Trends
in Streamflow of the San Pedro River—A U.S. Geological Survey
Study
“Trends in Streamflow of the
San Pedro River, Southeastern Arizona” is a study that was sponsored
by the Upper San Pedro Partnership and prepared by the United States
Geological Survey in cooperation with the Bureau of Land Management.
The complete study fact sheet is available
here.
What were the key findings of this study?
"Statistical analyses indicate
that seasonal pumping from wells near the river for irrigation in the
spring and summer was a major factor in the decrease in low flows and
that year-round pumping from wells in the regional aquifer away from
the river was not a major factor in the decrease in low flows…
These conclusions are for trends from 1913 to 2002, and regional U.S.
and Mexico pumping could affect streamflow at Charleston in the future…(Thomas,
2006)”
A review of the USGS study also tells us that:
- Annual rainfall followed the regional trends of other basins in the southwestern part of the state and was not a central factor in reduced stream flows for the San Pedro River.
- Changes in vegetation along the river system were major factors in reduced stream flows. The river system changed dramatically during the study period from a predominantly upland grassland system to the present day system of dense, woody, riparian vegetation.
- Groundwater pumping in the United States and Mexico had a mixed influence on stream flows, depending upon the proximity of the pumping to the River and the volume of water pumped.
- Seasonal pumping from irrigation wells near the River was a major factor in decreased stream flows.
- Year-round pumping from wells in the regional aquifer away from the River was not a major factor.
What does it all mean?
- The reduction in river flows at the Charleston gauge from 1913 to 2002 was not attributable to City and Fort groundwater pumping.
- Irrigated agriculture pumping from wells near the river had a major impact on stream flows.
- Precipitation is a critical component of our local water supply, but the trends toward decreased rainfall over time did not have a major impact on river flows.
- The transition from grasslands to dense and woody vegetation along the river significantly impacted flows in the river itself. Water consumption by the riparian vegetation has increased dramatically and has directly and substantially reduced stream flows. Over one-third of the annual water used in the basin is attributable to outflows from the River and riparian vegetation.
For a visual representation
of San Pedro River water use and recharge—visit
our “Water Use and Recharge Animation.”
For more information contact:
Blakemore E. Thomas
U.S. Geological Survey
Arizona Water Science Center
520 North Park Avenue, Suite 221
Tucson, Arizona, 85719-5035
bthomas@usgs.gov
www.az.water.usgs.gov
U.S.
Department of the Interior | U.S. Geological Survey
Persistent URL: http://pubs.water.usgs.gov/fs20063004
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