Sediment concentrations in run-off varying with spatial scale in an agricultural subwatershed of the Chinese Loess Plateau

Information is scarce on the spatial-scale effect on sediment concentrations in run-off. This study addressed this issue within an agricultural subwatershed of the Chinese Loess Plateau, using data observed at a hilltop plot, three nested hillslope plots, two entire-slope plots (a combination of hillslope and valley side slope) and the subwatershed outlet. Dominated by the splash and sheet erosions, the hilltop plot has a minimum Cae (mean sediment concentration for all recorded events) of 45 kg m−3. Unexpectedly, the high sediment concentrations at the hilltop do not occur at high rainfall intensities or large run-off events because of the protection of surface soils by relatively thick sheet flows. Because of the emergence of rills, Cae is as high as 310 kg m−3 even on the most upper hillslope. Downslope, both Cae and ESC (extreme large values of recorded sediment concentrations) increase; such a slope length effect attenuates with increasing slope length and event magnitude as a result of insufficient sediment availability associated with rill development. Active mass wastings ensure sufficient sediment supply and thus a spatially invariant Cae (approximately 700 kg m−3) and ESC (approximately 1000 kg m−3) at the scale of the entire slope and subwatershed. Detailed examination shows that most small events experience a decrease in sediment concentrations when moving from the entire slope to the subwatershed, indicating that the spatially invariant sediment concentration is valid only for large run-off events. This study highlights the control of the spatial scale, which determines the dominant erosional process, on erosional regime. Copyright © 2015 John Wiley & Sons, Ltd.