Hydraulics of interrill overland flow on a semi-arid hillslope, southern Arizona

Abstract

Simulated rainfall experiments were conducted on an 18-m-wide by 35-m-long runoff plot located in southern Arizona. By using a wide plot that extended from the divide to beyond the rill heads, it was possible to examine the full range of lateral variability and downslope changes in interrill overland flow on a shrub-covered, semi-arid piedmont hillslope. The mean depth d, mean velocity v and Reynolds number Re of overland flow were computed for two measured sections situated 12.5 and 21 m from the top of the plot. At-a-section d vs. Re and v vs. Re relations show that increases in Re are accomodated largely by increases in d. These hydraulic relations are principally the result of resistance to flow increasing with Re as overland flow spreads into new areas. Inasmuch as the maximum (equilibrium) discharge simulated in these experiments has a return period of about 7 yr, increasing resistance to flow as Re increases may be considered typical of most overland flow events on shrub-covered, piedmont hillslopes in southern Arizona. Downslope hydraulic relations differ strikingly from at-a-section relations. Under equilibrium runoff conditions, resistance to flow decreases rapidly as Re increases, permitting increases in Re to be accomodated entirely by increases in v. The decrease in resistance to flow is due to progressive downslope concentration of flow into fewer, deeper threads. Under non-equilibrium conditions, downslope hydraulic relations may be quite different from those obtaining at equilibrium, but resistance to flow always decreases downslope. This is the result of low flows following pathways formed by higher flows that concentrate downslope. Analyses of overland flow hydraulics at the scale of interrill areas produce results that are quite different from those obtained from small plots. A knowledge of the hydraulics at this scale is essential if models of interrill overland flow are to become more realistic and, hence, better predictors of runoff hydrographs and sediment yield.