| Scale issues, which have been extensively studied in the domain of soil erosion, are considerably significant in geomorphologic processes and hydrologic modeling.Nevertheless, relatively scarce efforts have been made to quantify the spatial scale effect on event-based water flow driven erosion and sediment delivery, and thus inter-scale runoff-sediment relationships and the effects of sediment-laden flow on soil erosion associated with sediment delivery and deposition have not been thoroughly revealed within slope/basin system. Therefore, profoundly addressing this issue will benefit deep perception of soil erosion mechanisms, further consolidation of conceptual basis for sediment yield prediction, promoted accuracy of extrapolation in sediment yield estimation across scales, improved hazard assessment on soil erosion, and proper design of anti-erosion strategies against different erosion patterns. The loess hilly gully region with a unique erosion environment produces pluvial erosion events that are driven by rainstorm floods on the Loess Plateau. Given the specificity of soil erosion in this region, the present study highlights the tempo-spatial scale effects on sediment behavior by regulating intra-event-based flood hydrographs. Analyzing these runoff–sediment events associated with their erosive energy characteristics under different hydrologic regimes will help further elucidate the mechanism of runoff regulation and control in soil and water conservation. Therefore, profoundly addressing this issue will benefit deep perception of soil erosion mechanisms, further consolidation of conceptual basis for sediment yield prediction, improved reliability of soil erosion prediction, promoted accuracy of extrapolation in sediment yield estimation across scales, improved hazard assessment on soil erosion, and proper design of anti-erosion strategies against different erosion patterns.Based on this, the thesis takes a typical small watershed-Chabagou watershed in theloess hilly gully region- as an example to study the issues on spatial scale effects on sediment-laden flows. Observed data on hydrologic observations and sediment measurements has been collected from No.2, No.3, No.4, and No.7 runoff plots in slope system within Tuanshangou sublateral and gauging stations including Tuanshangou,Shejiagou, and Caoping within basin system during 1959-1969. Therefore, spatial scale effects on water flow driven erosion and sediment delivery, as well as sediment flow behavior and energy flow driven by flood runoff of different patterns have been systematically analyzed in slope system and basin system under a near-natural condition without management. On the whole, based on the theory of runoff erosive energy underlying water erosion, combined methods of hydrological analysis and statistical analysis were used throughout the study. Major conclusions obtained are as follows:(1) Classifications of flood runoff regime and the regime-based erosion and sediment yield effectsOn the basis of flood hydrographs, combined methods of cluster analysis,discriminant analysis, and variance analysis have been used to divide flood events into different regimes at different spatial scales within slope/basin system. The complexity of flood runoff regimes diminish with the enlargement in spatial scale from slope system to basin system, and runoff-sediment relationships tend to be stable, therefore, greater and greater influence is exerted by flood runoff amount on sediment discharge. The variability of main sediment-related variables(area-specific sediment yield and sediment concentration) diminishes and the spatial scale effect on sediment-laden flow attenuates with the increase in drainage area, as a result, limited influence is exerted on downstream water flow driven sediment transport. The maximum sediment increment rate induced by the alteration of runoff-sediment relationships tends to decrease with the increase in spatial scale and total runoff amount: entire slope(790%), Tuanshangou sublateral(78%),Shejiagou branch(22%), Chabagou trunk(64%). Main sediment-related variables show an increasing tendency as the spatial scale enlarges in slope system, and the nonlinearity of water flow driven erosion and sediment yield becomes evident at the entire slope scale.Entering basin system, main controlling factor affecting sediment flow behavior tends to be single. Event-based area-specific sediment yield, suspended sediment concentration, as well as intra and inter scale flow-sediment relationship remains spatially constant withinrunoff erosion chains, especially for hyperconcentrated flows.(2) Construction of flow-sediment relationships and the effectiveness of dynamic parameters in describing water flow driven erosion and sediment deliveryThe effectiveness of runoff-related variables(total runoff amount, flood peak discharge, runoff depth, flow shear stress, stream power, runoff-energy factor, unit runoff-energy factor, stream energy) in intra and inter scale flow-sediment relationships is dependent on sediment-related variables(total sediment discharge, area-specific sediment yield, sediment concentration). Total sediment discharge can be described by total runoff amount, and variations in area-specific sediment yield can be interpreted by flow shear stress, while variations in event-based average and maximum sediment concentration can be explained by stream power. The equations for flow-sediment relationships mainly obey formulations of linear, power, and logarithmic functions. In runoff erosion chains within basin system, intra- and inter-scale based sediment concentration difference can be described with logarithmic function equations of main runoff-related variables, and total sediment discharge and total sediment discharge difference can be simulated with linear and power function equations of main runoff-related variable, respectively. The applicability of flow shear stress based on event runoff depth minimizes for depicting the process of water flow driven sediment delivery in basin system, and energy-related parameters show some advantages, among which stream energy factor prevails. To incarnate the process-specific conception of soil erosion and sediment delivery,process-based variables indicating runoff amount(depth) and flow strength should be incorporated the into the predictors for sediment yield estimation especially from small and medium sediment-producing events.(3) Sediment increment induced by flood flows and its regulative effectsIn slope system, differences in total sediment discharge across scales(downslope) are mainly dependent on variations in total runoff amount. On the average, the increment in sediment yield caused by per 1 m3 increase in runoff amount is 0.77 t, and the increment in sediment yield caused by per 1 m3/s increase in maximum water discharge is 402 t(theoretically) in the downslope direction. To eliminate the inter-scale based scale effect on sediment transport in slope system, the inter-scale based unit runoff-energy ratio should be decreased to lower than 1.8, or else, the inter-scale based runoff-energy difference shouldbe decreased to lower than 56.5 m6/s. In runoff erosion chains within basin system, flood peak discharge reflects the flow variability in the chains and is the major controlling factor causing significant difference in sediment export across different spatial scales. On the average, the increment in sediment yield caused by per 1 m3/s increase in flood peak discharge is at least 875 times of that caused by per 1 m3 increase in runoff amount in the downstream direction. To eliminate the inter-scale based scale effect on sediment transport within the erosion chain, the inter-scale based flood peak discharge ratio should be decreased to lower than 5‰, or else, the inter-scale based stream energy ratio should be decreased to lower than 600.(4) Flood runoff regime dependent characteristics of runoff erosive energyThe construction of energy-related parameters for flood runoff events of different regimes is mainly based on runoff-related variables including runoff depth, instantaneous discharge, flood peak discharge(maximum discharge), average discharge, and flow variability, etc. The complexity of energy-related parameters diminishes with the enlargement of spatial scale from system to basin system. Sediment concentration shows power and logarithmic function relationships with main energy-related parameters constructed. The erosive power index(effective runoff erosive power, average runoff erosive power, and runoff erosive power) based on water discharge(instantaneous discharge, mean discharge, maximum discharge, and flow variability) and runoff depth indicates the cumulative effect of runoff amount(depth) and the instantaneous effect of runoff intensity on sediment detachment and delivery as well as the unity in discontinuity and continuity of runoff erosivity effect on the flood-flow-driven erosion process. The erosive power index shows some applicability in constructing runoff-sediment transference relationships at the entire slope and sublateral scale to a certain extent.(5) Cutting down runoff erosive energy and cutting off the connectivity of energy flow are fundamentals for water and sediment regulationThe runoff erosion precaution in slope system should be aimed at the control of erosion pattern evolvement and the preclusion of hyperconcentrated flows to diminish the sediment increment effects induced by self-regulation of flood runoff hydrographs of different regimes. Therefore, steady flow-sediment relationships and low equilibrium of sediment delivery can be achieved. The runoff and sediment control in basin system shouldbe process-based. Flood runoff hydrograph should be regulated to cut off the connectivity of hyperconcentrated flows, reduce runoff erosivity, destroy the stability of flow-sediment relationships, and eliminate the constant conditions for sediment export. As a result, new equilibrium can be established. Comprehensive evaluation on eco-hydrology benefits brought by soil erosion alleviating systems should also be process-based. Equal importance should be attached to the direct and indirect effects induced by flood runoff regulation. In other words, direct sediment reduction(explicit benefit) caused by the decrease in runoff amount, and indirect sediment reduction(implicit benefit) induced by altered flow-sediment relationships driven by flood runoff regulation, should be quantitatively analyzed, respectively. |
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