A GEOPHYSICAL ASSESSMENT OF SEASONAL SOIL MOISTURE DYNAMICS IN A TEXAS VERTISOL

Vertisols are important soil resources for both agriculture and engineering applications. However, they are relatively difficult to manage. Their shrink/swell characteristics give rise to development of microrelief features (gilgai) and deep cracks. We present time-lapse analyses of data from two-dimensional resistivity monitoring of spatiotemporal soil-moisture variations in a Texas Vertisol. The field surveys were carried out at the USDA Research Station, Riesel, Texas, from May 1, 2005 to April 22, 2006, during which 32 resistivity profiles were collected. Laboratory data were used to generate soil moisture-resistivity relationships for the Vertisol, and to calibrate the inverted field resistivity data to generate apparent soil moisture sections. We used the time-lapse analyses to examine resistivity change in the soil within each interval over the duration of the field surveys. We also made vector plots of the apparent soil moisture data to generate apparent soil moisture gradient and flux in the soil; with the latter involving comparisons of successive sections. Earlier interpretations of these data have shown that there are three distinct soil-moisture regimes in the upper 1.4 m of the vertisol, where a relatively saturated middle layer appears to be underlain and overlain by relatively less saturated layers. This result is corroborated by the time-lapse analyses results which show that, during the wetting cycle in the Vertisol, following rainfall events, maximum (negative) resistivity change occurs in the middle layer, indicating preferential recharge of the layer, whereas the least change occur in the relatively less saturated underlying and overlying layers. The recharge of the middle layer appears to be enhanced by bypass flow through cracks in the soil. The apparent soil moisture gradient and soil moisture flux plots indicate concentrated soil-water flows towards the microlows and divergent flows away from the microhighs. This is in line with an earlier conclusion that the microhighs dry out faster than the microlow. Generally, the results of this study corroborate a conclusion that formation of gilgai in Vertisols may be due to periodic expansion of the relatively wet depth interval in a manner analogous to formation of mullions in contracting sedimentary sections.