Submitted to: International Symposium on Preferential Flow
Publication Type: Proceedings
Publication Acceptance Date: 10/1/2000
Publication Date: 1/3/2001
Interpretive Summary: Accurate predictions of rain infiltration is of critical importance for quantifying runoff processes and assessing the movement of nutrients, sediment and agrichemicals on agricultural fields. Most infiltration research in year past has been concerned with one-dimensional, vertical infiltration in uniform soils. This research has yielded a variety of analytical, numerical, and semi-analytical solutions, of which the specific form is determined by the boundary conditions and the applicable soil water characteristic functions of the problem at hand. In the real world, soils are usually not homogeneous nor are the water characteristic relationships of the soil time-invariant. Thus, most infiltration solutions are not accurate for predicting infiltration for real world situations. One important class of soils that substantially deviates from ideal behavior are swelling and cracking soils. These soils may yield extremely high infiltration rates through the cracks under dry conditions during the early stages of rainfall or may show negligible or non-detectable amounts of infiltration under wet conditions. This article describes a model that better describes infiltration into cracking soils. The model consists of a simple geometric structure of prismatic columns in which excess rainwater flows along the lateral surfaces of the columns, where it substantially is absorbed into the soil by horizontal infiltration. The equation obtained describes infiltration as a function of the incipient ponding time, crack morphology, and soil parameters. This relationship will be very useful for describing rainfall/runoff processes on landscapes with swelling/cracking soils.
Technical Abstract: An analytical solution of rain infiltration into dry cracked soil is presented and validated with experimental data. Particular emphasis is placed on the calculations of the post-incipient ponding phase and the cumulative infiltration amount. The morphology of the cracks is represented in a simple manner in terms of a length scale of crack spacing and geometric pattern. The crack volume is estimated in terms of an antecedent bulk density and a reference crack free density. The analysis utilizes the wetting front solutions of horizontal diffusion into the soil matrix governed by Richards' equation as recently developed by the present authors. The soil matrix diffusion characteristic parameters, which are needed in this solution, are determined experimentally for the cracked soil under study. In the simple geometric representation of the prismatic column structure assumed here, an accurate estimation of the crack volume is made in terms of bulk density measurements from which ponding time estimates are made in closed form. Closed form expressions are also derived for the cumulative infiltration amount. The analytical results are compared with the data being obtained at the National Sedimentation Laboratory from an experimental set up using a Mississippi delta clay soil.