We are applying the Darcian method, involving K measurement in the unsaturated zone at depths great enough that the flow may be steady and gravity-driven. We use the steady state centrifuge (SSC) method for the required accuracy. We have measured SSC unsaturated hydraulic property data on a total of 5 samples from 3 boreholes in the Oro Grande Wash.

In using this method to characterize recharge from ephemeral streams (thought to be the main source of recharge here), two serious complications are (1) that alluvial layers cause the moisture and pressure gradients to be far from negligible even for steady flow at great depths and (2) that most of the cores are unusable for measurements because of extreme dryness or coarseness. We are approaching these problems by developing upscaling techniques that extend our very sparse hydraulic property data to entire profiles. We have completed one-dimensional numerical simulations of the media penetrated by boreholes in the Oro Grande Wash to establish estimates of recharge rate from these sparse but accurate data. We have completes two-dimensional numerical steady-state simulations that help in estimating the total recharge from the channel and surroundings.

Depositional effects on unsaturated hydraulic properties:

Our main objective in this study is to determine the influence sedimentary structure, defined as arrangement of particles during depositional processes, has on the water retention curve.

Water retention curves were measured for 10 undisturbed core samples collected from washes with two distinct depositional environments in the western Mojave Desert. The adjacent fan at Sheep Creek Wash appears to be comprised mainly of debris flow deposits, while the fan at Oro Grande Wash appears to be more fluvial in nature, as observed along the incised channel walls at both washes. For the same particle size distribution within a given sample, fluvial samples are expected to have a greater abundance of both small and large pores than debris flow samples.

Core samples were collected from deep boreholes and from along incised channel walls. After measurement of water retention, samples were categorized as fluvial or debris flow by analyzing the stratigraphy of the cores and by calculating the spread of particle sizes within the cores. Some samples could not be categorized because intralayer sorting was difficult to discern visually for stratified samples. Comparisons of q_sat and the Rossi-Nimmo(1994)junction model parameters, y_o, and l, to the geometric mean particle size and standard deviation showed that the hydraulic properties of these samples were largely influenced by texture.

The amount air trapped during saturation and values of calculated structural porosity (after Nimmo, 1997) were used as structural indicators. Both of these parameters were higher for fluvial samples, suggesting that samples with a more ordered arrangement of particles possess greater structural pore space, and that this influence is greatest near the maximum water content of the retention curve. Although knowledge of the actual depositional environment for each sample is conjectural, based only on particle-size standard deviation and sample stratification, the results suggest that structure arising from systematic particle arrangement may affect q_sat or porosity (or the wettest parts of the retention curve) more than the water retention parameters y_o and l.

The immediate usefulness of the results is to characterize recharge for stream channels similar to those found in the Mojave Basin, especially where conditions do not permit the full set of unsaturated hydraulic conductivity measurements. A future goal includes the development of a theoretical hydraulic property model based on sedimentary structure, needed for the generalization of flow and transport processes over large areas or where unsaturated-zone properties have not been adequately studied.