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Location: National Soil Erosion Research

Title: The role of subsurface hydrology in soil erosion and channel network development on a laboratory hillslope

item Nouwakpo, S
item Huang, Chi Hua

Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/20/2012
Publication Date: 7/1/2012
Citation: Nouwakpo, S., Huang, C. 2012. The role of subsurface hydrology in soil erosion and channel network development on a laboratory hillslope. Soil Science Society of America Journal. 76:1197-1211.

Interpretive Summary: Ephemeral gully erosion contributes to a significant of sediments on the landscape. Yet, current erosion assessment technologies are inadequate in predicting where the gully channels initiate and how fast these channels deepen and expand on the hillslope. We believe gully channel development is controlled by a combination of surface and subsurface hydrology. In order to test our hypothesis, we designed a 9.8 m x 3.7 m laboratory hillslope where we can control different amount of surface and subsurface water going through slope segment. We found that the if the soil had subsurface seepage, i.e., soil is over-saturated and water flowing out of the soil, erosion rate is 2.1 to 1.6 times as higher when compared to the soil that water is allowed to drain through the profile. From analysis of photographs taken during the experiment, we found erosion from channel incision is about 1.5 times higher for the seepage condition. The photo analysis that shows areas of erosion and sediment deposition on the hillslope supports a modeling concept that both processes occur at the same time. This study demonstrated that subsurface hydrology is a controlling factor in the development of ephemeral gullies.

Technical Abstract: Ephemeral gully erosion is currently considered one of the dominant sources of soil loss from the agricultural landscape. It is assumed to be the result of surface flow concentration with hydraulic properties exceeding a given threshold for channel initiation. In this paper, we devised a laboratory experiment to show how subsurface hydrology impacts channel network development and soil loss. A series of rainfall+run-on experiments were conducted on a 9.75 m x 3.66 m laboratory hillslope set under drainage or oversaturation (seepage) condition. Soil loss was monitored by collecting runoff samples and by digitizing the soil surface at regular time intervals using digital photogrammetry. We found that the seepage condition produced erosion rates 2.1 times as high as those measured under drainage condition for a high rainfall+run-on intensity (34 mm/h rainfall equivalent) and 1.6 times as high for a low intensity (17 mm/h rainfall equivalent). After 1.2 cubic meter of runoff the soil under seepage condition lost on average 1.9 times more soil than the one under drainage condition. Digital photogrammetry performed well at quantifying elevation changes due to channel network development and suggested that channel erosion rates were 1.5 times higher for the seepage condition. We also found an effect of rainfall+run-on intensity on interrill sediment load, supporting a previously proposed model for interrill erosion relating sediment load to rainfall intensity and to the square root of runoff rate. Finally, elevation change patterns observed during each rainfall+run-on event were found to be more consistent with the simultaneous erosion deposition theory as opposed to the sediment transport capacity concept as a sediment transport mechanism. This study demonstrated that subsurface hydrology might be a controlling factor in the rate of development of ephemeral gullies.