainfall amount,short duration,heavy rainfall intensity,high frequency.Pattern C was characterized by high rainfall amount,long duration,low rainfall intensity,low frequency.Runoff and sediment load in Dongzhuanggou were much larger than those in Yangjiagou under each rainfall patterns.Runoff and sediment load in Yangjiagou decreased by 87.0%and 94.9%compared to Dongzhuanggou under pattern A,and decreased by 10.5%and 61.9%under pattern B,and decreased by 93.4%and 84.2%under pattern C.（4）The fitting degree of runoff and sediment load relationship of pattern A in Yangjiagou was better than that of Dongzhuanggou,and fitting degree of pattern B was slightly lower.Under the condition of the same runoff,sediment content of pattern A and B in Yangjiagou was lower,about 60%and 50%that of Dongzhuanggou.It shows that artificial afforestation in Yangjiagou significantly reduced the ability of surface runoff erosion and transport sediment.（5）GeoWEPP model were established for Yangjiagou and Dongzhuanggou watershed,and sensitivity analysis and calibration of parameters were carried out.Rill erodibility,effective hydraulic conductivity and soil critical shear stress were sensitive.The calibration results of effective hydraulic conductivity,rill erodibility and soil critical shear stress in Yangjiagou（Dongzhuanggou）watershed were 2.65 mm/h（2.3 mm/h）,2.0 s/m（3.5 s/m）,0.08 Pa（0.08 Pa）.The determination coefficient and Nash-Suttcliffe certainty coefficient were both greater than 0.7,proving the GeoWEPP model has good applicability in study area.（6）GeoWEPP model was used to simulated sediment yield under different rainfall patterns.Sediment yield of different rainfall patterns in Yangjiagou and Dongzhuanggou were significantly different.Compared with the single herb vegetation coverage in Dongzhuanggou,sediment yield in Yangjiagou was significantly lower than that in Dongzhuanggou through the optimal allocation of trees,shrubs and grasses.Spatial sediment yield was less than 250 t in Yangjiagou,and sediment yield increased to more than 2000 t in1.4%of areas in Dongzhuanggou under pattern A.Spatial sediment yield was less than 1500t in Yangjiagou,and sediment yield increased to more than 1500 t in 10.3%of areas in Dongzhuanggou under pattern B.The“grain for green project”plays an important role in restraining soil erosion on the Loess Plateau.Vegetation restoration has an obvious effect on reducing runoff and sediment,and the ability of artificial afforestation to reduce runoff sediment yield was significantly greater than that of natural restoration of vegetation. |
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