Estimation of soil cracking and the effect on surface runoff in a Texas Blackland Prairie watershed

Seasonal cracking of the soil matrix results in poor estimates of runoff and infiltration by simulation models due to the changing soil storage conditions. In this study, soil surface elevation changes were measured every two weeks and soil crack volume was calculated for a two-year period at the USDA-Agricultural Research Service, Riesel Y-2 watershed in central Texas. Soil anchors were placed in triplicate at depths of 0.15, 0.45, 0.90, 1.50 and 2.5 m and relative movement from a monument at 4.5 m was measured. Soil movement was translated into crack volume assuming isotrophic shrinkage. A crack flow model was developed for this study that computes crack volume from crack potential, soil depth and soil moisture. Simulated crack volume followed the seasonal trends found in the measured crack volume and was in general agreement with a regression R-2 = 0.84. The crack model was incorporated into SWAT (Soil and Water Assessment Tool), a comprehensive hydrologic model. Regression analysis was performed on measured and simulated daily surface runoff with an R-2 = 0.87 indicating good agreement. The model was able to simulate surface runoff accurately in winter months when cracks were swelled closed and in the fall recharge events of 1998 when crack volume went from 70 to 10 mm. The relationships between measured crack volume and hydrologic variables simulated by the model were also examined and discussed. Copyright (C) 2005 John Wiley Sons, Ltd.