UNDERSTANDING AND PREDICTING THE IMPACT OF AGRICULTURE ON THE ENVIRONMENTAL INTEGRITY OF MANAGED WATERSHEDS
Location: Water Quality and Ecology Research
Title: A discussion of assumptions and solution approaches of infiltration into a cracked soil
Submitted to: Proceedings of the International Yellow River Forum
Publication Type: Proceedings
Publication Acceptance Date: December 7, 2007
Publication Date: December 31, 2007
Citation: Romkens, M.J., Prasad, S.N. 2007. A discussion of assumptions and solution approaches of infiltration into a cracked soil. Proceedings of the International Yellow River Forum. Vol. I:392-400.
Interpretive Summary: Describing the hydrologic response of watersheds during storm events is very important for water management planning in agriculture. The most difficult component to estimate is the amount of water that enters the soil or that is stored in the surface drainage system. Soil is a highly complex porous medium, and infiltration rates vary in time and place depending on the local soil properties and soil profile conditions. Of particular challenge is the problem of estimating rain infiltration into swelling, shrinking, and cracking soils. Depending on the prevalent cracking condition, large quantities of rainwater may be absorbed into the cracks during periods of drought, while during relatively wet conditions, when cracks are small or closed altogether, rain infiltration is small and most rainwater becomes runoff. This article attempts to quantify the infiltration into a highly swelling/cracking soil. An infiltration model was developed that represents a two-component process consisting of Darcy flow in the soil matrix and Hortonian flow of excess rain along the vertical crack surfaces. It was shown that ponding time can be described as a function of the crack morphology (spacing, width, and depth), rainfall intensity, and soil characteristics. For the special case, when crack closure occurs when the wetting front tip on the crack surfaces reach the bottom of the crack, an exact solution for field ponding was obtained. Also, an expression for the cumulative infiltration was obtained as a function of rainfall intensity, field ponding time, and soil parameters. The derived equations allow for improved predictions of runoff initiation and volume on swelling/cracking soils. The article also discusses the assumptions made in the model formulations and possible approaches for improved predictions.
A model for predicting rain infiltration into a swelling/shrinking/cracking soil was proposed (Römkens, M.J.M., and S. N. Prasad., 2006, Agricultural Water Management. 86:196-205). Several simplifying assumptions were made. The model consists of a two-component process of Darcian matrix flow and Hortonian flow on the walls of the cracks. Model assumptions include: (1) Water enters into the soil horizontally from the vertical cracked surfaces exclusively by diffusive flow, (2) Vertical infiltration through the soil surface was negligible because of a severe surface sealing condition, (3) Water flow along the vertical surfaces of the cracks is uniform over its circumference, and (4) Wetting front advance interactions between adjacent vertical crack surfaces are assumed to be negligible. These assumptions will be examined and their effect on infiltration will be assessed. Also, alternative approaches involving different modular concepts for predicting infiltration and incipient ponding estimates will be examined.