Land Use and Spatial Cracking Dynamics of a Vertisol.

Understanding the dynamics of cracking and swelling of clayey soils improves the ability to predict the impact of land use on the hydrologic response of watersheds containing shrink-swell soils. The objective of this research was to characterize the impact of land use on spatial and temporal shrink-swell dynamics of a Vertisol. The land uses are grazing land (GL), native prairie (NP), and cropland (CL). The research was conducted at the USDA-ARS Grassland, Soil and Water Research Laboratory near Riesel TX. The soil at the site is Houston Black (Fine, smectitic, thermic Udic Haplustert). To monitor vertical soil movements, five measurement sites each on a GL and NP and four measurement sites on a CL (corn) were selected. Bi-weekly measurements of vertical soil movements and soil water were made beginning in June 2008. The change in absolute heights of soils at the surface and subsurface (0, 30, 60 and 90 cm) was used to track the temporal trends in thickness of soil layers. Near each set of soil height measurement, soil water content was measured using a neutron moisture meter. The study showed that maximum soil subsidence and the time of its occurrence varied with land use. The maximum soil subsidence ranged from 91 to 120 mm in the sites of GL; from 71 to 75 mm in the sites of NP, and from 67 to 76 mm in the sites of CL. The maximum soil subsidence in the CL was within the same range as that of the prairie (NP), but the maximum shrinkage occurred in mid August 2008 in the cornfield whereas at the end of August 2009 in the prairie. Grass roots, size, and shape of gilgai likely influence the observed variation and further studies are underway. Knowledge gained in these studies may be used to modify and refine hydrology models that simulate runoff, infiltration and solute transport across different land uses in a Vertisol landscape.