Current understanding of the movement of water and solutes in deep desert vadose zones represents a major knowledge gap among the hydrologic science community. A particularly critical aspect in closing this gap is supplying a hydrologic framework that explains both matric potential and chloride vadose zone profiles typically observed beneath interdrainage regions of desert floors. This research seeks to close that gap. The conceptual model of deep arid system hydrodynamics (DASH) formulated as part of this study relies on vapor transport and the hydrologic role of desert plants that have become established during the past 10 to 15 thousand years in the southwestern U. S. as critical elements in explaining the observations. According to the DASH model and supported by field observations, desert vegetation sustains very negative matric potentials at the base of the root zone and effectively buffers the deep vadose zone over very long time scales from most hydrologic near-surface transients, such as episodic precipitation events. A nonisothermal, multiphase flow and transport code, FEHM, is used to numerically simulate the DASH conceptual model and other conceptual flow models previously proposed. Model results generated using the DASH paradigm match both characteristic matric potential profiles and chloride profiles, whereas model results generated using the other conceptual models that were tested deviate dramatically from observed matric potential data. A unifying theory for the hydrology of desert vadose zones is particularly timely considering current water stresses and contaminant issues associated with desert regions.

A sensitivity analysis tests the applicability of the DASH model to a wide range of desert vadose zone parameters and conditions. The sensitivity analysis also enables assessment of the factors that control moisture movement in deep vadose zones. The results indicate that most thick desert vadose zones have been locked in slow drying transients for many thousands of years, since the desert vegetation became established. A hydrodynamic condition, as opposed to a hydrostatic condition, characterizes deep vadose zones in equilibrium with the dry hydraulic conditions imposed by desert vegetation. Long response times, on the order of 10^4 – 10^6 years, are required to reach this hydrodynamic equilibrium, which exceeds the typical time scale of major climate shifts.

Two case studies demonstrate the application of the DASH model to interpretation of measured hydraulic and solute profiles from deep arid vadose zones. The DASH model approach enables paleohydrologic reconstruction and yields information about current vapor and liquid fluxes between the base of the root zone and the water table. Both case studies emphasize the influence of desert vegetation on deep vadose zone hydrodynamics. Desert vadose zone hydraulic profiles cannot be resolved apart from their climate and vegetation histories.