Spatial scale dependence of sediment dynamics in a semi-arid badland drainage basin

Abstract

Problems of spatial scale transference are of key interest in drainage basin research. The objective of this study was to investigate the spatial scale dependence of sediment dynamics in semi-arid badland drainage basins. Simulated rainfall experiments indicated that on microscale plots runoff generation and sediment entrainment occurred when total rainfall exceeded 0.5 to 6 mm on pediment surfaces, 1.5 to 4 mm on sandstone surfaces, and 8 to 25 mm on shale surfaces. The sediment concentration/discharge relationship for two mesoscale basins displayed a change from clockwise to counterclockwise hysteresis when total rainfall exceeded the threshold rainfall for the initiation of flow in deep tunnel systems. The initiation of tunnel flow causes counterclockwise hysteresis due to a delayed increase in sediment concentrations during a runoff event. This contrasts with the clockwise hysteresis caused by flushing in the early stages of runoff for smaller rainstorms. Comparison of micro- and mesoscale sediment dynamics indicates that microscale thresholds are not by necessity in evidence at the mesoscale. In addition, at the mesoscale elements exist which are non-existent at the microscale, e.g. a deep tunnel system. The behaviour of the mesoscale basin is dominated by these elements rather than by the microscale components it contains.