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Assessment of Urban-Enhanced Recharge in the Sierra Vista Subwatershed, Arizona

Project Number: 9671BUW (Special Initiative)
Project Chief: James Callegary
Cooperating Agencies: The Army Garrison at Fort Huachuca
Period of Project: October 2006 – September 2008

INTRODUCTION

For the past several years the Arizona Water Science Center has received Special Initiative funding annually from USGS Headquarters to conduct relatively small-scale projects designed to develop innovative approaches to hydrologic investigations or improve new strategies for data collection. In addition, the Arizona Water Science Center provides funding for a student, who participates in the research and can use the reports or journal articles produced as part of the requirements to complete a Master's Thesis or a Ph.D. Dissertation. These projects can be used as proof-of-concept for research ideas that can later be scaled up in cooperation with other federal, state or local agencies.

NEED FOR STUDY AND BACKGROUND

Stakeholders in the Sierra Vista Subwatershed of the Upper San Pedro Basin are currently under significant pressure from the U.S. Congress and other groups to attain a sustainable level of ground-water development. A partial solution to the problem has been identified as increased ground-water recharge caused by urbanization. Impervious surfaces created by urbanization concentrate additional runoff in ephemeral stream channels where, in semi-arid to arid climates, recharge is more likely to occur. Previous estimates of enhanced recharge due to urbanization are large compared with engineered recharge projects in the Sierra Vista area. It has not been suggested that the volume of increased recharge exceeds the increased pumping associated with urbanization, simply that the increase partially mitigates pumping withdrawals. Owing to the difficulty in estimating recharge, the values determined for post-development increase have high levels of uncertainty. These values have, however, been used as part of a list of mitigation credits included in the annual report to congress regarding progress toward sustainable yield in the subwatershed as required in P.L. 108-136, Section 321.

Figure 1. Map of study area showing Coyote, Graveyard and Woodcutter’s washes, the town of Sierra Vista and the San Pedro River, as well as the ten ephemeral-stream channel gages.

SCOPE

The objective of this project is to use numerical modeling, and monitoring of precipitation and ephemeral-stream channel flow and losses to improve estimates of increased recharge resulting from urbanization in the Sierra Vista-Ft Huachuca area. This work will build upon and integrate previously unrelated field and modeling efforts by a number of entities including the USGS, ARS, and two consulting firms, Stantec and GeoSystems Analysis. The study location is the ephemeral-stream channel system in and near the City of Sierra Vista and the Army Garrison at Ft Huachuca. In this area, a large number of detailed data types have already been collected including ephemeral-stream vegetation and channel characteristics, housing-development-scale rainfall-runoff studies and monitoring and estimation of hydraulic parameters at six unsaturated zone sites. The use of these diverse data sets will give the modeling effort a more reality-based foundation than was available to previous workers. The data were collected between 2000 and the present, and the proposed study is expected to take two years to complete. The first year will include compilation and analysis of data, installation and monitoring of precipitation and stream gages, and initiation of modeling. In the second year, modeling will be completed, and a journal article written and published.

APPROACH

The first stage involved the collection, integration and analysis of already existing data sets. Three examples of which are described here. First, USGS and GeoSystems Analysis collectively have installed about 6 exploration boreholes/monitoring wells in or downstream from urbanized areas to monitor infiltration and percolation. These holes have provided valuable, but highly site-specific information that has been the basis of ephemeral channel recharge estimates made to date. Second, USGS has completed an ephemeral-stream channel reconnaissance that has resulted in a map of recharge potential in two channels within and downstream of Sierra Vista. The reconnaissance has allowed an extrapolation of the properties measured in the boreholes mentioned previously to a larger area. Third, USGS has operated a nested pair of streamflow-gaging stations in a housing development in Sierra Vista allowing for a comparison of an undeveloped and a developed site having similar drainage areas. The outcome is an improved understanding of the manner in which urban development and impervious surfaces change total runoff and peak flows under the climatic conditions and development styles specific to the Sierra Vista area.

In the second stage of the project, precipitation and ephemeral-stream channel flow are being monitored. Precipitation and flow data are required to calibrate numerical models and to quantify uncertainty about the flow system. If flow is continuous between two gages and no inflow occurs from side channels or floodplain runoff, the difference between discharge at the two gages can be used to estimate the volume of water that infiltrates. To measure urban-enhanced streamflow and recharge in the Sierra Vista-Ft Huachuca area, we have installed 10 continuous-stage gages and plan to install streamflow-duration loggers and scour chains. These have been installed in Woodcutters’ Wash (four in the main channel and one in an un-urbanized tributary), one in Coyote Wash, and four in Graveyard Wash (including two in an un-urbanized tributary). The locations of the monitoring sites were based upon factors such as access, maximizing spatial coverage, site characteristics, and owner permission. At each gage, three logging pressure transducers have been deployed in the thalweg (deepest portion of channel). Transducers were installed in stainless steel casings to protect them from damage and to minimize movement during flow events (fig. 2). To estimate volumetric flow, the continuous-slope area method will be applied to the runoff data collected by these instruments. At locations between gaging sites, the presence or absence of flows will be monitored with stream-duration loggers.

Figure 2. Diagram and photos of ephemeral-stream channel gage installation.

The third stage of the project is enhanced-recharge estimation modeling. This will be based on the software GSFLOW, which allows for the combined modeling of ground-water and surface-water. Input to the model will include the data sets just described as well as previously developed flow models and GIS data sets.

Figure 3. Estimates of increased runoff due to urbanization from a comparison of two small watersheds, one urbanized and one un-urbanized, will be used to estimate enhanced recharge.

RELATION OF PROJECT TO OTHER STUDIES

The outcome of this study has direct bearing on the evaluation of ground-water sustainability in the Sierra Vista Subwatershed which is a key component of other USGS work in the area. Although the scope of this project does not include the entire Subwatershed, there is keen interest on the part of the Upper San Pedro Partnership in this work, and a reasonable possibility that this study could serve as a proof of concept for a larger study that would encompass the entire Sierra Vista Subwatershed.