Size characteristics of sediment in interrill overland flow on a semiarid hillslope, Southern Arizona

Abstract

This study examines the size characteristics of sediment removed from a semiarid hillslope by interrill overland flow. Rainfall simulation experiments were conducted on a runoff plot 18 m wide and 35 m long established on a piedmont hillslope in southern Arizona. The top of the plot coincided with the hillslope divide, and its outlet was located within a shallow rill. Samples of runoff were obtained from two cross-sections located in the interrill portion of the plot upslope of the rill and from a calibrated flume through which was directed interrill overland flow reaching the bottom of the plot. Analyses of sediment contained in these samples showed that sediment in interrill flow is finer than the matrix soil. The fineness of the interrill sediment compared to the matrix soil appears to be due to the inability of interrill overland flow to transport the coarser fraction of the sediment supplied to it by raindrop detachment. This finding implies that the rate of soil erosion in interrill areas is not, as is commonly supposed, limited by the rate at which raindrops can detach sediment but by the rate at which they detach sediment of a size that the overland flow is competent to transport. The relative fineness of sediment eroded from this hillslope is consistent with other evidence for the recent evolution of shrub-covered hillslopes in southern Arizona.

Sediment Morphology, Distribution, and Recent Transport Dynamics in a Reforested Fragment

“…This micromorphological evidence reinforces the results obtained from the morphological descriptions in the field concerning formation of the A horizon on the surface of the deposited sediments after implementation of the reforestation project. The presence of moderately-to well-sorted coarse material in the sediments indicates that runoff was not high in energy since only selective, non-turbulent flows would be able to transport and deposit particles of similar sizes (Parsons et al, 1991;Shi et al, 2012). These results corroborate the statistical results of the sand fraction, which also indicated uniformity in the size of the sand grains.…”

Particle size characteristics of suspended sediment in hillslope runoff and stream flow

“…There are virtually no data on the characteristics of sediment eroded under natural conditions and there has been no examination of these characteristics over entire hillslopes. A notable exception is the work of Parsons et al (1991) in southern Arizona, although in that study, sediment was still dispersed in the laboratory because the sandy catchment soils were not well aggregated and tended to erode mostly as primary particles. There have also been few attempts to assess the grain size composition of the sediment moving through the delivery system of entire drainage basins, despite recognition that such data may provide the information needed to understand the transfer processes better (Walling, 1990).…”

Size fractions resulting from an aggregate stability test, interrill detachment and transport

“…These erosion processes depend on size distribution of available soil fragments provided by soil breakdown. However, size selectivity of interrill erosion detachment and transport had been studied in comparison with size distribution of the total soil (Gabriels and Moldenhauer, 1978;Poesen, 1981;Miller and Baharuddin, 1987;Parsons et al, 1991;Sutherland et al, 1996;Wan and El-Swaify, 1998). The previous effect of aggregate breakdown mechanisms had not been taken into account.…”

“…From the comparison with size distribution of the total soil, the size selectivity of deposition and transport has been well demonstrated (e.g. Poesen and Savat, 1981;Parsons et al, 1991). There are conflicting results concerning the effects of size selectivity on detachment.…”