Ephemeral-stream channel and basin-floor infiltration and recharge in the Sierra Vista subwatershed of the upper San Pedro Basin, southeastern Arizona

Abstract

The timing and location of streamflow in the San Pedro River are partially dependent on the aerial distribution of recharge in the Sierra Vista subwatershed. Previous investigators have assumed that recharge in the subwatershed occurs only along the mountain fronts by way of stream-channel infiltration near the contact between low-permeability rocks of the mountains and the basin fill. Recent studies in other alluvial basins of the Southwestern United States, however, have shown that significant recharge can occur through the sediments of ephemeral stream channels at locations several kilometers distant from the mountains. The purpose of this study was to characterize the spatial distribution of infiltration and subsequent recharge through the ephemeral channels in the Sierra Vista subwatershed. Infiltration fluxes in ephemeral channels and through the basin floor of the subwatershed were estimated by using several methods. Data collected during the drilling and coring of 16 boreholes included physical, thermal, and hydraulic properties of sediments; chloride concentrations of sediments; and pore-water stable-isotope values and tritium activity. Surface and subsurface sediment temperatures were continuously measured at each borehole. Twelve boreholes were drilled in five ephemeral stream channels to estimate infiltration within ephemeral channels. Active infiltration was verified to at least 20 meters at 11 of the 12 borehole sites on the basis of low sediment-chloride concentrations, high soil-water contents, and pore-water tritium activity similar to present-day precipitation. Consolidated sediments at the twelfth site prevented core recovery and estimation of infiltration. Analytical and numerical methods were applied to determine the surface infiltration flux required to produce the observed sediment-temperature fluctuations at six sites. Infiltration fluxes were determined for summer ephemeral flow events only because no winter flows were recorded at the sites during the monitoring period. Four boreholes were drilled in the basin floor to estimate infiltration in areas between ephemeral channels. Infiltration fluxes through the basin floor ranged from less than 1 centimeter to 6 centimeters per year. At a site in semiconsolidated to consolidated basin-fill conglomerate, the long-term infiltration fluxes were very low (less than 1 centimeter per year). Chloride, tritium, and stable-isotope data indicate long periods of no net deep downward percolation flux beneath the basin floor. At a site in unconsolidated to semiconsolidated basin-fill sand and gravel, infiltration fluxes were high (2 to 6 centimeters per year). Chloride, tritium, and stable-isotope data indicate active infiltration to 8 meters, and a decrease in infiltration below 8 meters. The change in the infiltration rate below 8 meters is controlled by an increase in the silt and clay content of the sediment. Ephemeral-channel recharge for the entire subwatershed was estimated by upscaling the calculated infiltration fluxes and weighting the fluxes by streamflow duration, evaporation, and transpiration. In contrast to previous assumptions, recharge from ephemeral-streamflow infiltration occurs not only near the mountain fronts, but also along significant lengths of ephemeral channels. Although most of the ephemeral streams in the subwatershed flow less than a few days per year, the available streamflow quickly infiltrates past depths where it is available for evapotranspiration. This water likely stays in the unsaturated zone until it is vertically displaced by infiltrated water from subsequent streamflows and eventually recharges the regional aquifer. Ephemeral-channel infiltration during 2001 and 2002 was estimated to account for about 12 to 19 percent of the estimated average annual recharge in the Sierra Vista subwatershed.