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ARS Home » Pacific West Area » Tucson, Arizona » SWRC » Research » Publications at this Location » Publication #351977
Research Project: Understanding Water-Driven Ecohydrologic and Erosion Processes in the Semiarid Southwest to Improve Watershed Management

Location: Southwest Watershed Research Center

Title: Overland flow velocities on semiarid hillslopes 2471

Author
item Polyakov, Viktor
item Nearing, Mark
item STONE, J.J. - Retired ARS Employee

Submitted to: Earth Surface Processes and Landforms
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/30/2018
Publication Date: N/A
Citation: N/A

Interpretive Summary: The results of this study show that velocity of overland flow on rangelands is related primarily to discharge and is independent of slope gradient. This supports the slope-velocity equilibrium hypothesis, which suggests the existence of negative feedback between flow rate and surface morphology. Namely, steeper slopes with initially high flow velocities tend to develop a rougher surface, which acts to decrease the velocity. Previous studies applied this hypothesis to rill flow, but according to our data it also holds true for a wide range of inter-rill flow conditions on rangelands. The results also highlight the importance of biological factors in the slope surface morphology. Vegetation and organic litter significantly affect flow hydraulics, but unlike soil do not directly respond to erosive forces. Hence, these factors have to be accounted for in the discharge-velocity relationship. Our results also suggest that hydraulic friction is a dynamic, discharge dependent property. This raises the question whether the use of Manning or Chezy equations in their present form is appropriate for overland flow modeling.

Technical Abstract: A series of 188 rainfall plot simulations was conducted on grass, shrub, oak savanna, and juniper sites in Arizona and Nevada. A total of 897 flow velocity measurements was obtained on 3.6% to 39.6% slopes with values ranging from 0.007 m s-1 to 0.115 m s-1. The experimental data showed that flow velocity on inter-rill areas of rangelands was related to discharge and ground litter cover and was independent of slope gradient or soil characteristics. A power model was proposed to express this relationship. These findings support the slope-velocity equilibrium hypothesis. Namely, eroding soil surfaces evolve such that steeper areas develop greater hydraulic roughness. As a result overland flow velocity becomes independent of the slope gradient over time. Our findings have implications for soil erosion modeling suggesting that hydraulic friction is a dynamic, slope and discharge dependent property.