Modeling Retention of Water In a Vertisol with Circular Gilgai.

Surface cracking, surface microtopography (circular gilgai), and differences in subsoil horizonation associated with gilgai create distinct spatiotemporal variability in soil physical properties that affect small-scale (meter) surface ponding.  Only a few soil hydrology models can include surface cracking phenomena and fewer include information on meter-scale surface and subsurface heterogeneity of soil properties and topography that are important to modeling retention of storm water.  Our overall objective was to adapt the Precision Agricultural-Landscape Modeling System (PALMS) to meet the need for a model that can accurately predict water flow into and across a Vertisol.  PALMS contains two modules that are essential for simulation of surface hydrology on Vertisols.  First, PALMS has a module that addresses seperation of mesopore and matrix infiltration. This model can be modified to simulate cracking and infiltration into a Vertisol. Second, PALMS has a diffusive wave module for simulation of water runoff and ponding of water in closed depressions.  PALMS was tested on a 10 by 10 meter area of a Vertisol that contains circular gilgai.  Testing was done using 1 x 1-m resolution in an attempt to accurately represent the surface variability present on a Vertisol with gilgai. Infiltration and ponding was modeled on a 100 m² area for dry and wet antecedent moisture conditions and under various intensities of rainfall events. Results of these simulations will be compared to observations of soil water content and ponding in gilgai. Knowledge gained will be used to modify cracking module and scale soil information for PALMS simulations over larger areas, for example agricultural fields.