Timing and Duration of Recharge to a Shallow Aquifer Using Continuous-Slope-Area Streamflow Data and Water Levels from Monitoring Wells

Monday, April 20, 2009: 4:30 p.m.
Joshua Tree (Hilton Tucson El Conquistador Golf & Tennis Resort )
Jeffrey T. Cordova , Arizona Water Science Center, USGS, Tucson, AZ
Jesse E. Dickinson , Arizona Water Science Center, USGS, Tucson, AZ
The San Pedro River is one of the largest remaining undammed rivers with extensive perennial reaches in the semiarid southwestern United States.  Riparian areas along the river support several endangered species and provide important habitat for migratory birds.  Most reaches of the San Pedro River are dry throughout most years; however, some reaches are perennial where ground water discharges to the surface.  Monsoon rain storms from mid June through September produce ephemeral streamflow events lasting several hours to about a week in some reaches.  In 2005, the U. S. Geological Survey, in cooperation with the Arizona Department of Water Resources, began an investigation of the hydrogeology of parts of the Upper and Lower San Pedro basins.  One objective of the study is to characterize ground-water and surface-water interactions and evaluate system responses to monsoon rain storms and dry periods.  Continuous ground-water level measurements at shallow, near stream piezometers are compared to streamflow records to infer the timing and magnitude of recharge to alluvial sediments at two reaches of the San Pedro River.  Streamflow is calculated by an application of the continuous slope-area method using continuous stream-stage data.   Data collected during monsoons in 2007 and 2008 indicate that ground-water levels generally do not increase except during and after streamflow events greater than 500 cubic-feet per second.  The largest ground-water level responses (on the order of several feet) are caused by streamflow events lasting a week or more, and remain elevated for several weeks after cessation of streamflow.  These data can be used to infer the locations and magnitude of recharge along the San Pedro River and to constrain hydrologic parameter values needed for numerical models of ground-water/surface-water interactions.