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Simulating soil moisture dynamics in coarse-textured alluvial soils under high evaporative demand and infrequent rainfall challenges both characterization and numerical methods. For this study, three geomorphically young disturbed soils in the central Mojave Desert were characterized using direct, indirect, and inverse parameter estimation methods for numerical simulations using HYDRUS. Observations of soil water content (θ) and matric head ( h ) were used to formulate our multiobjective framework optimized using the AMALGAM algorithm. Both the direct and indirect parameterization methods reproduced θ but proved inadequate for h data. Sensitivity analysis found the parameters related to the dry end of the soil hydraulic function—including residual water content (θ r ) and pore-size distribution ( n) —most sensitive but often conflicting between the two criteria while soil temperature was effectively simulated but insensitive. At the sites, a large dynamic range of h (1–400 m) was accompanied by a relatively small range of θ (0.04–0.10 m3 m−3), limiting our ability to resolve the wet-end of the hydraulic functions. Direct parameterization from infiltrometer data and indirect pedotransfer functions had good agreement with θ but largely underestimated h . Pareto analysis determined the trade-off between fitting both criteria and revealed model structure inadequacies as well as bifurcations into parameter populations, favoring either θ or h error minimization. The Pareto solutions compensated for the underestimation of h by forcing the α and n parameters to uncharacteristically low values for such coarse textures. This compensation kept the water capacity sufficiently high at lower θ to maintain conductivity toward the surface. These results are likely a consequence of either the disturbed soil system or hysteresis between measurements of θ and h.

A reconnaissance tool that can estimate the clay content and saturated hydraulic conductivity ( K s) of well-developed desert pavements has immediate applications for researchers and practitioners who work in arid environments. We examined the use of surface-based ground penetrating radar (GPR) to rapidly approximate these properties on six, 100-m-long transects for two different aged (∼100 000 vs. 4000 yr) desert pavement surfaces. We determined early-time amplitudes from GPR transects, ground electrical conductivity, hydraulic properties, clay content, water content, and soil salinity at regular intervals along each transect. Both surfaces were low in water content and salinity; however, the older pavement contained substantial amounts of silt and clay in the surficial soil horizon. Using multivariate linear regression, which included GPR amplitude and a nominal measure of soil structure ascertained by visual field inspection, we show significant correlations between measured and predicted values of both silt plus clay content ( r = 0.84, P < 0.0001) and K s ( r = 0.73, P < 0.0001) for the older surface. No significant correlations were found on the younger surface. This is probably due to the low concentrations of clays in the young soil. Including the metric of soil structure improved the predictive capabilities on the older surface. The GPR method provides higher spatial resolution than electromagnetic measurements. The results suggest that, at this location, the approach is not influenced by heterogeneities in lower soil horizons. The method can be used for reconnaissance surveys as a means of estimating the clay content, and in some cases K s, of surficial soils on certain well-developed desert pavements.

Desert pavements are prominent features in arid environments and consist of a surface layer of closely packed gravel that overlies a thin, gravel-poor, vesicular A (Av) soil horizon. Well-developed Av horizons form distinct and highly structured columnar peds. These structures, along with their silt- and clay-rich texture, are hypothesized as controlling infiltration and hence the overall hydrologic conditions in the soil profile. The objectives of this study were to (i) evaluate how pedological development in near-surface soil horizons in an arid alluvial fan complex affects the soil hydraulic characteristics, and (ii) to compare the use of Wooding9s equation and inverse modeling for evaluating hydraulic conductivity in highly layered, near-surface soils. These objectives were approached through field tension infiltrometer studies, soil sampling, and laboratory analyses of soil texture, water content, and soluble salt concentrations. Soils at five sites were studied at the Mojave National Preserve, California, representing a soil chronosequence (50–100000 yr) with varying degrees of desert pavement development. Results indicated 100-fold and threefold declines, respectively, in saturated hydraulic conductivity ( K s ) with both analytical methods, and α w using Wooding9s method, as the soils aged. No clear trends in K s or α w were detected in the underlying horizon, indicating that the controlling feature at these sites, in terms of water entry, was the K s of the surface (Av) horizon. Soluble salt concentrations within the profile indicated reduced infiltration with increased pavement development. Results showed that surface age can be used as an excellent predictor of saturated hydraulic conductivity ( r 2 = 0.9254). Further, results suggest that Av horizon development represents a key process controlling water cycling, potentially influencing ecosystem function in arid lands.

Accurate determination of water content is an important aspect of most vadose zone monitoring programs. Real-time, continuous, in situ measurements of water content in relatively undisturbed conditions are usually limited to shallow soil horizons. We present a new methodology using time domain reflectometry (TDR) for water content monitoring in deep vadose zone horizons. The method uses flat, flexible, waveguides pressed against the wall of a borehole. The flexible TDR waveguides are attached to the outer side of a flexible sleeve filled with a liquid resin. The resin (e.g., a two-component urethane) generates hydrostatic pressure that forces the flexible waveguides against the borehole wall, ensuring a close fit to the irregular shape of the borehole walls. The probe can be used with either a standard TDR technique, which uses a cable tester (e.g., Tektronix 1520, Tektronix, Beaverton, OR) for collecting waveforms, or a water content reflectometer (e.g., model CS505, Campbell Scientific, Inc., Logan, UT), which provides a direct electrical output, which may be sampled using a data logger. Laboratory calibration experiments and a full-scale field experiment showed that the method is reliable and capable of providing accurate water content measurements in deep vadose zone horizons.