| In order to control the serious soil erosion on the Loess Plateau,the state and local governments have invested a lot of financial and human resources to restore vegetation,build terraces and silt dams in small watersheds,resulting in significant changes in regional land use and surface cover,and the ecosystem has become more favorable and the soil erosion area has been effectively curbed.The vegetation cover and net productivity of the Loess Plateau have increased significantly since 1999 with the promotion of the policy of "returning farmland to forest(grass)" and ecological civilization construction,and the vegetation has influenced the flow production and sink process on the slope through the canopy,dead leaves layer and root system.Soil physicochemical properties and infiltration capacity are direct factors affecting the flow production patterns in the watershed,but there is still a lack of in-depth and systematic studies on how vegetation restoration on the Loess Plateau affects soil physicochemical properties,its extent and consequently the flow production mechanism.In this paper,we investigate the effects of different vegetation types and their restoration years on soil physicochemical properties and infiltration performance through field experiments and indoor analyses,and also analyze the changes of runoff processes in small watersheds under different surface vegetation cover,in order to reveal the effects of vegetation restoration on flow production mechanisms and patterns in small watersheds on the Loess Plateau.The main conclusions are as follows:(1)Vegetation restoration significantly improved the hydro-physical properties of the soil such as water storage and water holding.In particular,the composition of soil particles has undergone significant changes,which has also had a certain impact on the soil texture.In the Shenmu area,the soil texture has changed from loam to silt loam.In addition,under the conditions of vegetation restoration,the bulk density of the soil is significantly reduced,while the porosity,organic matter,and water-stable aggregates content of the soil are significantly increased(P < 0.05).These changes are most evident in the 0-20 cm soil layer.In grassland,the organic matter and aggregate content of shallow soil are significantly increased,while in shrubs and forests,the effect on the organic matter and aggregate content of deep soil is significant(P < 0.05).By fitting the soil moisture characteristic curves of the Suide and Shenmu areas with the Van Genuchten model,it is found that the water holding capacity of the soil in the soil layer of more than 40 cm has been enhanced after vegetation restoration.(2)Vegetation restoration significantly improves soil infiltration capacity.The initial infiltration rate of grassland in both Suide and Shenmu regions increased significantly(P<0.05).The stable infiltration rate of grassland,shrubland,and forest in Suide region increased by 76%,11%,and 19% respectively,while in Shenmu region,the stable infiltration rate of grassland,shrubland,and forest increased by 53%,128%,and 58%,respectively.The soil infiltration capacity at different depths of the Qiaogou 1st Gully and 2nd Gully(vegetation restoration sites)was greater than that of farmland,and vegetation restoration increased the difference in infiltration capacity between soil layers.Compared with the surface soil,the initial infiltration rate of the 10 cm soil layer in the Qiaogou 1st Gully,2nd Gully,and farmland decreased by 19%,12%,and 0%,respectively,while the stable infiltration rate decreased by23%,25%,and 5%,respectively.Soil bulk density and porosity are the main factors affecting soil infiltration capacity.(3)After vegetation restoration,the mean values of flood ephemeris and flood lag time of multi-year field floods in the Qiaogou watershed increased,and the mean values of runoff depth,runoff coefficient,percentage of surface runoff,and mean value of flood flow decreased.Prior to vegetation restoration,of the 17 floods in the Qiaogou watershed,8 had sharp-rising and sharp-falling hydrographs,accounting for 47.1% of the total,while 9 had sharp-rising and gradual-falling hydrographs,accounting for 52.9% of the total.However,in the 11 rainfall events after vegetation restoration,only 1 had a sharp-rising and sharp-falling hydrograph,accounting for 9.1% of the total,while 10 had sharp-rising and gradual-falling hydrographs,accounting for 90.9% of the total.It shows that with the recovery of vegetation,the flow production in the sub-watershed has a tendency to change from hyperinfiltration flow production to storage full flow production mode.(4)Comparing the two typical floods,it was found that after vegetation restoration,the flood ephemeral time and flood lag time increased,while the flood flow,runoff depth,surface runoff percentage and runoff coefficient decreased by 82.0%,78.2%,3.59% and 76.5%,respectively.Due to the increase in the proportion of interflow in the runoff generation process,the symmetry of the flood process also changed from rapid rise and rapid fall before vegetation restoration to rapid rise and slow fall after vegetation restoration. |
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