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PEDOLOGY

Vertisol Crack Extent Associated with Gilgai and Soil Moisture in the Texas Gulf Coast Prairie

^a Dep. of Soil and Crop Sciences, Texas A&M Univ., College Station, TX 77843-2474
^b USDA Natural Resources Conservation Service, Victoria, TX 77902-2252

^* Corresponding author (akishne{at}ag.tamu.edu).

Long-term observations of in situ crack formation and closure in shrink-swell soils are rare, but important to understanding hydrology in shrink-swell soils. To analyze spatial and temporal variability of crack development in a Vertisol with gilgai, soil cracks were measured on a 100-m² area of Laewest clay (fine, smectitic, hyperthermic Typic Hapludert) with native tallgrass vegetation on 42 dates from 1989 to 1998. Our objectives were to (i) report the distribution of Vertisol cracking across gilgai microtopography; (ii) estimate crack depth as a function of crack width considering gilgai; (iii) investigate the relationship of surface cracking and soil moisture considering gilgai and hysteresis. All surface cracks were mapped on scaled diagrams with width categorized, and some crack depth measured. Gravimetric soil moisture corresponding to crack measurements was measured on 18 dates, and on an additional 32 dates without crack measurements. Drying, wetting, and uniform soil moisture conditions were classified from the difference in soil moisture from 10- to 25-cm depths. Microtopography was quantified using a digital elevation model. Results showed that crack area density was greatest on microhighs and microslopes, though microlows had the largest cracking potential. The linear correlation between crack depth and width was moderately strong (r² = 0.5), and not affected significantly by gilgai and hysteresis. However, taking hysteresis into account improved the linear regression models of crack area density versus soil moisture (up to r² = 0.69) on both microhighs and microlows. Antecedent soil moisture seemed to impact in situ crack area density. Further field studies are recommended.