Effects Of Vegetation Patches On Hydrological Connectivity And Soil Erosion On Dryland Hillslopes

Drylands cover more than 40%of the earth’s surface area.Vegetation patch patterns are widespread in these areas due to water shortages.Climatic extremes may increase water scarcity and moisture deficits for drylands in the context of global warming,resulting in a further increase in dryland areas.The vegetation coverage on the hillslopes is insufficient due to the severe water shortage in drylands,and the hillslopes usually exhibit complex patch patterns,which makes the soil prone to water erosion,leading to infertile and shallow soils and decreasing plant productivity.The lack of research on the contribution of vegetation patches to controlling erosion on dryland slopes and their impact on hydrological connectivity limits our understanding of the soil erosion processes of dryland.The dryland slopes with vegetation patches in the Loess Plateau region were selected as the research object of this study to address these issues.The soil hydrological characteristics and erosion resistance under different vegetation patches were analyzed to clarify the role of vegetation patches in the local soil-vegetation-hydrological process of the hillslope.Meanwhile,machine learning models were used to simulate sediment loss.The soil erosion process of drylands was explained from the perspective of hydrological connectivity,and the effect of vegetation patches on hydrological connectivity was evaluated.The main control factors of the soil erosion process and the critical role of vegetation in the patch scale on the soil erosion process were investigated.（1）The experiment on the soil detachment capacity of soil samples from different vegetation patches on the hill slope showed significant differences in the soil detachment capacity values of different vegetation patches.Notably,the maximum value of soil detachment capacity in the bare patch was higher than in other patches.In contrast,the grassland and shrub patches had lower average soil detachment capacity values.The vegetation patches reduced the average soil detachment capacity values by97.45 to 98.08%.Furthermore,the correlation analysis indicated that soil detachment capacity was closely related to clay%（6.67%）,sand%（13.36%）,saturated hydraulic conductivity（7.69%）,and total porosity of the soil.The effect of vegetation patches on soil detachment capacity and rill erodibility showed that vegetation patches increase soil resistance to erosion.The sediment and nutrients were more likely to depose in vegetation patches rather than detachment.（2）The analysis of the soil hydraulic parameters for all types of patches showed that significant differences in the saturated soil water content was found under different types of patches.For example,the maximum and mean values of saturated soil water content in the grassland and shrub patches were higher than those of the bare patch.The minimum and average saturated hydraulic conductivity values of the shrub patch and tree patch were higher than those of all samples.Conversely,the saturated hydraulic conductivity mean value of the bare patch was significantly lower than that of all samples.The mean values of shrub and grassland patches increased by 200%and152.4%,respectively,compared with that of the bare patch.The spatial analysis of saturated hydraulic conductivity showed that the value of C/（C0+C）is 41%,and the saturated hydraulic conductivity showed moderate spatial dependence.The structural equation model and principal component analysis indicated that the soil property parameters and soil hydraulic property parameters were key influential factors for saturated hydraulic conductivity.The total effect of the soil field capacity（-0.580）on Sqrt Ks was slightly greater than those of the bulk density（-0.355）,while the silt（0.369）showed strong total effects.The effect of spatial characteristics on saturated hydraulic conductivity is further amplified when the spatial distribution range of sampling points increases.The saturated hydraulic conductivity on a hillslope can be more accurately predicted by the Random Forest model compared to other machine learning models and the stepwise regression model.（3）The vegetation patches effectively reduce the structural connectivity,functional connectivity,and flow velocity of the hillslope and alter the water flow regime.The flow velocity of shrubs,trees,grassland shrubs,and shrubs-grasses patches increased by 23 to 40%,compared with that of the bare patch.The vegetation patches could significantly affect the total flow-path length,total flow-path width,and average width per flow path.Higher values of the total flow-path length were observed in the bare and shrub-covered plots.The total flow-path width was positively related to the infiltration rate,indicating that the distribution of surface runoff affects the infiltration rate.The infiltration rate increased with the increase of total flow-path width.The total flow-path length was positively related to the runoff and negatively related to the average width per flow path.However,this relationship gradually weakened due to changes in soil properties（such as soil water content,hydraulic head,and surface roughness）during the experiment.Principal component analysis and correlation analysis indicated that the flow-path characteristics were mainly affected by hydrodynamic factors and soil properties.Taylor diagrams showed that Support Vector Machines and Random Forest models had higher predictive accuracy for flow-path characteristics than the stepwise regression model.（4）The results of plot experiments showed that the vegetation significantly reduced sediment loss.Structural and functional connectivity are important factors influencing the erosion process of the hillslope.Compared with the bare plot,the sediment yields of vegetation patch-covered plots with shrub,shrub-grass,tree,and grass were reduced by 46.9 to 94.4%.Furthermore,it is observed that the mean value of D₅₀ for sediment in grass patch-covered was lower than that of other vegetation patch-covered plots,and the percentage content of silt was higher than that of other vegetation patch-covered plots.Correlation analyses and structural equation model showed that the sediment loss by runoff was mainly affected by soil properties,hydraulic characteristics,connectivity characteristics,biological factors,and hydrodynamic parameters.The average flow path length（-0.584）has the most significant total effect on sediment yield,followed by vegetation coverage（-0.568）and flow path length（0.569）.As an important biotic factor affecting soil erosion of hillslopes,vegetation coverage mainly indirectly influences hill slope erosion by affecting structural connectivity.Functional connectivity primarily affects sediment loss by directly impacting hill slope erosion via water flow path characteristics.Hydraulic parameter characteristics,represented by flow velocity,are mainly controlled by structural and functional connectivity to influence sediment loss.Soil characteristics parameters primarily influence the soil erosion of hillslopes by controlling hillslope runoff via infiltration rate.Compared with other models,the Random Forest model showed better results for predicting sediment loss.Although vegetation patches affected water flow patterns during the experiments,the proposed model shows high accuracy.In conclusion,the results of this study provide important information for developing a soil erosion model on dryland hillslopes and understanding the roles of various influencing factors in the soil erosion process of dryland hillslopes.To summarize,the effects of vegetation patches on hillslope hydrological connectivity and their significant role in soil erosion were investigated.Vegetation patches primarily affect flow paths and soil erosion of hillslopes by altering the structure and functional connectivity.Additionally,vegetation can reduce sediment loss and increase nutrient enrichment,resulting in a"fertility island"effect in vegetation patches and promoting positive feedback for vegetation growth.In the areas between vegetation（bare land）,low soil infiltration rate and insufficient water supply can inhibit plant growth,resulting in a negative feedback loop.The combined positive and negative feedbacks promote the formation of dryland patch patterns,which have long-term impacts on soil hydrological characteristics and erosion resistance and affect soil erosion on the hillslope.This study provided an important reference for explaining soil erosion and hydrological characteristics on the hillslopes of drylands.