Geoelectric Studies of Seasonal Wetting and Drying of a Texas Vertisol

Vertisols are complex soils with high clay content (>30%), high shrink–swell potential, and microrelief features known as gilgai . We applied field and laboratory electrical-resistivity measurements to characterize seasonal wetting and drying of a Texas Vertisol, and to quantify the effects of gilgai and cracks on seasonal hydrodynamics of the soil. Thirty-two multielectrode resistivity-profiling lines were collected along the same profile from 1 May 2005 to 22 Apr. 2006, using combined dipole–dipole and Schlumberger electrode configurations. The profiles were 17.5 m long and intersected two sets of microlows and microhighs of the gilgai. The resistivity data were inverted using the RES2DINV program and the inverted data were corrected for temperature. We measured variations of resistivity with soil moisture in the laboratory and the results were used to calibrate the field data. To evaluate the resistivity results, in situ measurements of soil moisture were made using auger sampling. During the wetting cycle, three distinct soil moisture regimes were recognized in the upper 1.4 m of the Vertisol: an upper zone (0–0.5-m depth), which is the most dynamic with regard to wetting and drying; a middle zone (0.5–1.1 m), which is relatively saturated and less dynamic; and a lower zone (below 1.1 m), which is relatively less saturated compared with the middle layer. The saturation of the middle layer appears to be enhanced by preferential flow through cracks. Also, the microrelief topography exercises a control on spatiotemporal variations in soil moisture in that the microhighs dry faster than the microlows.

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