Journal cover Journal topic
Hydrology and Earth System Sciences An interactive open-access journal of the European Geosciences Union
doi:10.5194/hess-2017-83
© Author(s) 2017. This work is distributed
under the Creative Commons Attribution 3.0 License.
Research article
24 Feb 2017
Review status
A revision of this discussion paper was accepted for the journal Hydrology and Earth System Sciences (HESS) and is expected to appear here in due course.
Slope–Velocity–Equilibrium and evolution of surface roughness on a stony hillslope
Mark A. Nearing1, Viktor O. Polyakov1, Mary H. Nichols1, Mariano Hernandez1, Li Li2, Ying Zhao2, and Gerardo Armendariz1 1USDA – Agricultural Research Service, Southwest Watershed Research Center, Tucson, AZ, 85719, USA
2School of Natural Resources and the Environment, University of Arizona, Tucson, AZ, 85705, USA
Abstract. Slope–velocity–equilibrium is hypothesized as a state that evolves naturally over time due to the interaction between overland flow and surface morphology, wherein steeper areas develop a relative increase in physical and hydraulic roughness such that flow velocity is a unique function of overland flow rate independent of slope gradient. This study tests this hypothesis under controlled conditions. Artificial rainfall was applied to 2 m by 6 m plots at 5 %, 12 %, and 20 % slope gradients. A series of simulations were made for each treatment with measurements of runoff rate, velocity, rock cover, and surface roughness. Velocities measured at the end of each experiment were a unique function of discharge rates, independent of slope gradient or rainfall intensity. Physical surface roughness was greater at steeper slopes. The data clearly showed that there was not a unique hydraulic coefficient for a given slope, surface condition, or rainfall rate, with hydraulic roughness greater at steeper slopes and lower intensities. This study supports the hypothesis of slope–velocity–equilibrium, implying that use of hydraulic equations, such as Chezy and Manning, in hillslope scale runoff models is problematic because the coefficients vary with both slope and rainfall intensity.

Citation: Nearing, M. A., Polyakov, V. O., Nichols, M. H., Hernandez, M., Li, L., Zhao, Y., and Armendariz, G.: Slope–Velocity–Equilibrium and evolution of surface roughness on a stony hillslope, Hydrol. Earth Syst. Sci. Discuss., doi:10.5194/hess-2017-83, in review, 2017.
Mark A. Nearing et al.
Mark A. Nearing et al.

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Short summary
This study presents novel scientific understanding about the way that hillslope surfaces forms when exposed to rainfall erosion, and the way those surface interact with and influence runoff velocities during rain events. The data show that hillslope surfaces form such that flow velocities are independent of slope gradient, and only dependent on flow rates alone. This result represents a shift in thinking about surface water runoff.
This study presents novel scientific understanding about the way that hillslope surfaces forms...
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