Modeling Soil Crack Volume at the Pedon Scale using Available Soil Data

Core Ideas Desiccation cracks make runoff and infiltration predictions difficult in expansive soils. A crack‐volume model based on water content and soil properties was developed. Crack volume was measured in situ using digital photography of excavated soil layers. The new model improved crack volume estimates over a model based on layer thickness. A critical gap in hydrology knowledge is predicting the partition of runoff and infiltration during rainfall events in shrink‐swell soils with desiccation cracks. Knowledge of surface cracking and crack volume is needed, but field measurements of these vertical soil cracks are time and labor intensive, and the results cannot be easily translated to another location. Our approach to predict soil crack volume at the pedon‐scale uses an existing soil shrinkage model, which has been modified to include soil water content and the coefficient of linear extensibility (COLE). To validate the model, measurements of soil layer thickness, water content, and crack volume were made for seven soils with COLE values from 0.01 to 0.17 m m–¹. Soil crack volume was estimated by filling cracks with a cement slurry and photographing excavated soil layers at the end of the study. Over two drying and wetting cycles, the relationship between soil layer thickness and water content was linear. The modified crack volume equation, using COLE and water content, was a better fit to cement‐estimated crack volume, r² from 0.06 to 0.61, than the existing shrinkage model. However, crack volume estimates by both models were six‐times higher than the cement‐estimated crack volume. It is possible the models of crack volume are including all changes to soil porosity from mega‐cracks to mesopores, while direct techniques only measure the larger‐scale cracks. The new, modified crack volume equation is a pedon‐validated equation that advances predictions of cracking extent in landscapes where shrink‐swell potential is variable in space.