Abstract. We quantified unsaturated flow beneath playa and adjacent interplaya settings at a site in the Southern High Plains (United States) to resolve issues related to where and how water moves through the unsaturated zone. This is the first study in which the data density (39 boreholes) and the variety of techniques used (physical, chemical, and isotopic) were sufficient to quantify spatial variability in unsaturated flow. Water contents, water potentials, and tritium concentrations were much higher and chloride concentrations were much lower beneath playas than in interplaya settings, which indicated that playas focus recharge. These results refute previous hypotheses that playas act as evaporation pans or that recharge is restricted to the annular region around playas. Water fluxes estimated from environmental tracers ranged from 60 to 120 mm yr -1 beneath playas and were -•0.1 mm yr -1 during the past 2000-5000 years beneath natural interplaya areas not subjected to ponding. To evaluate the apparent inconsistency between high recharge rates and thick clay layers beneath playas, we applied bromide and FD&C blue dye to evaluate flow processes. These applied tracer experiments showed preferential flow along roots and desiccation cracks through structured clays in the shallow subsurface in playas.

Abstract. Because geomorphology can readily be mapped, our ability to characterize unsaturated flow over large areas would be greatly enhanced if relationships between geomorphic settings and unsaturated flow could be identified. The purpose of this study was to evaluate relationships between geomorphic settings and spatial and temporal variability in unsaturated flow at a field site in the Chihuahuan Desert of Texas. This study differs from most previous studies in the variety of geomorphic settings studied, including drainage areas (Blanca Draw and Grayton Lake playa) and interdrainage areas (basin-fill deposits, eolian sheets, alluvial fans, and a fissure), density of data (-- Noninvasive techniques, such as electromagnetic (EM) induction and ground-penetrating radar, are becoming increasingly popular for evaluating unsaturated flow because they can be used to evaluate unsaturated flow rapidly over large areas and because they provide an evaluation of conditions between point measurements from boreholes. In an Australian study the correlation coefficient r between apparent electrical conductivity and recharge estimated according to unsaturated zone chloride data was 0. The geomorphic evolution of the landscape was described by Scanlon et al. [1999]. The study area has been subdivided into interdrainage and drainage areas. The interdrainage area consists of fine-grained basin-fill deposits and eolian sheets surrounded by a narrow rim of alluvial fans at the margin of the basin. An earth fissure is also found in the interdrainage area. The drainage area includes Bianca Draw and Grayton Lake.-The floor of Eagle Flat basin consists mostly of muds overlain by the Arispe Surface, which has well-developed soils. The basin-fill deposits are overbank deposits from the braided streams and from the toes of the alluvial fans. These deposits are stable, vegetated landforms that do not exhibit channels or erosional or depositionai features resulting from fluvial or alluvial activity. Three calcic soil horizons are found at depths of 0 to 1, 3, and 6 m, which suggest extremely stable conditions Boreholes were drilled with a hollow-stem auger without any drilling fluid, and samples were collected with a split spoon. Particle-size analyses were conducted on sediment samples from 37 boreholes using sieving and hydrometer analyses [Gee (Figure 1 and Table A1). Many samples were collected from the same boreholes as those that had been sampled for texture. To determine Samples were collected for tritium analysis from boreholes EF 79 and EF 117 in the interdrainage eolian sheet, EF 92 beneath the fissure, EF 96 10 rn from the fissure, and GL 2 in Grayton Lake. Water was extracted from core samples in the laboratory by toluene azeotropic distillation and purified using paraffin wax [Ingraham and Shadel, 1992]. The samples were enriched and analyzed using liquid scintillation methods at the University of Arizona Tritium Laboratory or using gas proportional counting at the University of Miami Tritium Laboratory. 4.Results and Discu...

The use of apparent electrical conductivity (ECa) measured with electromagnetic (EM) induction was examined as a reconnaissance tool for characterizing unsaturated flow in a semiarid region in the Chihuahuan Desert of Texas. Aboveground conductivity meters (EM31 and EM38) were used to measure ECa along transects in various geomorphic settings. Eight boreholes were drilled at different locations along the transects, and a downhole conductivity meter (EM39) was used to measure ECa. Samples were collected for analysis of clay, water, and chloride content to evaluate factors affecting spatial variability in ECa. Variations in ECa measured with the aboveground EM31 meter were affected by variations in clay content in a playa/interplaya setting, water content in a fissure, and chloride content adjacent to a drainage system. These factors affecting ECa were confirmed by comparing ECa measured with the downhole EM39 meter and clay, water, and chloride content of soil samples from boreholes. The hydrologic significance of parameters controlling ECa was evaluated. Variations in clay content are not hydrologically significant in this basin. High correlations between ECa and water content are difficult to interpret because in some areas water content variations simply reflect variations in clay content, as in the playa/interplaya setting, whereas in other areas higher water contents reflect higher water flux, as in the fissure. In some areas water content was below threshold values; therefore, ECa did not respond to water content or salinity in these areas. Although EM induction alone cannot distinguish causes of ECa changes, it provides a valuable tool for delineating variations in ECa that can be used to guide borehole locations and to provide valuable information for interpolating and extrapolating from point estimates provided by borehole data.

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