A new report from the USGS finds that regional groundwater contributes about 3/4 of the base flow of the San Pedro River near Tombstone, with 1/4 coming from "summer storm runoff stored as alluvial groundwater for the 2000 to 2009 period."The authors conclude that "Continued regional groundwater pumping will, however, eventually lead to a decline in the contribution of regional groundwater to base flow."
Ref: Kennedy, J.R., and Gungle, Bruce, 2010, Quantity and sources of base flow in the San Pedro River near Tombstone, Arizona: U.S. Geological Survey Scientific Investigations Report 2010-5200, 43 p.