January-February 2009

Commentary - Giving Credit Where Credit Is Due

In the November/December 2008 issue of Environment, the article “Saving the San Pedro River: Science, Collaboration, and Water Sustainability in Arizona” (pages 30–42) contained a draft version of the map on this page that was incorrectly attributed. The map, recently published by Stan Leake, Don Pool, and Jim Leenhouts of the United States Geological Survey (USGS), shows potential capture by groundwater wells of surface water that would otherwise be available to the river and riparian vegetation in the upper San Pedro basin, illustrating the differential effect on river flows of groundwater withdrawals in various parts of the basin. The report, USGS Scientific Investigations Report 2008-5207, is titled Simulated Effects of Ground-Water Withdrawals and Artificial Recharge on Discharge to Streams, Springs, and Riparian Vegetation in the Sierra Vista Subwatershed of the Upper San Pedro Basin, Southeastern Arizona, and can be found at http://pubs.usgs.gov/sir/2008/5207/.

We appreciate this opportunity to set the record straight and point out how useful this capture map is as a tool to communicate to land use planners and other land and water managers the outcomes of a large and complicated groundwater flow model. It conveys a clear message: there are places in the San Pedro basin where groundwater pumping will have a much more immediate impact on the flows of the San Pedro and the water available for riparian vegetation than others. Though all groundwater pumping is likely to have an impact on river flows at some point in time and space, this map shows that all pumping is not equal—there is a major lag between groundwater pumping and river flow impacts in the blue and green areas of the map, while pumping in the red and orange areas should be limited as much as possible. Responses to groundwater withdrawals (and similarly, to groundwater recharge) are greatest near the river, but the presence of clay layers in some portions of the basin delays the response. As a key partner in the San Pedro Partnership, the USGS has been providing major scientific insights that form the underpinnings of decision support tools and evaluation of conservation options.

Katharine L. Jacobs
Arizona Water Institute
Tucson

George Saliba
Baltimore County Department of Environmental Protection and Resource Management
Maryland

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