Drive Effect Of Vegetation Restoration On The Mode Of Production And Flow Pattern Of The Loess High Ravine Area

Vegetation restoration has significant impacts on the hydrological cycle,particularly the process of runoff generation and confluence within watersheds.Investigating the driving effects of vegetation restoration on changes in watershed runoff patterns is fundamental to understanding watershed hydrological processes and holds important theoretical and practical implications for the rational utilization of water resources and ecological environment development in watersheds.This study focused on two contrasting gullies within the Nanxiahegou Watershed,located in the loess hilly-gully region.It employed various methods such as field sampling,in-situ experiments,slope-scale simulated rainfall experiments,and analysis of long sequence measured data to investigate the impact of vegetation changes on soil physical properties,hydrological characteristics,and infiltration properties,as well as the transfer relationships among them.Using the"spatial substitute for temporal"approach,the study conducted a comparative analysis to explore the effects of vegetation changes on slope-scale water infiltration and runoff processes and their characteristics based on hydrological principles.It statistically analyzed the response of rainfall-runoff characteristics to changes in vegetation restoration during rainfall events,identified the reasons for variations in the characteristics of typical storm floods and runoff types,and revealed the mechanisms underlying the changes in watershed runoff patterns driven by the impact of vegetation restoration on soil hydrological,physical,and infiltration properties.The main results were as follows:（1）Vegetation restoration mainly affects the hydrological and physical properties of surface soil（0-40cm）,significantly reduces soil bulk density,increases porosity,significantly improves soil hydrological properties,and have no significant effect on soil texture.The change of soil hydrological properties is also closely related to physical properties.It is significantly positively correlates with total porosity and capillary porosity,and significantly negatively correlates with soil bulk density.Forested and grassland areas have shown an increase in saturated water content of the topsoil by 20.65%to 44.63%,an increase in field water-holding capacity by 19.76%to 44.20%,and an increase in saturated hydraulic conductivity by 92.01%to 191.12%compared to agricultural land.Soil saturates hydraulic conductivity is most affected by vegetation restoration,which is mainly affectes by soil bulk density and aggregate content.On the whole,the dominant factors affecting the change of soil hydrological properties are soil porosity and bulk density.Among them,soil aggregates,porosity,organic matter and sand content are closely related to bulk density.（2）Vegetation restoration significantly improves soil infiltration characteristics and infiltration capacity.The soil infiltration characteristic values of forest land and grassland after vegetation restoration are 2～3 times higher than those of farmland,and the infiltration capacity of the three is also significantly different,which is shown as artificial forest land>natural grassland>corn farmland.In the process of soil infiltration in woodland and grassland,the time when soil moisture reaches infiltration stability is later,and the stability rate is also significantly improves.The four infiltration models have good fitting effects on the infiltration process（R²>0.89,RMSE<4.30）,and the Kostiakov model is the best.Soil infiltration characteristics are closely relates to soil hydro-physical properties,positively correlates with saturated water content and field capacity,positively correlates with soil saturated hydraulic conductivity,porosity and organic matter content,negatively correlated with soil bulk density and soil water content,and complicates with particle composition.（3）By changing the soil hydro-physical properties,vegetation restoration indirectly changes the mechanism of slope runoff generation,from mostly hyperosmotic surface runoff generation to hyperosmotic surface runoff and interflow.In the slope infiltration runoff,the grassland increases the conversion of rainfall to soil storage,reduces the generation of surface runoff,and even the interflow of different layers.In addition,the water content of soil layers at different depths of grassland changes earlier and faster than that of bare land,and the initial time of runoff is also later.The runoff components are more abundant,the runoff yield is less,and the stable runoff rate is reduced by about 0.02 mm?min^-1.（4）The interannual variation of precipitation in the study area from 1954 to 2017 is not significant,with only an insignificant downward trend of 0.76 mm?a^-1.The slope of the rainfall-runoff double accumulation curve of the two vegetation restoration methods is 2～4times different.Under no significant change in rainfall,vegetation restoration resulted in less runoff.The restoration of artificial forest land is more effective than the natural restoration of grassland to reduce runoff.The distribution of rainfall duration intensity of 750 flood-producing rainfalls from 1954 to 2017 under the two vegetation restoration methods is basically the same,but the characteristics of floods are significantly different.The flood characteristics such as total flood volume,flood duration and runoff coefficient have changed significantly.Among them,the total flood volume in Dongzhuanggou（grassland）is mostly concentrated in the range of 50～1000 m³,and Yangjiagou（forest land）is concentrated in the range of 0～200 m³,with the most obvious difference.（5）Vegetation restoration will significantly reduce flood peak flow,increase flood peak lag,reduce runoff depth,surface runoff depth ratio and runoff coefficient.Under different rainfall types,the two vegetation restoration methods of natural grassland restoration and artificial planting forest land will greatly reduce the peak flow and change the shape of the flood.The peak flow before and after recovery can usually differ by dozens of times,or hundreds of times,or even more than 1000 times in the case of short duration and small intensity rainstorm.Vegetation restoration will also significantly affect the change of runoff components in the recession section of flood hydrograph.Before vegetation restoration,the runoff type is mainly’R_s type’（i.e.,overland flow）,In the early stage of artificial restoration of forest land and natural restoration of grassland,it gradually becomes’R_s+R_int type’（i.e.,overland flow and interflow）.In the case of long recovery period,forest land will continue to develop into’R_sat+R_int type’（i.e.,saturated overland flow and interflow）.This makes the runoff pattern in the study area change from’excess infiltration’to’Surface saturation excess runoff’.