In desert soils, water infiltration and retention are strongly influenced by features characteristic of different developmental stages, such as biotic crusts, desert pavement, vesicular horizons, and downward illuviation of fine-textured material. Our purpose is to measure the effect of such features on basic soil-water behavior and on the water retention curves and unsaturated hydraulic conductivities (K) needed for large-scale modeling of soil moisture. The results are used in conjunction with surficial geologic mapping of the Mojave Desert in evaluations of ecological habitat quality.

We conducted infiltration/redistribution experiments in three different-aged deposits on an alluvial fan in the Mojave National Preserve: (1) recently deposited active wash sediments, (2) a soil of early Holocene age, and (3) a highly developed soil of late Pleistocene age. In each experiment we ponded water in a 1-m-diameter infiltration ring for 2.3 hr, generating as much as 1.8 m of infiltration. For several weeks we monitored water content and matric pressure to depths as great as 1.5 m, as far as 6 m from the ring. Measuring techniques included surface electrical resistance tomography, dielectric-constant probes, heat-dissipation probes, and tensiometers. Analysis of the subsurface measurements using an instantaneous-profile technique gives the retention and K properties for predictive modeling.

In each experiment the infiltration rate was nearly constant in time, decreasing slightly in most. Infiltration capacities were lower in older soil, consistent with the expectation that the more highly developed layer contrasts impede flow. In some cases, however, the quantified soil-water behavior showed less contrast among surface types than expected; active wash deposits had significant depositional sedimentary stratification to impede downward flow, and the horizonation of the early Holocene-age soils was only a modest impediment to downward flow, causing only a factor-of-2 reduction of infiltration capacity from that of the wash (830 mm/hr).