Deep Temperature Profiles to Evaluate Recharge in the Desert Southwest, United States

Natural recharge is one of the most poorly constrained aspects of hydrologic budgets in the southwestern United States. One component of natural recharge to alluvial aquifers in many basins in the desert Southwest occurs through infiltration beneath ephemeral stream channels. Shallow temperature methods have been successfully used to estimate recharge beneath these channels. These methods rely on variations in shallow (less than 5 meters) temperature changes, which are due to advective flow of heat from infiltrating water, and are often applied in areas where depth to ground water is shallow. In cases where shallow temperature methods are used in thick unsaturated zones, it must be assumed that flux beyond the depth of the measurement is equivalent to recharge. For thick unsaturated zones, deep temperature profiles can be used to estimate recharge. This method relies on the alteration of temperature profiles deeper in unsaturated zones by percolating water. Reduced gradients deep beneath active channels indicate downward heat transfer by percolation of water. This heat transfer perturbs the geothermal gradient, resulting in a heat deficit in the vertical profile. This investigation uses deep temperature profiles below stream channels for analysis of the effects of percolation on the geothermal gradient. Recharge estimates made on the basis of heat deficits are compared with recharge estimates made using shallow-temperature methods. Utility of the deep temperature-profile method in areas where shallow-temperature methods may not be appropriate is examined. The analysis evaluates temperature profiles from boreholes within and adjacent to ephemeral stream channels in the Sonoran and Mojave Deserts.