Influence Of Soil Structure On Cracking Behavior.

Accurate estimates of soil crack volume for shrink-swell soils are important for partitioning precipitation for hydrology models.  Soil scientists have attempted to relate changes in vertical thickness with soil crack volume using field observations, however, these measurements are time and labor intensive, and are often not validated with direct measurements of soil crack volume.  To build a mechanistic model of soil shrinkage and crack volume, soil subsidence and water content were measured over time at seven sites with COLE values from 0.01 to 0.17 m m^-1, and soil crack volume was measured at the end of the study period.  Vertical subsidence could be predicted using COLE, initial soil layer thickness, initial water content, and the wilting point water content with reasonable results and no fitting parameters. 

There was evidence of the influence of soil structure in these simple models.  Layer thickness was slightly overestimated using the mechanistic model, influenced by stronger, more developed, structure in these soil horizons.  The strong structure may produce more horizontal cracks or macropore space, decreasing vertical shrinkage.  When modeling areal volume of cracks (m), the model over predicts by 7 times the measured values, and had a 0.01 m intercept, implying that soil shrinkage does not always result in measurable crack volume.  The over prediction of crack volume may provide a useful partitioning of total shrinkage into macropore space and crack volume.  There also may be a critical COLE value (shrink-swell potential) for a soil to crack, in this study it was 0.10 m m^-1.  These models may be improved by including structure information to improve estimations of macropore space for soil water infiltration and redistribution.