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Research Project: Understanding Water-Driven Ecohydrologic and Erosion Processes in the Semiarid Southwest to Improve Watershed Management

Location: Southwest Watershed Research Center

Title: Soil loss from small rangeland plots under simulated rainfall and run-on conditions

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

Submitted to: Geoderma
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/4/2019
Publication Date: 12/1/2019
Citation: Polyakov, V.O., Nearing, M.A., Stone, J. 2019. Soil loss from small rangeland plots under simulated rainfall and run-on conditions. Geoderma. 361.

Interpretive Summary: Soil erosion on rangeland hillslopes was studied using a rainfall simulator. Erosion was best predicted by surface water runoff rate for both rainfall and surface water only flow conditions (R2 = 18% to 75%). There was a statistically significant difference between two input treatments. Rainfall generated 2 to 44 times more sediment than surface water run-on at the same discharge rate. Differences in soil loss under rainfall and runoff were attributed to raindrop impact and flow concentration patters. Overland flow on rangeland slopes did not form rills with dendritic topology. Instead, it follows micro depressions between vegetated micro-topographic highs, and is often divergent. While an increase in surface cover diminishes splash it may increase channel scour. If surface organic litter is abundant it could be mobilized by overland flow to create debris dams, pond water and thus further diminish detachment. Other factors, such as canopy, rock, bare soil cover, or canopy gap, significantly affected the erosion. However, there was no single secondary predictor common across all ecological sites in the study. This was likely due to the heterogeneity of the data set, and low magnitude and large variability of sediment yields. There was no evidence of rain drop impact affecting flow velocity on rangeland slopes.

Technical Abstract: In order to predict and mitigate soil erosion it is necessary to understand the mechanism of detachment by raindrops and by shallow flow. The relative contribution of these drivers to the overall erosion process is not well understood and field experimental data is limited. The experiment was conducted using simulated rainfall on 56 small natural plots located on 7 arid rangeland sites in Arizona, USA. The goals of the study were (i) to compare raindrop driven and shallow flow driven erosion rates on arid rangeland, and (ii) assess the role of flow hydraulics, vegetation, and cover on attenuation of erosive impact of raindrops. A total of 520 measurements of steady state flow under two treatments (rainfall and run-on) were obtained. Flow discharge on the plots varied between 3 and 355 mm h-1 and unit sediment yield varied between 0.02 and 30 g m-2 min-1. Sediment yield was best predicted by flow discharge for both rainfall and run-on treatments on all sites explaining 18% to 75% of its variability. There was statistically significant difference between two treatments. Rainfall treatment generated 2 to 44 times more sediment than run-on at the same discharge rate. We found no strong evidence of raindrop impact affecting overland flow velocity. Among 19 variables related to surface conditions a weak correlation was found between sediment yield and plant foliar cover, structure, and surface litter. However, there was no single best cover predictor common for all ecological sites tested.