Measured temperature profiles to evaluate recharge In the semiarid southwest

Abstract

Natural recharge is one of the most poorly constrained components of the water budget. In semiarid environments, quantifying natural recharge is difficult due to the relatively low fluxes and the thick unsaturated zones. Measuring deep recharge to great depths in multiple locations is impractical and cost prohibitive for most measurement methods. An alternative method to estimate deep percolation utilizes temperature profiles measured from the ground surface to the water table. Temperature measurements taken in existing cased wells provide an economical and useful vantage for accessing many locations within a basin. A controlled infiltration experiment was conducted to test the feasibility of using downhole temperature measurements to estimate the infiltration flux. VS2DHI (Healy and Ronan, 1996), an interactive numerical model, was used to predict and interpret coupled water-flow and heat-transport occurring in the subsurface during infiltration and redistribution. A sequential sensitivity analysis showed that downhole temperature profiles were most sensitive to the temperature of the applied water, the water flux, and the heat capacity of the dry soil. This downhole temperature method was extended to a natural setting. Temperature profiles were measured at six sites within the Tucson Basin. The temperature profiles varied throughout the basin, and the factors affecting the temperature profiles were numerous. Profiles from the in-channel and intrabasin boreholes showed effects of focused recharge, mountain-front recharge, and stream-channel recharge. Results suggest that this temperature method may be transferable to other basins in semiarid regions.