2012 Modeling / Water Sustainability Study
Projecting our water demand and supplies for the next 100 years
As part of efforts to update our Designation of Adequate Water Supply (AWS), in 2012, the City initiated a study to quantify Flagstaff’s current water supplies and project how they can meet our needs over the next 100 years. This study considered variables such as water conservation, growth rate, reclaimed water use, groundwater availability, and surface water availability. It relied on conservative assumptions and examined the possible impacts of future pumping, including the likelihood that pumping would “capture” water that would otherwise flow to springs along the Mogollon Rim and the south rim of the Grand Canyon.
A two-fold approach was used to evaluate our surface water, groundwater, and reclaimed water supplies. The first step entailed identifying the “sustainable yield” of the City’s well fields — in other words, how much we can pump without significantly impacting the aquifer system. It then applied the criteria identified by the Arizona Department of Water Resources (ADWR) for its AWS program.
Groundwater flow modeling
This study relied on a groundwater flow model — a digital representation of the Coconino Plateau aquifers and flow system — to simulate aquifer conditions in and around Flagstaff. This model was based on the Northern Arizona Regional Groundwater Flow Model (NARGFM), which was developed by the USGS in 2011, and used to predict the physical availability of groundwater over the next 100 years. It incorporates information such as water levels, spring flow rates, historical pumping, and measure of aquifer hydraulic properties.
To develop this model, the consultants, AMEC Environmental & Infrastructure (now AMEC Foster Wheeler) — along with Utilities staff, local professionals, and scientists — reviewed the NARGFM’s input variables and parameters to adapt for the goals of this project. A focus area for the Flagstaff model was defined within the NARGFM domain [view study area map]. A “conceptual model” was then developed to define the regional subsurface geology and hydrology of the Coconino (C) and underlying Redwall-Muav (R) aquifers. The following information was used to develop the conceptual model and to calibrate the numerical groundwater flow model to 2010 conditions:
- Water levels in both the C- and R- aquifers, where available, prior to the 1950s were used to represent pre-development conditions. Water levels from the 1950s to 2009 were used to represent pumping conditions. These data show historical trends over time.
- Aquifer thickness maps were developed for the C- and R- aquifers.
- Aquifer hydraulic properties were estimated based on the results of testing over the past several decades at numerous wells owned by the City and other local water providers. These properties include transmissivity, hydraulic conductivity, and storage within the C- and R- aquifers.
- Aquifer recharge (from rain and snow) was estimated for the C-aquifer. Other components of recharge include leakage from Upper Lake Mary; reclaimed water that is discharged to the Rio de Flag by the City and released at Flagstaff Ranch and Forest Highlands; and localized stream flow within the Rio de Flag, Clay Avenue Wash, and known limestone sinkholes such as the “Bottomless Pits” near Continental Country Club.
- Aquifer discharge was estimated from the Coconino Plateau via pumping, springs, and downward flow from the C-aquifer to the R-aquifer. Pumping, which represents the most significant outflow from the groundwater system, was estimated based on records from the City, ADWR, the USGS, and local water providers.
Revisions and updates were also made to the NARGFM groundwater budget.
The results of this modeling effort have not only helped us develop and implement sound water management decisions, but they were also instrumental in convincing ADWR to update Flagstaff’s 1973 AWS Designation, which did not require a physical determination of water supply. This new Designation is significant because it provides long-term assurances to residents, businesses, investors, and others that our water supplies will be sufficient to meet our needs.