Safe Yield & Water Resource Management

The Arizona Department of Water Resources (ADWR) is responsible for managing our surface water and groundwater. Groundwater and surface water are administered separately in Arizona under differing legal principles and are not legally recognized as hydrologically connected. However, basic principles of hydrology demonstrate that groundwater flows down gradient, usually emerging as the base flow of a river or other surface water body. Thus, when groundwater is withdrawn, surface water flows are diminished, potentially harming surface water right holders and the environment. The legal but unscientific separation of groundwater and surface water drives the controversy surrounding groundwater pumping in the Big Chino Valley that will eventually diminish the upper Verde River.

Under Arizona water law, the prior appropriation doctrine (first in time, first in right) governs surface water rights. The first person to use water (a "senior appropriator") acquires a superior right to future water use and can constrain water use by junior appropriators.

Groundwater is managed under the 1980 Arizona Groundwater Management Act (AGMA), which established five Active Management Areas (AMAs) covering 13% of the state where groundwater supplies were threatened. The Prescott Active Management Area (PrAMA) is the smallest of the AMAs and is entirely dependent on groundwater.

The Prescott AMA was officially declared to be out of safe yield in January of 1999 (See "What is Safe Yield?" below), thus activating the Assured Water Supply (AWS) Rules. These rules attempt to prevent the overdraft (see "How big is the overdraft" below) from increasing by requiring new subdivisions to obtain "alternative" water, which is any water other than AMA groundwater, such as any surface water, recharged effluent, or groundwater from outside the PrAMA. The subdivision sponsor must demonstrate continuous legal, physical, and financial capability for a 100-year supply of this alternative water.

The requirement for alternative water does not apply to smaller private wells that produce less than 35 gallons per minute and irrigate less than two acres (exempt wells). Additionally, 32,000 plats that were filed prior to and in anticipation of the declaration of "out of safe yield" are grandfathered under the old rules. Thus the overdraft has increased despite the AWS rules.

In the 87% of Arizona outside the AMAs, groundwater is governed by the Beneficial Use rule: groundwater is available without limit to any landowner who can demonstrate beneficial use on their property, even though their pumping might someday diminish flow to a neighbor's well or to a spring or stream whose water rights are held by someone else. In some areas, developers and water providers are regulated by the Adequate Water Supply rules which allow subdivisions that prove a 100-year water supply is continuously, legally, and physically available . If unable to so demonstrate, the subdivision may proceed with the available supply or as a dry lot subdivision (lot owners provide their own water) after notifying the first purchasers of the lots that a 100-year supply has not been proven. Or, the developer may divide the property by lot splits with no constraints or assurances as to water use.

A city or county may adopt the Adequacy Rules for all new subdivisions only upon unanimous approval of the Council or Board of Supervisors. To date Yavapai County supervisors have not adopted the Adequacy Rule, so the Beneficial Use rules apply. As of 2014, only the town of Clarkdale, Cochise County, and Yuma County have adopted the Adequate Water Supply rule.

The 1980 Arizona Groundwater Management Act (AGMA) states: "Safe-yield is a groundwater management goal which attempts to achieve and thereafter maintain a long-term balance between the amount of groundwater withdrawn in an active management area and the annual amount of natural and artificial recharge in the active management area" (A.R.S. § 45-561 Section [12]).

The AGMA defines safe yield as a goal and not as a requirement. There are no regulatory penalties for not meeting safe yield and there are no regulatory incentives for meeting safe yield. ADWR has no obligation to develop, or require the communities to develop, a plan to reach safe yield. Therefore, all water providers and users in the Prescott Active Management Area (PrAMA) must voluntarily and jointly reach safe yield or suffer the eventual physical penalties and economic hardships of a depleted aquifer.

The amount of annual overdraft varies during wet or dry cycles usually lasting for several years, and the overdraft can be calculated a few different ways. Thus it is important to understand the assumptions in the calculation.

The long-term (70 years) natural recharge is about 10,000 acre-feet per year. The natural outflow has diminished over time to its current level of about 5,000 acre-feet per year. The long-term natural recharge and the current natural outflow combine to yield a net natural recharge of about 5,000 acre-feet per year. Current water use in the Prescott AMA is about 20,000 acre-feet per year. Combining the current use and the current net natural recharge results in an overdraft of 15,000 acre-feet per year for planning purposes. Put another way, we are withdrawing about four times what we should. (An acre-foot is 325,850 gallons, roughly a football field covered with one foot of water, enough for 3-4 homes' annual use.)

However, Prescott and Prescott Valley calculate the overdraft by allowing for zero natural outflow, which would result in further damage to our riparian areas. With that undesirable assumption, the overdraft is calculated at about 10,000 acre-feet per year.

Prescott and Prescott Valley recharge effluent to the aquifer at about 4,000 acre-feet per year, which could further reduce the calculated overdraft. However, the communities choose to retain the right to use that recharge quantity to supply new subdivisions with groundwater. ADWR does not include effluent recharge in their determination of safe yield unless the recharged water is dedicated to the aquifer.

CWAG believes the current natural outflows should be maintained. Consequently, for the purpose of planning to achieve safe yield, we consider the current overdraft to be 15,000 acre-feet per year.

A diminished aquifer will have both direct and indirect economic consequences that are complex, multifaceted, catastrophic, and difficult to estimate.

Indirect costs include the loss of our riparian areas, diminished wildlife, fewer recreational opportunities, and a generally lower quality of life. Additionally, without a secure water supply, businesses and industries may be reluctant to locate or expand here, diminishing future economic growth.

Direct economic losses will occur as shallow wells on the outer edges of the aquifer dry up; this is already happening, usually causing property values to shrink. The Safe Yield Workgroup of the Upper Verde Watershed Protection Coalition looked at the potential loss in property value if domestic wells go dry and estimated a decline of about $500 million in today's dollars. Additional direct losses would accumulate from land subsidence (see topic below).

Small private water providers, over time, will likewise be unable to supply sufficient water of acceptable quality, so their customers will also suffer a loss in property values and quality of life. Large municipal providers, such as Prescott Valley and the city of Prescott, will be the last group to suffer from water shortages, but eventually that will happen if we continue to overdraft the aquifer. Economic costs would be very large because many more homes and businesses would be affected.

Not unless a significant AMA-wide plan is developed and implemented very soon. In fact, our overdraft is expected to increase to over 20,000 acre-feet per year by 2025 if no actions are taken. There are no current efforts by the tri-cities to develop a safe yield plan.

If we continue to overdraft our aquifer by pumping more than the safe yield value, groundwater levels will continue to fall. Achieving safe yield will halt the decline, and eventually groundwater levels will stabilize. The longer we overdraft and delay achieving safe yield, the lower the level at which groundwater stabilizes will be. Thus, the sooner our communities act to achieve safe yield, the better for our groundwater resources. CWAG's position paper "Safe Yield with River Flow" explains these concepts in more detail.

When more groundwater is removed than is recharged, the water table declines. Pores in formerly saturated alluvium are no longer held open by water pressure, allowing the alluvium to compress, thus permanently lowering the land surface. This is land subsidence. The Arizona Geological Survey monitors known subsidence zones statewide using InSAR satellite technology.

Beginning in the 1940s in central and southern Arizona and continuing today, land subsidence in Arizona has already cost millions from damaged pipelines, collapsed wells, and cracked canals, roads, bridges, buildings, homes, and dams. Active subsidence regions include the McMullen Valley, Scottsdale, Queen Creek, Luke AFB, and south central agricultural areas. A statewide risk assessment estimated potential damages at nearly $200 million.

ADWR hydrologists state, "At this time the amount of historic water level decline in the PrAMA would be considered generally too small to have caused any significant subsidence (compared to conditions in other areas of the state). However, that is more of a general observation than a hard and fast rule." (Frank Corkhill, private communication). Satellite images obtained by CWAG from ADWR for the Little Chino Sub-basin show no detectable subsidence for the period 2005-2008.

Due to the potentially disruptive and expensive effects of subsidence, long-term subsidence monitoring is needed. Achieving safe yield will eventually stabilize groundwater levels and prevent expensive, substantial damage to infrastructure and property.