Study On The Slope And Scale Effects During Rainfall-runoff And Soil Erosion Processes

Soil erosion is a common phenomenon in nature,which has become a critical environmental challenge worldwide.As one of the most important elemental and fundamental process in soil erosion,the slope rainfall-runoff and soil erosion processes attract global attention.However,there are still some shortcomings.For example,the dynamic processes of slope rainfall-runoff and soil erosion have not been fully summarized in the previous studies.The slope and scale effects on flow erosivity and soil erosion still remain controversial issues.Moreover,there is no numerical model can simulate the slope rainfall-runoff and soil erosion processes on complex hillslope,which is constituted with rill and interrill area.To overcome these shortcomings,the dynamic processes,the slope and scale effects,and the hydrodynamic characteristics during rainfall-runoff and soil erosion processes have been revealed by the theoretical and numerical studies in the present study.The main contents of this dissertation are as following:From the perspectives of hydraulics and sediment dynamics,the fundamental dynamics of slope rainfall-runoff and soil erosion processes are firstly discussed in detail.Then,the effects of rainfall,soil characteristics,slope gradient,and slope length on the raindrop splash erosion and runoff erosion have been analyzed qualitatively.The theoretical framework explained and quantified the direct slope effect by coupling the modified Green-Ampt equation accounting for slope effect on infiltration,1D kinematic wave overland flow routing model,and WEPP soil erosion model.The flow velocity,runoff rate,shear stress,interrill,and rill erosion were calculated on 0°-60° isotropic slopes with equal horizontal projective length.The results show that,for short duration rainfall events,the flow erosivity and erosion amounts exhibit a bell-shaped trend which first increase with slope gradient,and then decrease after a critical slope angle.The critical slope angles increase significantly or even vanish with increasing rainfall duration but are nearly independent of the slope projective length.The soil critical shear stress,rainfall intensity and temporal patterns have great influences on the slope effect trend,while the other soil erosion parameters,soil type,hydraulic conductivity,and antecedent soil moisture have minor impacts.The relative slope effect on soil erosion in physically-based model WEPP was compared to those in the empirical models USLE and RUSLE.The trends of relative slope effect were found quite different,but the difference may diminish with increasing rainfall duration.The theoretical framework explained and quantified the direct scale effect on slope runoff by coupling the two-layer conceptual infiltration model and 2D diffusion wave overland flow routing model.The results show that,for steady rainfall,an approximately power-law decrease between the runoff depth and slope length can be obtained for short duration rainfall events.With an increasing of rainfall duration,the power-law trend of runoff depth versus slope length will shift to linear relationship eventually.For natural rainfall,the runoff may generate for several times and both power-law relationship and linear relationship can be obtained during the intermittent rainfall process.Moreover,the rainfall intensity and temporal patterns,soil infiltration parameters and soil crust have great impacts on the scale effect trends.On the basis of 2D slope rainfall-infiltration-runoff model,a 2D hillslope scale soil erosion model is developed to reveal the scale effect of hillslope soil erosion and the hydrodynamic parameters of overland flow.This numerical model is able to simulate the complex hillslope that is constituted with rill and interrill area.The results show that an approximately power-law increase between the erosion amount and slope length can be obtained for different rainfall intensity,rainfall patterm,slope gradient,soil anti-erosion capability and soil infiltration parameters.The relative scale effect on soil erosion in physically-based model WEPP was compared to those in the empirical models USLE and RUSLE.At event time scales,WEPP and RUSLE may describe the scale effect on soil erosion more accurately,while all these models reflect similar trends of scale effects for long term cases.Moreover,with an increasing of rainfall intensity,slope gradient and slope length or a decreasing of soil infiltration capacity,the turblence and flow erosivity of overland flow will increase while the flow resistence will decrease to a certain extent.