Relationship between geomorphic settings and unsaturated flow in an arid setting

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 (∼50 sampled boreholes 3–31 m deep), and variety of techniques (physical, chemical, and electromagnetic) used to quantify unsaturated flow. Spatial variability in unsaturated flow parameters is related to geomorphic settings. The various geomorphic settings form distinct groups on a plot of chloride versus water potential. Interdrainage areas have low water potentials and high chloride concentrations, indicating low water fluxes. Mean water fluxes estimated from chloride data ranged from 0.02 to 0.05 mm yr−1. In contrast, localized topographic depressions (fissure, gully, and borrow pit) have high water potentials and low chloride concentrations which indicate high mean water flux (∼100 mm yr−1). These topographic depressions occupy <1% of the basin area. Drainage areas have low water potentials, which indicate low water fluxes and low to moderate chloride concentrations, which indicate higher water fluxes in the past (≤40 mm yr−1). Short‐term variability in response to precipitation events was only monitored in topographic depressions. The age of the pore water in the interdrainage areas spanned paleoclimatic variations (≤136,000 years at 25 m depth); however, most interdrainage profiles show negligible response to past climate fluctuations. Some profiles in eolian sheets showed increased water flux in response to Pleistocene climate change. The findings from this study indicate that geomorphology can provide valuable information on unsaturated flow and underscore the importance of localized topographic depressions for focusing unsaturated flow.