Effects of initial soil moisture on rill erodibility and critical shear stress factors in the WEPP model across diverse soil types

Authors: ARI, FIKRIT - Ankara University; SAYGIN, SELEN - Ankara University; TEMIZ, CAGLA - Ankara University; ARSLAN, SEFIKA - Ankara University; UNAL, MEHMET - Ankara University; ERPUL, GUNAY - Ankara University; Flanagan, Dennis

Submitted to: International Soil and Water Conservation Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/3/2024
Publication Date: 10/6/2024
Citation: Ari, F., Saygin, S.D., Temiz, C., Arslan, S., Unal, M.A., Erpul, G., Flanagan, D.C. 2024. Effects of initial soil moisture on rill erodibility and critical shear stress factors in the WEPP model across diverse soil types. International Soil and Water Conservation Research. https://doi.org/10.1016/j.iswcr.2024.10.001.
DOI: https://doi.org/10.1016/j.iswcr.2024.10.001

Interpretive Summary: Soil erosion by water is a serious problem throughout the world, as the loss of fertile topsoil can affect agricultural production levels and sediment that moves offsite can cause multiple problems, including clogging of waterways, loss of reservoir capacity, and pollution of lakes and other water bodies. Soil can be eroded both by falling raindrops and flowing channelized water. In this study, we examined the effect of initial soil subsurface hydrologic conditions on the susceptibility of different soils to detachment by flowing water in small rill channels, and how these effects can be mathematically gauged using equation parameters for a soil erosion computer model. Three different initial soil moisture conditions were tested: initially dry soil, saturated soil, and soil that had been saturated then allowed to drain for 24 hours. All soils were initially dry and placed in small mini-flumes, and water at different inflow levels was added, and water and sediment were collected from the end of the flumes to measure soil loss rates. We tested 20 different soils with widely different textures from Turkey, and found that for all soils their erosion rates were least when they were initially saturated then allowed to drain for 24 hours. The drainage and drying caused cohesive bonds that strengthened the soil. Soils that had higher clay contents exhibited greater erosion when the soil was saturated, and soils having greater silt contents had greater soil loss under initially dry conditions. We also found that a measurement of soil cohesion level may also help to predict a soil’s susceptibility to detachment by water flow in rills. These results will improve soil erosion prediction models, and impact scientists and users of these technologies.

Technical Abstract: Rill erosion, a significant issue in agricultural regions, is intricately linked to initial soil moisture conditions, affecting the development of concentrated flow erosion processes. However, understanding its dynamics amidst varying subsoil hydrological conditions remains challenging. This study aimed to assess the impact of different soil moisture levels on rill erodibility parameters in the Water Erosion Prediction Project (WEPP) model and to evaluate soil cohesion across a spectrum of soils. Through laboratory experiments employing a small V-shaped rill channel, we investigated rill erodibility (Kr) and critical hydraulic shear stress (tcr), under three subsurface hydrologic scenarios: dry, saturated, and drainage, with incremental surface inflow rates. Additionally, we examined the efficiency of a soil cohesion factor obtained from an Automated Soil Cohesion Measurement Apparatus in predicting Kr and tcr across various soil textures. Our analysis encompassed twenty soils representing nine texture classes, revealing significant correlations between basic soil properties, cohesion parameters, and WEPP model rill erodibility. Notably, initial soil moisture conditions exerted substantial influence on erodibility potentials. We observed the lowest Kr values in initially drained soils, while soils with higher clay content exhibited the highest values under saturated conditions. Conversely, soils with higher silt content showed elevated Kr values under initially dry conditions. The inverse relationship between Kr and tcr was pronounced, with the highest tcr values recorded under drainage conditions. Furthermore, strong correlations between rill erodibility, silt contents, and soil cohesion values were evident. Soils with higher silt contents demonstrated better fits in terms of Nash-Sutcliffe model efficiency, particularly under dry and saturated conditions. However, predictions for initially drained soils yielded poor fits, emphasizing the intricate interplay between soil properties and hydrological conditions. In conclusion, our findings emphasize the critical role of subsoil hydrology in rill erodibility. We propose that mechanical soil cohesion serves as a valuable predictor, complementing friction forces within the soil and enhancing simulations of rill erodibility under shallow flow conditions in channels, particularly in next-generation process-based modeling approaches.