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Water Sustainability Study
2012 Sustainability Study Purpose
Economic vitality and community sustainability starts with a reliable, long-term water supply. This sModel Schematictudy provided the City of Flagstaff and its decision makers an evaluation of its water supplies and their long-term sustainability and reliability for citizens here today and future generations. More simply put, the purpose of the study determined “what is Flagstaff’s bucket of water that it can objectively rely upon over the next 100-years?”
The study represents a conservative approach that assumes variables such as water conservation, growth rate, reclaimed water use, groundwater availability and surface water on which to base future projections for water supply. Also incorporated as a focus of the study are spring flows across the region and the effect pumping future water by the City of Flagstaff may have on capturing flow to springs in northern Arizona (primarily along the south rim of the Grand Canyon and along the Mogollon Rim.)

Study Process

The criteria used to evaluate the sustainability and reliability of Flagstaff’s surface water, groundwater and reclaimed water supplies was two fold. First, this study identified what is the “sustainable yield” of groundwater pumping within the City’s well fields. The second criteria evaluated the city’s water supplies against those criteria identified in the Arizona Department of Water Resources (ADWR) Adequate Water Supply program including its proposed revisions and Hydrologic Guidelines (see them here). Interestingly, ADWR does not have criteria for areas of the state with an initial starting water level that is deeper than the criteria in statute, which is drawdown exceeding 1,200 feet below land surface after 100 years pumping. Therefore Flagstaff applied for a depth-to-water exemption as explained in rule R12-15-716(C) in the Arizona Administrative Code. Staff worked with NAU and the USGS to develop criteria for this situation, recommending a not-to-exceed of 50% of the saturated thickness of the aquifer after 100 years of pumping. This rule will be suggested to ADWR for consideration as an adoption into rule once the Governor lifts the moratorium on rule making in Arizona.

In order to better understand and predict the reliability of local groundwater supplies, a major aspect of this study was to construct a numerical groundwater flow computer model of the Coconino Plateau aquifers in and around the City of Flagstaff. This tool was based upon the 2011 U.S. Geological Survey’s Northern Arizona Regional Groundwater Flow Model (NARGFM) and was used to determine the physical availability of groundwater in and around Flagstaff over the next 100-years. The results of this study have helped the City and its decision makers develop and implement sound water management programs and policies.

Modeling Limitations and Assumptions
It is important to remember that groundwater modeling is a tool scientists use to simulate previous and current groundwater conditions, and then to analyze future scenarios and how the groundwater system may respond to a change in conditions, whether pumping conditions or climate conditions for example. Groundwater models are calibrated to past and current conditions, that is, model input includes the information we know about a groundwater system, such as water levels over time in wells, metrics that quantify how the aquifer responds to pumping, the discharge history at springs, or how easily the aquifer transmits water. Specific inputs as part of this study are discussed below.

The USGS NARGFM model was the first publically available three-dimensional regional numerical groundwater flow model for northern Arizona. The consultants on this project, AMEC Environmental and Infrastructure (now AMEC Foster Wheeler), reviewed the model with staff and a number of local professionals and scientists, reviewed the input variables and parameters in the NARGFM in more detail to address in the specifics of the project. Out of this process the following became a focus of the modeling effort: 
    • development of a hydrogeologic conceptual model that utilizes city, consultant, and public (USGS and ADWR) existing hydrological data in order to define the regional subsurface geology and hydrology as it pertains to the Coconino (C) and underlying Redwall-Muav (R) Aquifers within the Flagstaff area. The following information was used to develop the hydrogeologic conceptual model and the information required in order to calibrate the numerical groundwater flow model to 2010 groundwater conditions:
      • the area within the NARGFM model domain in which to focus revision efforts - view study area map
      • water levels - this study utilized both pre-development (prior to 1950s) and transient conditions (1950s to 2009) water level information from wells completed in both the C and R-Aquifers where available. These data represent historical trends in water level changes with time.
      • aquifer thickness - geologic and hydrostratagraphic unit isopach maps for the C and R-Aquifers were developed that reflect the best known historic and current thickness of the aquifers.
      • aquifer hydraulic properties - this study utilized the results of numerous aquifer pumping tests from the City’s water supply wells and from other local water providers that have occurred over the past several decades. The purpose of this analysis was to develop initial estimates of aquifer hydraulic properties including transmissivity, hydraulic conductivity and storage within the C and R aquifers.
    • revisions and updates to the NARGFM groundwater budget
      • aquifer recharge - this study will attempt to estimate the historical annual volume of water that has recharged the C-Aquifer in the Flagstaff area from precipitation as snow and rainfall. Understanding the areal distribution of the recharge and infiltration rates is also important and will attempt to be correlated with the type of geology units at land surface. Other components of recharge include leakage from Upper Lake Mary; infiltration from reclaimed water discharge to the Rio de Flag by the City; reclaimed water disposal from Flagstaff Ranch and Forest Highlands; localized stream flow within the Rio de Flag, Clay Avenue Wash and known limestone sink holes such as the “Bottom-less Pits” near Continental Country Club.
      • aquifer discharge - There are several ways groundwater exits the Coconino Plateau in the vicinity of Flagstaff which includes groundwater pumping, spring discharge and downward migration from the C-Aquifer to the RAquifer. Groundwater pumping is the most significant anthropogenic outflow from the groundwater system and detailed records will be used from the City, Arizona Department of Water Resources, U.S.Geological Survey and local water providers. This study will also attempt to summarize the known spring flows along the Mogollon Rim as well as attempt to estimate the downward movement  of groundwater from the C to R-Aquifer, if possible.

Model Results & Water Planning for Flagstaff

In February 2013, the Arizona Department of Water Resources modified the City of Flagstaff’s original Designation of Adequate Water Supply from 1973, which was not based on the physical water supply determination. The City was first granted a Designation of Adequate Water Supply in 1973, from the Arizona Water Commission, the predecessor to the Department of Water Resources. This designation did not require the City physically prove a water supply. Today’s designation assures the citizens of Flagstaff that for 30 years, the City is financially, legally, physically and continuously capable of meeting current, committed (platted and not served), and projected demands, with a quality of water that meets Arizona Department of Environmental Quality standards.  Under Adequacy rules, the City is required to submit on an annual basis an update of water use and future water demands.  This “checks and balances” approach to water management, coupled with the 100-year groundwater flow modeling exercise, are two ways the Utilities Division contributes to a management strategy that considers economic, environmental and social elements in its planning.