No abstract

Pinal Creek is an intermittent stream that drains a 200‐square‐mile alluvial basin in central Arizona. Large changes in water levels and aquifer storage occur in an alluvial aquifer near the stream in response to periodic recharge and ground‐water withdrawals. Outflow components of the ground‐water budget and hydraulic properties of the alluvium are well‐defined by field measurements; however, data are insufficient to adequately describe recharge, aquifer‐storage change, and specific‐yield values. An investigation was begun to assess the utility of temporal‐gravity surveys to directly measure aquifer‐storage change and estimate values of specific yield. The temporal‐gravity surveys measured changes in the differences in gravity between two reference stations on bedrock and six stations at wells; changes are caused by variations in aquifer storage. Specific yield was estimated by dividing storage change by water‐level change. Four surveys were done between February 21, 1991, and March 31, 1993. Gravity increased as much as 158 microGal ± 1 to 6 microGal, and water levels rose as much as 58 feet. Average specific yield at wells ranged from 0.16 to 0.21, and variations in specific yield with depth correlate with lithologic variations. Results indicate that temporal‐gravity surveys can be used to estimate aquifer‐storage change and specific yield of water‐table aquifers where significant variations in water levels occur. Direct measurement of aquifer‐storage change can eliminate a major unknown from the ground‐water budget of arid basins and improve residual estimates of recharge.

A numerical groundwater model was developed to simulate seasonal and long-term variations in groundwater flow in the Sierra Vista subwatershed, Arizona, United States, and Sonora, Mexico, portions of the Upper San Pedro Basin. This model includes the simulation of details of the groundwater flow system that were not simulated by previous models, such as groundwater flow in the sedimentary rocks that surround and underlie the alluvial basin deposits, withdrawals for dewatering purposes at the Tombstone mine, discharge to springs in the Huachuca Mountains, thick low-permeability intervals of silt and clay that separate the groundwater flow system into deep-confined and shallow-unconfined systems, ephemeral-channel recharge, and seasonal variations in groundwater discharge by wells and evapotranspiration. Steady-state and transient conditions during 1902-2003 were simulated by using a five-layer numerical groundwater flow model representing multiple hydrogeologic units. Hydraulic properties of model layers, streamflow, and evapotranspiration rates were estimated as part of the calibration process by using observed water levels, vertical hydraulic gradients, streamflow, and estimated evapotranspiration rates as constraints. Simulations approximate observed water-level trends throughout most of the model area and streamflow trends at the Charleston streamflow-gaging station on the San Pedro River. Differences in observed and simulated water levels, streamflow, and evapotranspiration could be reduced through simulation of climate-related variations in recharge rates and recharge from flood-flow infiltration. GroundWater Flow Model, Sierra Vista and Sonoran Portions of the Upper San Pedro Basin, Arizona and Mexico Member agencies and individuals of the Upper San Pedro Partnership were helpful in collecting and disseminating information used to construct the groundwater flow model. Individual knowledge of historical water use was often useful in developing model input when documentation was insufficient. The Morris K. Udall Center was instrumental in arranging for the dissemination of hydrogeologic information that was collected in support of mining operations in Mexico. Recent studies and data made available by the U.S.

[1] Significant variations in interannual and decadal recharge rates are likely in alluvial basins of the semiarid southwestern United States on the basis of decadal variations in climate and precipitation and correlation of El Niño with high rates of winter precipitation and streamflow. A better understanding of the magnitude of recharge variations in semiarid and arid regions would reduce water budget uncertainty. Variability of ephemeral channel recharge with climate in southeastern Arizona was investigated through analysis of hydrologic monitoring near three ephemeral streams in southeastern Arizona during the middle to late 1990s and by relating the results to long-term hydrologic and climatic trends. The analysis used precipitation, streamflow, water levels in wells, estimates of groundwater storage change from repeat gravity surveys, and two climatic indicators of El Niño-Southern Oscillation (ENSO), Southern Oscillation index, and Pacific Decadal Oscillation (PDO). Results indicate that variations in winter recharge are related to ENSO. El Niño conditions correspond with a greater probability of high rates of winter precipitation, streamflow, and recharge. La Niña conditions are almost exclusively associated with below-average recharge. Rates of recharge along Rillito Creek near Tucson during 1977-1998, a period of frequent El Niño conditions and positive PDO values, were 3 times recharge rates during 1941-1957, a period dominated by La Niña conditions and low PDO values. Quantification of recharge variability with decadal climate cycles should improve estimates of rates of aquifer drainage and replenishment in the region. Similar methods are applicable to other regions where thick unsaturated zones can accept significant periodic recharge.

zone until it is vertically displaced by infiltrated water from subsequent streamflows and eventually recharges the regional aquifer. Ephemeral-stream 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.