Water Resources Research Act Program

Details for Project ID 2007AZ190B

Riparian Vegetation Response to Cessation of Groundwater Pumping, Lower San Pedro River, Arizona

Institute: Arizona
Year Established: 2007 Start Date: 2007-03-01 End Date: 2008-02-28
Total Federal Funds: $11,990 Total Non-Federal Funds: $24,156

Principal Investigators: Juliet Stromberg

Project Summary: Hundreds of millions of dollars are being spent on the restoration of riparian ecosystems throughout the Southwest, often without sufficient scientific background to inform the efforts and ensure success. A novel restoration approach has been pioneered along the lower San Pedro River by the Nature Conservancy of Arizona and other collaborating groups. The lower San Pedro River Basin constitutes an important conservation landscape in southeastern Arizona, but, on some reaches, groundwater and surface water have declined below threshold levels needed to sustain cottonwood-willow forests and emergent wetlands. In some areas, tamarisk shrublands now dominate the floodplain, and the stream channels are wide and dry. The restoration approach involved purchase of farms that pumped large quantities of alluvial groundwater for crop irrigation, and subsequent reduction of pumping rates to negligible levels. The assumption is that the biotic components of the riparian ecosystems will establish on their own accord, following restoration of the hydrologic regime, thereby obviating the need for restoration plantings. There is a need to document the results of this hydrologic restoration strategy to assess its effectiveness on the San Pedro River as well as its applicability to other settings. In 2002 and 2003, we initiated baseline monitoring at seven restoration research sites and five reference sites on the Lower San Pedro. We are requesting research funds to allow for another year of data collection and for data analysis and synthesis. Data will be collected on metrics that should change rapidly in response to hydrologic restoration (herbaceous vegetation composition and diversity along the low-flow channel; annual growth increments of Fremont cottonwood and Goodding willow trees; woody tree seedling densities) and those that will change over a longer time span (woody vegetation density, composition, and age structure; floodplain patch structure; stream geomorphology). Our results will provide managers with feedback about the effectiveness and time-span of this restoration approach. Another important benefit relates to informing approaches to the management of tamarisk, considered by some to be a problematic species. We expect that the hydrologic changes at the Lower San Pedro restoration sites will drive shifts in woody riparian plant composition from tamarisk to cottonwood/willow; if so, this study will provide a demonstration of an alternate approach to traditional invasive species management.