Soil hydraulic parameters used in estimating soil water and energy fluxes are included in a lookup table in the Noah Land Surface Model (LSM). The purpose of the study was to examine the Noah default soil hydraulic parameters and compare them to soil measurements across Texas, USA. These default soil parameters were compared to measured soil properties from a soil database including 6,749 soil samples located within and around Texas. Mean differences between measured and default soil parameters were tested using a t-test (α = 0.05). To assess the proposed changes to default soil parameters, water retention curves were created using updated parameters and compared to measured soil moisture content at field capacity (θ fc) and permanent wilting point (θ wp). Spatial trends across major land resource areas in Texas were also demonstrated for water retention parameters. Our findings indicate that 95% of the default soil parameters were significantly different from the region-specific measured values. Measured soil water content at air-dry, θ wp , and θ fc are better replacements than default values. Consequent changes in θ wp and θ fc yielded a 35 to 76% decrease in plant available water compared to default. Updated water retention curves showed improved agreement between estimated soil water and measured values in 95% of cases. For three texture classes, the standard deviations of parameters for water retention parameters ranged 30% for the slope of the water retention curve and 65% for saturated hydraulic conductivity. These results indicate the importance of accounting for spatial variability of soil parameters rather than combining these parameters into texture classes alone. The revised parameter table improves modeling of soil hydraulic properties. Ultimately spatially distributed databases of hydraulic soil parameters will better capture variability and spatial structure of soil processes modeled by Noah LSM.

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.

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Vertisols have unique morphology and the capacity to change volume with changes in water content; therefore, they are well known for their spatial and temporal variability. Our objectives were to address the spatial variability of Vertisol surface and subsurface features, as well as the spatial and temporal variability of Vertisol cracking. First, we discuss surface and subsurface features of Vertisols based on the literature and more than three decades of interagency soil investigations in the Texas Gulf Coast Prairie. Second, we report on a 10‐yr field study of location, length, width, depth, and duration of seasonal crack openings in a 100‐m2 area of Laewest clay (fine, smectitic, hyperthermic, Typic Hapludert) with gilgai in a native grassland south of Victoria, TX. Results of the crack monitoring showed that cracks always started on, and crack density was greater on, microhighs or the upper part of microslopes. Under prolonged drying periods, cracks formed on microlows and were, on average, deeper in microlows. Cracks generally occurred in the same location within a year. Between years, crack locations shifted slightly, although they were clustered in the same general areas during the 10 yr of study. Vertisol cracking was affected not only by seasonal fluctuation of precipitation, but also by multiple‐year precipitation cycles. Analysis of precipitation over several decades indicated that the Ustert/ Udert classification criteria based on cracking in normal precipitation years was never met in this region of Texas because there were never eight normal months within a normal year. Normal yearly precipitation standards and months on a regional basis would provide better precipitation norms for soil classification.

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