Author
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NOUWAKPO, SAYJRO - Purdue University |
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Huang, Chi Hua |
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Submitted to: Proceedings of the American Society of Agricultural and Biological Engineers International (ASABE)
Publication Type: Proceedings Publication Acceptance Date: 1/31/2011 Publication Date: 9/18/2011 Citation: Nouwakpo, S.K., Huang, C. 2011. Quantifying intrinsic and extrinsic factors affecting soil erodibility. Proceedings of the American Society of Agricultural and Biological Engineers International (ASABE), September 18-21, 2011, Anchorage, Alasks, 2011 CDROM. Interpretive Summary: Technical Abstract: Soil erodibility has traditionally been conceived as a soil dependent parameter that can be quantified from intrinsic soil properties that usually stay constant. Development of erosion prediction equations, from the empirical-based Universal Soil Loss Equation (USLE) to a more processed-based Water Erosion Prediction Project (WEPP), all used erodibility terms based on the same perception that the erodibility is a soil dependent property that remains constant. Both USLE and WEPP erodibilities are derived either experimentally or through statistical regression from field plots under the most erodible condition which is defined as leaving the plot bare fallow under natural rainfall or freshly tilled seed-bed condition under simulated rain storms. This most erodible condition only considers the state of soil aggregates or particles without the influence of soil water that may in fact impose additional forces on the soil particles causing the soil to respond differently under erosive forces. If soil erodibility is defined as a measure of a soil’s susceptibility against erosive forces, then this erodibility concept will encompass both intrinsic and extrinsic factors affecting the strength of the soil. Many researchers have shown that soil erosion is affected by soil pore water pressure and subsurface hydraulic gradient. We have found that an increase in pore water pressure resulted in an increase in soil erodibility and a decrease in critical shear stress under concentrated flow erosion which is traditionally quantified by a linear excess shear stress equation. Despite these findings, most soil erosion models do not adjust soil erosion parameters for the hydraulic gradient effect. In this research, we define the experimentally measured erosion parameters as apparent quantities that can be further partitioned into intrinsic and extrinsic terms. For concentrated flow erosion, the critical shear stress is the flow stress imparted on the soil aggregates or particles to initiate particle detachment. In other words, this term is a measure of soil strength or resistance against flow shear. We propose to partition critical shear stress into an intrinsic term encompassing a buoyancy corrected weight term added to a cohesion term and an extrinsic term which depends on the soil moisture condition and hydraulic gradient at the soil surface. In the case of infiltration through a given soil, the downward infiltration flow velocity results in a positive effect on the extrinsic term while an upward seepage scenario on the same soil results in a negative effect on the extrinsic term. We also developed a simple laboratory procedure based on the fluidized bed concept to measure soil cohesion. Results from a concentrated flow experiment where we measured sediment flux and critical shear stress under different hydraulic gradients, suggest that the erosion parameters measured under zero hydraulic gradient (i.e. when the water table is set at the soil surface) reflect the intrinsic properties of the soil while the parameters measured under a positive or a negative hydraulic gradient (drainage or seepage condition) include the effect of the extrinsic term. This research shows that soil erodibility derived under a drainage gradient may not represent the most erodible condition of the soil. The erodibility term needs to be adjusted as the moisture gradient shifts from a drainage condition to a seepage condition. |
