Spatial Scale Dependency Of Erosive Runoff And Sediment Flow Behavior In The Loess Hilly-Gully Region

The spatial variation of erosive runoff energy in the slope-gully system of the Loess Plateau governs the spatial differentiation pattern of soil erosion.It is necessary to effectively identify the runoff and sediment dynamics in the erosive runoff events,thereby understanding the relationship between sediment transport and erosive energy on slopes.However,the current identification of erosive events is mostly based on the division of erosive rainfall,and there is a lack of research on identifying erosive events based on erosive runoff.Based on the energy perspective,this study used multidisciplinary theories of soil science,hydrology,and soil and water conservation to investigate the erosive energy and sediment transport characteristics of the slope-gully system in the Tuanshangou small watershed of the Chinese Loess Plateau.Key threshold parameters were then selected to determine thresholds for erosive runoff and analyze the variation in these thresholds.Subsequently,the dynamics of runoff and sediment transport driven by erosive runoff were explored.Furthermore,the relationship between energy and sediment in erosive and non-erosive events was then discussed at both inter-and intra-event timescales.Finally,the study quantified the potential sediment regulation efficiency that could result from regulating erosive energy.The main conclusions are as follows:（1）Based on the 369 observed runoff events in the slope-gully system,statistical analysis was used to analyze the erosive energy and sediment transport characteristics in the system and explore the correlation between runoff-and sediment-related parameters.Threshold parameters for delineating erosive runoff were ascertained,including runoff duration（T）,stream power（ω）,stream energy factor（SE）and area-specific sediment yield（SSY）.The statistical results showed that the coefficient of variation for runoff duration was 1.04.Larger coefficients of variation were observed for per unit runoff energy factor,stream power,and stream energy factor（2.70-4.24）.In contrast,the coefficient of variation for SSY,average sediment concentration（S_m）,and maximum sediment concentration(S_max)were smaller（0.74-1.88）.Furthermore,the strongest correlation was found between SSY and SE.The S_m and S_max was most closely related withω.（2）Based on threshold parameters for delineating erosive runoff,the frequency analysis was used to determine the thresholds for erosive runoff.The variability of thresholds was then explored across spatial scales.The range of variation in the threshold for runoff duration was 9 to 31 minutes,with little variation on the hillslope.The range of variation in the threshold for stream power was 10.5 to 26.8 W/m,and the threshold linearly increased with the increasing runoff plot area.The threshold for stream energy factor varied from 0.059 W to 1.01 W,exhibiting a logarithmic functional relationship with runoff plot area.The minimum value of the threshold for SSY occurred in runoff field No.12 at 0.54 kg/m²,while the maximum value occurred in runoff field No.7 at 1.23 kg/m².（3）Erosive events and non-erosive events were distinguished based on the thresholds for erosive runoff.Energy-sediment relationships were then established at the inter-and intra-event time scale.The sediment regulation efficiency from regulating erosive energy was quantified from these thresholds and energy-sediment relationships at the inter-event scale.A power function was primarily exhibited in the relationship between area-specific sediment yield and per unit runoff energy factor.Similarly,a power function predominantly governed the relationship between area-specific sediment yield and stream energy factor.There was a linear function relationship was found between area-specific sediment yield and steam power.R² of regression equations for erosive events were greater than those for non-erosive events.The relationship between average sediment concentration and stream power followed a logarithmic function.A logarithmic function relationship was also observed between maximum sediment concentration and stream power.R² of the regression equations for erosive events were smaller than those for non-erosive events.The sediment regulation efficiency from regulating the unit stream power（10 W/m）ranged from 0.5%to 37.3%.Variation in efficiency showed an inverted‘N’-shaped distribution on the hillslope,with little variation across the entire slope.The sediment regulation effect from regulating the unit stream energy factor（0.1 W）varies from 7.7%to 65.2%.The variation in the efficiency showed a downward trend from hillslope to entire slope.On average,regulating unit stream energy factor yielded 12.6%higher sediment regulation efficiency compared to regulating unit stream power.（4）Regression analysis was used to establish energy-sediment relationships at intra-event timescales for both erosive and non-erosive events.Sediment transport capacity and sediment increment capacity were derived from these fitted relationships for runoff events of different erosivity.There was a logarithmic function relationship between instantaneous sediment concentration and instantaneous stream power.A proportional function relationship existed between instantaneous sediment delivery rate and instantaneous stream power.There was also a proportional function relationship between increments in area-specific sediment yield and increments in stream energy factor.The sediment delivery capacity of erosive runoff ranged from 0.068 to 0.115 kg·m/J.The sediment delivery capacity of non-erosive runoff ranged from 0.053 to 0.088 kg·m/J.The difference between them decreased as scale increased.The sediment increment capacity of erosive runoff varied from 0.11 to 4.47kg/（m²·W）.The sediment increment capacity of non-erosive runoff varied from 1.13 to 17.62kg/（m²·W）.The difference between them increased as scale increased.The results of this study will provide a basis for distinguishing erosive runoff events in the Loess Plateau slope-gully system.These findings will facilitate insights into the effects of erosive runoff energy on sediment yield.They will also promote precise control of runoff energy and optimal implementation for soil and water conservation.