Assessing the State of San Pedro Water Resources

Water availability in the middle San Pedro watershed in southeast Arizona is strongly dependent on climate and water use, according to a new water resource assessment by the U.S. Geological Survey, in cooperation with the Arizona Department of Water Resources.

Groundwater is the primary source of water for municipal, domestic, industrial and agricultural use in the middle San Pedro watershed. Demand for water resources is projected to increase to meet the water needs of the growing population in the region. An improved understanding of available water resources and how water moves through the watershed can improve the capability of resource managers to optimally use this important resource and minimize or mitigate the effects of development.

Findings characterize the current state of water resources and reveal how water resources have changed in the past. The new report describes how surface water and groundwater move throughout the system and evaluates water quality and chemistry throughout the watershed. The full report can be accessed online.

“Recognizing the effects of water use in the watershed are likely to be difficult and complex,” said lead author of the report and USGS scientist Jeffrey Cordova. “The information gathered in this report lays a strong foundation for the potential to develop a hydrologic model, which would be a useful for simulating the potential effects of groundwater use on the aquifer, streams and surrounding areas.”

The study analyzed how the groundwater system changed after the middle San Pedro watershed was developed as a water resource. During predevelopment times, it’s assumed that the total average inflow of water into the watershed was balanced by the average total water outflow. In the Benson subarea for the period after development from 2001–2009, the average total outflow exceeded the average total inflow by an amount of water equivalent to around 3,000 Olympic swimming pools per year (6,000 acre-feet).  During predevelopment times, base flow leaving the Benson subarea was equivalent to 2,700 Olympic swimming pools per year (5,500 acre-feet). As of 2009, the base flow leaving both the Benson and Narrows-Redington subareas is now zero as groundwater discharges only in select reaches of the San Pedro River and only for part of the year.

Climate has also greatly influenced the amount of water available throughout the San Pedro watershed over the past few decades. The study found that groundwater and streamflow responded to periods of higher precipitation in the mid-1980s to mid-1990s, as well as to periods of overall lower precipitation in the 1960s through mid-1980s and mid-1990s to 2009. The median annual streamflow of the San Pedro River near Tombstone decreased by 50 percent between the periods of 1968–1986 and 1997–2009. The amount of water entering the soil from the San Pedro River, also known as streamflow infiltration, has also decreased 44 percent from 1914–2009. 

The middle San Pedro watershed includes the northern portion of the Upper San Pedro Basin of Cochise County and the southern portion of the Lower San Pedro Basin of Cochise, Pima and Graham counties in southeastern Arizona. The Upper and Lower San Pedro Basins are groundwater basins designated by the state of Arizona. The middle San Pedro watershed covers an area of about 2,100 square miles and is about 70 miles from south to north, and about 20 to 30 miles wide.

This is the second USGS report that describes the various aspects of the hydrology of the middle San Pedro watershed. The first report describing the hydrogeologic framework is available online.

For more information on water studies conducted in Arizona, visit the USGS Arizona Water Science Center website.