Spatial scale effects on sediment delivery within runoff erosion chains in basin system

Scale issues are of great significance in studies concerning geomorphologic processes and hydrologic modeling, which have been extensively studied in the domain of soil erosion. Nevertheless, studies are relatively rare on spatial scale effect on sediment delivery within event-based runoff erosion chains. Based on recorded sediment-runoff yield data from 3 gauging stations including Tuanshangou, Shejiagou, and Caoping station at varying spatial scales in Chabagou river basin, 44 runoff erosion chains across scale sequences of sublateral, branch, and trunk were extracted. Spatial scale effect on event-based sediment delivery was investigated within runoff erosion chains in a typical agricultural basin system of hilly loess region on the Chinese Loess Plateau. The results showed that: 1) event-based area-specific sediment yield, sediment concentration, as well as intra and inter scale flow-sediment relationship remained spatially constant within runoff erosion chains, and thus almost all the sediment-laden flows could reach erosion thresholds across scales, especially for hyperconcentrated flows. Average area-specific sediment yield, average sediment concentration, and maximum sediment concentration at Tuanshangou, Shejiagou, and Caoping station at different spatial scales were 3912, 3285, 3522 t/km2; 497, 524, 679 kg/m³; and 639, 634, 800 kg/m³; respectively; 2) Compared with single indicators including peak discharge and runoff depth as well as hydrodynamic indicator such as flow shear stress, composite energy parameters such as stream power, unit runoff energy, and stream energy, all of which incorporated the flood peak discharge term, could better describe intra- and inter-scale based runoff-sediment relationships within runoff erosion chains; 3) Sediment production was mainly dependent on total runoff volume, however, flood peak discharge, rather than total discharge could better interpret the difference in sediment yield across scales within runoff erosion chains. Therefore, the reliability of sediment estimation would be improved for small and medium sediment-producing events if such variables expressing runoff variability were introduced into indicators for soil erosion and sediment yield prediction. On the average, the increment in sediment yield caused by per 1 m³/s increase in flood peak discharge was at least 875 times that caused by per 1 m³ increase in total discharge in the downstream direction. Statistically, to eliminate inter scale-based scale effect on sediment discharge, the inter-scale flood peak discharge ratio must decrease to be lower than 5‰ or the inter-scale runoff energy ratio must reduce to be lower than 600; 4) The influence of upstream sediment-laden flow on downstream sediment output was limited and spatial scale effect on sediment-laden flow decreased with the increasing drainage area, especially for major sediment-producing events with area-specific sediment yield larger than 300 t/km2. The study highlighted the event-based process-specific characteristics of soil erosion by water flows in basin system, revealing great potentials for sediment reduction caused by flood regulation and control. Thereby, event-based sediment control should be process-oriented and benefit evaluation and efficiency assessment of sediment control measures should also be process-based especially for hyperconcentrated flows. The results may be helpful to fully reveal the significance of runoff regulation systems in soil and water conservation, and provide supports for evaluation of erosion control strategies within runoff erosion chains across different spatial scales.