Testing a Mesopore and Matrix Model for Use on Shrink-Swell Soils

Void space caused by drying of shrink-swell soils forms desiccation cracks and mesopores which conduct water from the soil surface, influencing water redistribution and complicating the partitioning infiltration and runoff. These paths impact hydrology, but often are not included in models because the standard method of modeling flow through soil (Richards? equation) assumes a continuous matrix of particles-this assumption is invalidated by voids, the changing volume of paths is difficult to characterize, and parameters to simulate water flow are difficult to obtain for preferential flow routines. The Precision Agriculture Landscape Modeling System (PALMS) contains a Mesopore and Matrix (M&M) module, which was tested on cracking soil at the plot and field scale.

The M&M module predicted 10 times more mesopore area than, but was linearly related to, measurements of crack area. Four irrigation events on plots of cracked soil and volumetric water content (VWC) output for the M&M module was compared to neutron moisture meter readings. Previous measurements of VWC and runoff on a 4.4 ha subwatershed were compared to predictions. The M&M module moved water down the profile quickly and eliminated unobserved ponding (plot) and runoff (field) that were simulated without the mesopores, modeling mesopore flow did produce more drainage. Simulations of water content of the soil profile were generally improved when the M&M module was used. The M&M module had easily obtainable and physically relevant parameters. The M&M module is a useful tool for model assisted decision making on landscapes where preferential flow occurs.