Wet/Dry Mapping as a Monitoring Technique
Wet/dry mapping is a simple concept for monitoring changes in surface water extent, and has been relatively straightforward to apply. Despite some turnover among organizers and participants, the methods have remained consistent for more than a decade, aside from minor refinements in the data forms and training. The resulting data have allowed both quantitative analysis and spatial representations of conditions.
Wet/dry mapping has provided several insights that were different from those offered by other monitoring techniques. The total wetted length within SPRNCA was correlated with stream flow at the Charleston gage, but those flow measurements could never describe the spatial arrangement of wet and dry stream channel, and wet/dry mapping provided no details on the rate of stream flow. Wet/dry mapping also showed that identification of perennial reaches depends critically on which years are examined.
Project costs included a one-time expense of $2,000 for 9 GPS units, along with annual costs of approximately five days of BLM and TNC staff time for training and survey events. Data processing and creation of display maps require about another seven days of staff time each year. Expansion of the project beyond the SPRNCA to the entire length of river in the US and Mexico has benefitted from efficiencies of scale, requiring very little increase in annual costs.
The Role of Citizen Science
The term “citizen science” has been used for projects in which volunteers, who may have no scientific training, collect data for research or monitoring projects directed by scientists. The approach has been used for a variety of efforts, including the Audubon Society’s Christmas Bird Count, which has run annually since 1900. Citizen scientists have worked on several other river conservation efforts in Arizona, including water quality sampling on the Santa Cruz and Verde Rivers. Such efforts often include social goals, such as education and public participation in politically-charged technical issues (Clark and Illman 2001). There is a growing need for tools to resolve conflicts associated with water management, and this type of participatory, citizen science may be helpful for basins where conflicting opinions exist in the absence of adequate streamflow data, or as a supplement to standard stream gages.
Citizen scientists have been critical to the wet/dry mapping project, and this volunteer approach had social and scientific benefits. This project engaged hundreds of volunteers in understanding and appreciating the flow variability in their local river. They included elected officials, military personnel, environmental activists, ranchers, business representatives, land managers, academics, and regulatory agency staff getting their feet wet and sharing an interest in the river’s life. As a result of their involvement, we have been able to create a useful data set with a small investment of time and money.
Implications for San Pedro River Management
Adaptive management toward the sustainable yield of groundwater in this region will require that monitoring programs, such as the wet/dry mapping effort, provide feedback on the effectiveness of a wide spectrum of management measures. Only by considering how surface flows are changing spatially, in a comprehensive manner, can we fully understand the combined effects of management efforts across the watershed.
As an example, the longest permanently wet reach, occurring in Segments 5 and 6, coincided with an area of shallow clay and near-surface bedrock (Fig. 2; Gettings and Houser 2000; Pool and Dickinson 2007). While continual base flows in these segments were documented in the earliest years of this project, they are also almost perfectly coincident with rising regional groundwater elevations from a City of Sierra Vista effluent recharge facility which began operations in 2002 (Schmerge and others 2009). While recharged effluent is likely augmenting the magnitude of base flows in that reach, no statistically-significant change in the spatial extent of flows is evident. Changes in the magnitude of flows are still best detected by conventional stream gage monitoring.
The increasing length of wetted channel in Segment 2, in contrast to other segments, suggests a localized change in groundwater availability. During the period of this study, conservation land purchases retired 114 ha of irrigated farm fields near or adjacent to the river in Segments 1 and 2, likely reducing water consumption in that vicinity (TNC unpublished data). Wet/dry mapping cannot prove causality for changes in baseflows within a complex hydrologic system. However, we do believe the method holds great promise for distinguishing system-wide influences on flows, such as climate change, from the localized influences of groundwater withdrawals or artificial recharge. These insights can be helpful for refining conservation strategies.
Results of the wet/dry mapping project have had several applications during the past decade. The organizing board of the Upper San Pedro water district included the 2008 wet/dry map in their comprehensive water resources plan (ADWR 2010). The USGS used wet/dry data to delineate study reaches for riparian community conditions (Leenhouts and others 2006). The BLM has used the data to better understand the distribution and effects of beaver activity on surface flows since their reintroduction, and academic researchers have used the data to help design various research and monitoring efforts (e.g., Stromberg and others 2006).
Implications for Ecological Research
Recent studies have noted that spatial intermittency in surface flow has large effects on ecological conditions within aquatic and riparian systems (Boulton and Lake 2008; Larned and others 2010; Stanley and others 1997; Stromberg and others 2005). Repeated measurements in a few systems have shown complex longitudinal patterns in flow permanence (the proportion of time that water is present) leading to identification of long-term average flow permanence as a driver of community structure in intermittent streams (see review by Larned and others 2010). One measure of this has been the percentage of time with surface flow at a site in a given year (Stromberg and others 2005).
This study shows that flow permanence during a predictably dry season can vary across time as well as space in a groundwater-fed, main stem river. While some reaches of various lengths were consistently wet and thus perennial, total wetted length varied widely from year to year within most analysis segments and across the study area as a whole. Thus, using a single-year assessment of flow status may give an inaccurate basis for analyzing the ecological community in a spatially-intermittent stream. It also suggests the need to test whether average flow permanence or a percentage of days with flow is the best metric for the ecological effects of intermittent flow, or if some minimum value has greater explanatory value.
Other Applications of the Wet/Dry Mapping Technique
This approach has been used in other stream systems around Arizona, including Cienega Creek (conducted by TNC, BLM, and Pima County), Agua Fria River (Arizona NEMO), and several tributaries of the San Pedro (TNC).
Wet/dry mapping to monitor seasonal changes in flow length and groundwater conditions on part of Cienega Creek have been conducted by the Pima Association of Governments since 1999 (Fonseca 2008). Surface flow was mapped for ten years prior to that by a consultant using aerial surveys, but they changed to walking surveys after the riparian forest canopy closed over perennial reaches. Pima County has also used their wet/dry data to identify discharge cells for MODFLOW hydrologic models, and to justify an instream flow water right application by documenting the extent of perennial flow (J. Fonseca, personal communication).