Wet/Dry Mapping in Arizona
Many people in the arid Southwest care about the fate of perennial streams and their associated riparian communities. The loss of flows in streams and rivers has social, economic, and ecological consequences, so managers and concerned citizens seek ways to track their status. This need has grown in importance and awareness as drought conditions have deepened.
In 1999, the Nature Conservancy launched a volunteer program involving scientists, managers, and citizen volunteers in southern Arizona who participated in a low-cost, comprehensive, and enjoyable method for monitoring our rivers—wet/dry mapping. The timing to begin an effort to track water presence change over time in the iconic, binational San Pedro River was coincidentally the start of megadrought conditions, and resulting aridification, in the Colorado River Basin, of which the San Pedro River is a tributary.
Every year on the third Saturday in June, people walk or ride horses along desert streams to map where the streams have water present and where they are dry. With a decades-long, uninterrupted dataset, this work is helping scientists and managers better understand and manage our riparian and aquatic habitats. The data have been consistently collected at the end of the dry summer months, right before the monsoon rains typically begin. Mapping at that point captures a point in time when any water will be present due to either a gaining groundwater-fed reach or where previous year’s precipitation events were of adequate frequency and/or magnitude for water to persist months later.
Wet/dry mapping uses citizen scientists, TNC staff and conservation partners to map the extent of water presence in a river where certain reaches dry up during the spring and summer. It provides a snapshot of conditions along the entire river at the same week each year, allowing comparisons of year-to-year variability.
Applications of wet/dry mapping
Wet/dry mapping has been used to:
- quantify long-term trends in surface water patterns
- better understand groundwater/surface water interactions
- identify near-steam properties for conservation projects, such as easements and recharge
- identify study reaches to design and implement ecological research and monitoring
- manage fish and wildlife populations and riparian habitat
Wet/dry mapping in the San Pedro River watershed
The Nature Conservancy has conducted wet/dry mapping every June since 1999 on the San Pedro River and its tributary streams. The extent of river and tributary miles mapped has grown from the 50 miles protected by the San Pedro Riparian National Conservation Area (SPRNCA) in 1999, to well over 100 miles of reaches in Mexico, the middle and lower reaches, and nearly as many tributary miles, such as the Babocomari River, Ramsey, Miller, Hot Springs, and Redfield Canyons, and Los Fresnos in Mexico. Partners in the U.S. have included the Bureau of Land Management, Arizona Game & Fish Department, Arizona Department of Forestry and Fire Management, Community Watershed Alliance, Friends of the San Pedro River, Fort Huachuca, National Park Service, Cascabel Volunteers, Salt River Project, and Pima and Cochise Counties. Partners mapping the headwaters in Sonora, Mexico include Comisión Nacional de Áreas Naturales Protegidas,, BIDA, Reserva Forestal Nacional y Refugio de Fauna Silvestre Ajos-Bavispe, and Naturalia.
Data from wet/dry mapping in the San Pedro Riparian National Conservation Area (SPRNCA) reveals that wetted length varies from year to year in the 14 hydrologically distinct reaches along that 50-mile section of the river, and has been critical to help inform research and monitoring projects, including studies of riparian conditions within the SPRNCA. Wet/dry mapping data is analyzed as one of 14 indicators of groundwater sustainability that are of interest to members of the Upper San Pedro Partnership who manage various aspects of water in the Upper San Pedro Basin. Wet/dry mapping data is featured as one dataset readily available on the Web-based Hydrological Information Portal for the Upper San Pedro Basin (WHIP) launched in early 2022.
The length of wetted channel through the SPRNCA correlates with the volume of flow at the USGS Charleston stream gauge on the day of each wet/dry survey, validating a strong relationship between these approaches. However, the two monitoring methods tell different, but complimentary stories. Stream gauges provide an understanding of hourly changes throughout the year at a fixed point on the stream, while wet/dry mapping helps us grasp landscape-scale changes throughout the entire river on a fixed date.
Additional information on how wet/dry data have been analyzed can be found in a published paper by TNC scientists Dale Turner and Holly Richter, Wet/Dry mapping: using citizen scientists to monitor the extent of perennial surface flow in dryland regions.
Wet/dry mapping on other rivers
The wet/dry mapping approach has been applied in other stream systems around Arizona, including Agua Fria River (Arizona NEMO), and Cienega Creek (conducted by TNC, BLM, and Pima County), and tributaries of the San Pedro River such as the Babocomari River, Ramsey, Miller, Hot Springs, and Redfield Canyons, and Los Fresnos in Mexico.
Wet/dry mapping conducted since 2006 along Cienega Creek has revealed locations of previously-unmapped perennial flow. While locals knew better, official state maps did not recognize a major portion of Upper Cienega Creek, and two flowing reaches of its tributary, Empire Creek, as perennial. These locations were subsequently omitted from mapping efforts such as Arizona Game and Fish Department's statewide inventory of riparian vegetation in the mid-1990's. Further downstream where the creek has been tracked since 1984, once-reliable reaches now go dry in some years.
The value of citizen science
Wet/dry mapping turns a walk through the cottonwood forest into meaningful science. Participants in the past have included ranchers, realtors, regulatory agencies, environmentalists, City Councilmen, children, and reporters. It gives interested citizens a chance to learn more about their rivers and get their feet wet in the ecosystem. Along the way, mapping volunteers have encountered species such as coatimundi, mountain lion, leopard frogs, bear, Gila monsters, bobcats, gray hawks, and longfin dace.
Wet/dry mapping is simple. Using hand-held GPS units, people record the start- and end-points of every wet portion along a stream or river. Using GIS software, these points are translated to lines on a map for display and analysis. To learn more, download detailed instructions and sample data forms for conducting wet/dry mapping.