Hydrologic controls on wash load sediment concentrations within a low-ordered, ephemeral watershed

This study explores the influence of rainfall and runoff production on wash load sediment transport in a low-order, ephemeral watershed in southeast Arizona. Wash load concentrations measured at the watershed outlet generally decreased throughout a runoff event and were better correlated with the rainfall intensity and the rainfall volume that fell within the 6 min (the watershed’s ‘time of concentration’) before the sediment concentration was measured than with the runoff discharge. The difference between the rainfall volume per unit time and the runoff discharge was primarily due to infiltration and transmission losses, processes that reduce runoff volume but do not appear to affect the wash load concentration within the runoff. Infiltration and transmission losses are significant in this watershed and vary considerably in space and time but are generally described by an inverse relationship between the runoff coefficient and drainage area. Runoff coefficients vary amongst runoff events and are correlated to the peak rainfall intensity during each event. Results indicate that wash load concentrations are set by the sediment entrainment processes occurring on the hillslopes (e.g., rainsplash, Hortonian overland flow) and do not greatly fluctuate as water is routed through the watershed. This suggests that wash load concentration may be better predicted by metrics of rainfall than flow discharge in certain environments. Using rainfall as a predictor of wash load could be useful for estimating sediment fluxes in semiarid regions where flow discharge is often difficult to monitor and sediment transport concentrations can be high (i.e., >10,000 mg L−1).