Determining the riparian groundwater use within the San Pedro Riparian National Conservation Area and the Sierra Vista Sub-Basin, Arizona

The Upper San Pedro Basin in Cochise County, Arizona, hosts a growing population of human residents as well as several Federal establishments including the Fort Huachuca Army Installation, Coronado National Monument and National Forest, and the Bureau of Land Management’s San Pedro Riparian National Conservation Area (SPRNCA). Ground water is currently the primary source of water for human residents. It also sustains the health of the riparian ecosystem and provides base flows for the San Pedro River. The Upper San Pedro Basin is divided into the Sierra Vista and Benson Subwatersheds. Most of the SPRNCA is contained within the Sierra Vista Subwatershed. The largest municipalities in the Sierra Vista Subwatershed are Sierra Vista, Bisbee, Tombstone, and Huachuca City. Development distributed in rural parts of the watershed is increasing. Fort Huachuca is also an important component of the economy within the Sierra Vista Subwatershed and in southern Arizona in general. The total population in the subwatershed is about 72,500. Water outflow from the subwatershed, including water withdrawn by pumping, exceeds natural inflow to the regional aquifer within the subwatershed. As a result, ground-water levels in parts of the subwatershed are declining and ground-water storage is being depleted. The continued decline of ground-water levels upgradient from perennial river reaches will eventually diminish the base flow of the San Pedro River and imperil the riparian vegetation within the SPRNCA. Residents of the subwatershed have responded to these water issues by forming the Upper San Pedro Partnership (USPP). The USPP is a consortium of 21 agencies and organizations formed to ensure long-term water needs are met, both for the residents of Sierra Vista Subwatershed and for the portion of the SPRNCA within the subwatershed. The USPP determined that specific information regarding the water needs of the riparian system is needed in order to reach the goal of meeting the water needs of the people and the riparian system. The USPP assembled a multidisciplinary team of researchers and initiated a study, detailed in this document, to evaluate the water needs of the riparian system within the SPRNCA. The term â€øe}water needs,” in this context, refers both to riparian water use through evapotranspiration (ET) and to the hydrologic conditions needed to sustain levels of riparian condition. The term â€øe}riparian” refers to transitional areas between terrestrial and aquatic ecosystems that depend on the existence of surface or subsurface water flows. The riparian corridor in the SPRNCA is a band along the San Pedro River that encompasses low-flow channel bars, streambanks, and post-entrenchment flood plains, as well as pre-entrenchment terraces at a higher elevation than the current flood plain. The corridor is vegetated, in part, by phreatophytic plants that use ground-water from the stream alluvium. This study is a coordinated effort by the U.S. Geological Survey (USGS), the U.S. Department of Agriculture, Agricultural Research Service (USDA–ARS), and Arizona State University, with assistance from the U.S. Army Corps of Engineers, the University of Wyoming, and the University of Arizona. The specific objectives of the study were: to determine the water needs of riparian vegetation through the riparian growing season and throughout the SPRNCA to ensure its long-term ecological integrity; to quantify the total water use of riparian vegetation within the SPRNCA; and to determine the source of water used by key riparian plant species within the SPRNCA. To meet these objectives, the study was divided into three elements: (1) a characterization of the status and variability of hydrologic factors within the riparian system (USGS), (2) a riparian biohydrology study to relate spatial and temporal aspects of riparian changes and condition to the hydrologic variables (Arizona State University), and (3) a water-use evapotranspiration (ET) study to quantify annual consumptive ground-water use by riparian transpiration and direct evaporation from the stream channel (USDA–ARS) in cooperation with the U.S. Army Corps of Engineers, the University of Wyoming, and the University of Arizona. Twenty-six sites within the SPRNCA were selected for collection of vegetation data from three primary streamflow regimes (perennial, intermittent-wet, intermittent-dry), which include the principal vegetation communities. Detailed hydrologic-condition data were collected at a subset of 16 of these sites, called the SPRNCA biohydrology sites. Water-use and water-source data were collected at a subset of 5 of the 16 biohydrology sites. Vegetation data also were collected at supplemental sites within the SPRNCA boundary in the Upper San Pedro Basin and in the Lower San Pedro Basin. In addition to information about vegetation and geomorphic conditions, hydrologic data collected at the 16 biohydrology sites were used to delineate 14 reaches that were internally homogenous in terms of streamflow hydrology (spatial intermittence of streamflow) and geomorphology (channel sinuosity and flood-plain width). Although this overall study consisted of three elements, the elements were closely coordinated to derive integrated results. Specifically, the connection between water demand, water availability, and riparian functioning represents a synthesis of the study elements. The effects of intra- and inter-annual as well as spatial variability of hydrologic and riparian factors were observed in each of the three study elements.