Chapter 5. Managing scarce groundwater resources to ensure long-term supply in Tucson, Arizona1

This chapter examines groundwater allocation in Tucson, Arizona. Tucson provides an example of how developing a diversified water resources portfolio along with water banking and demand management has helped to eliminate groundwater mining. The case also highlights the importance of flexibility in groundwater allocation and of concerted stakeholder engagement.


Intensive groundwater pumping has led to depletion and land subsidence

In the state of Arizona in the U.S., groundwater provides approximately 43% of the total water supply (Towne and Jones, 2011). In the city of Tucson, as much as 88% of the total water demand was met by groundwater resources as of 2002 (Tucson Water, 2015). The strong dependence on groundwater has resulted in depletion, with groundwater levels declining by 90-150 meters since predevelopment in Tucson and the surrounding area. Consequently, land subsidence of approximately 3.8 meters as compared to 1940 levels has been observed (Ponce, 2006).

Tucson, a desert city, is also subject to a severe risk of surface water shortage. Historical average rainfall has been about 300 millimetres annually. The Colorado River is over-allocated among seven U.S. states and Mexico, and the Colorado River Basin has experienced drought for 14 years. Further, downscaled climate models project that the region will become hotter and possibly drier (Megdal, 2014).

Most residents in and surrounding Tucson are served by Tucson Water, which is a public water utility under the auspices of city authorities (Megdal, 2014). Out of its 709 000 clients, as of 2012, approximately 25% are commercial and industrial water users with the residential users accounting for the remainder (Megdal, 2014).

New surface water sources were introduced to reduce the pressure on groundwater

In response to groundwater depletion, the federal government funded the construction of a 540 km long lined and open canal in Arizona, called the Central Arizona Project (CAP). The CAP was built by the U.S. Bureau of Reclamation and completed in the early 1990s. Operations and repayment are the responsibility of the Central Arizona Water Conservation District, an elected body established by law. Water from the Colorado River is pumped into the CAP from near sea level to a maximum elevation near Tucson of about 730 m. Built to transport approximately 1 850 million m3 of water annually, the CAP is the largest consumer of electricity in Arizona (Megdal, 2014). In addition to providing a much needed alternative to groundwater, the introduction of CAP water was a way to ensure consistency with the safe-yield management goal for the region. Tucson was granted the largest municipal allocation (approximately 178 million m3 per year) within the CAP system (Megdal, 2014).

Historically reliant on the region’s good quality groundwater, which did not require much treatment prior to delivery to customers, integration of the CAP water through direct delivery required the construction of a large, centralised treatment plant. This was built using a combination of rate-payer charges collected in advance of operation and revenue bond financing. In 1992, Tucson Water delivered treated CAP water to half of its consumers. This first real infusion of surface water into the Tucson Water system turned out to be fraught with difficulties. The CAP water had a different chemistry from that of groundwater and travelled in a different direction through old water mains. The corrosivity of the CAP water was particularly challenging. As a result, the calcium coating on the inside of the water pipes dissolved, allowing rust and soil into the home distribution lines. Many of the galvanised pipes in older parts of the city failed and leaked (Wilson, 2016).

Storage and recovery was implemented as an alternative to direct use of CAP water

The damage caused by the CAP water corrosivity when supplied through the regular network, coupled with the utility’s hesitancy to acknowledge the problems, led to lack of confidence and customer activism to restrict the way in which this new water source could be used. In order to respond to consumers’ opposition to direct delivery of treated CAP water and to the risk of shortage of surface waters, as well as to comply with the relevant legislation, Tucson Water adopted an indirect approach to utilising CAP water. Rather than treating the water in a large treatment facility and then directly delivering the water to its customers, the utility deployed a Storage and Recovery approach (S&R), in compliance with Arizona State regulations (Megdal, 2014).

Arizona state law has authorised the use of aquifers for water storage and groundwater replenishment and S&R programmes make up an important water management tool in many parts of the state (Megdal, 2014). A system of permits and accounting administered by the Arizona Department of Water Resources (ADWR) governs the construction and use of water storage facilities as well as the recovery of stored water. The permit system allows the ADWR to ensure that the recharge is hydrologically feasible and that no harm is done to water and land resources (Megdal, 2014).

The S&R approach allowed utilising CAP water, first by storing it underground, mostly in large, shallow spreading basins, where it mixes with groundwater in the aquifer, and then by recovering it for distribution (OECD, 2015). Tucson Water is not currently delivering its full allocation of CAP water to its customers, but it is taking delivery of the full allocation. Water over and above that needed to supply current demands is being stored underground for future use. Such storage is extremely important to Tucson Water’s ability to withstand Colorado River shortage declarations. Through 2013, Tucson Water invested USD 134 million in the facilities required for its S&R system, with another approximately USD 180 million planned. Annual investment is approximately USD 38.6 million (Megdal, 2014).

The implementation of the S&R programme was facilitated by a number of factors

A number of early decisions by the state and local authorities helped lay the foundation for the implementation of the S&R programme. For example, the state of Arizona had the foresight to establish the Arizona Water Banking Authority (AWBA). In anticipation of a Colorado River shortage declaration. The AWBA has been storing CAP water underground in Tucson Water’s storage facilities, since 1997. Consequently, Tucson Water has essentially developed a “drought-proof” system, allowing it to rely on its own storage, as well as that of the AWBA, should there be future curtailment of CAP surface water deliveries. Tucson Water can also increase its use of groundwater if or when needed (Megdal, 2014).

Moreover, in the 1960s and 1970s, Tucson purchased some agricultural lands northwest of the city, with the expectation that the water rights associated with the lands would be used to meet Tucson Water’s future demands. What was not envisioned at the time was that these lands would become the site of the large storage facilities that are the backbone of today’s S&R system. The ownership of these lands enabled Tucson Water to avoid land acquisition costs when constructing the S&R system (Megdal, 2014).

The implementation of the S&R programme was also aided by Tucson Water’s engagement of stakeholders in its planning efforts. In addition to being proactive in its outreach to communities, the water utility embarked on a partnership with a local farming entity at the early stages of implementing the programme. The agricultural partner helped construct some water conveyance infrastructure that was used to deliver CAP water to farm lands and to recharge basins. Tucson Water accrued water storage credits, pursuant to state law and ADWR permitting, for the utilisation of CAP water on the agricultural fields in lieu of groundwater. The state’s Groundwater Savings Storage programme, which is incorporated in the statutory framework, is a good example of a mutually beneficial and voluntary partnership (Megdal, 2014).

Local water conservation projects, rainwater harvesting and use of grey water have also contributed to reduce demands on the potable water system and promote conservation (Megdal, 2014). Water conservation has long been a focus of Tucson Water’s activities. Water banking is recognised as an important strategy for addressing the long-term needs of the region, and the importance of conservation and wise water use has been a consistent component of Tucson Water’s public messaging (Megdal, 2014). Demand management is also promoted through water pricing. Water pricing in Tucson is designed to recover costs of providing water, including extraction, diversion, treatment, delivery, debt service, and administrative charges. The pricing structure for residential customers is based on increasing block tariffs. Commercial customers face higher rates in summer than they do in winter, providing a price signal designed to reduce water consumption during periods of scarcity (Megdal, 2014).

In 2015, Tucson Water reported no mined groundwater use: 84% of the water consumption was supplied by CAP water resources through the S&R approach, 10% was reclaimed water and 6% of total water supply came from remediated (cleaned to a very high standard) groundwater (not considered groundwater use by the regulatory authorities), which is fed into the potable water system (TW & CoT, 2015). This reflects the extent to which the introduction of CAP water as well as the S&R approach has succeeded in altering water consumption patterns in Tucson, resulting in a significant decline in the risk of groundwater depletion.

Tucson Water had to work hard to overcome the loss in confidence that was associated with the failed introduction of CAP water to the Tucson community, and learnt the importance of consulting and communicating with its stakeholders. The water utility has made a particular effort to engage stakeholders in the implementation of its 2013 Recycled Water Master Plan, and regularly informs and engages its governing body – the Tucson Mayor and Council (Megdal, 2014). In addition, Tucson Water’s capital investment plan undergoes rigorous review by stakeholders. It is funded through revenue bonds, which through 2005 were submitted to City of Tucson voters for approval. It is worth noting that despite the poor community experience associated with introduction of CAP water, City of Tucson voters approved sizable bond issues to fund replacement of large transmission pipelines and other capital needs. The voters understood that replacing old infrastructure was necessary and supported the higher rates associated with USD 380 million in bonds between 1994 and 2005 (Megdal, 2014).

Lessons learned

Tucson Water serves a desert community, which has, in the past, relied on mining groundwater to support a growing population and economy. Arizona took action as early as 1980 to require use of renewable water supplies to reduce groundwater overdraft. The construction of the CAP enabled the importation of new water supplies to the region. Although strict federal drinking water quality regulations prevail at all times, state groundwater regulations allow utilities flexibility in their approach to utilising renewable surface water supplies. This allowed Tucson Water to adapt from direct delivery of CAP water to an S&R system, avoiding the costs of centralised treatment facilities, and enabling the storage of water for future use (Megdal, 2014). This highlights the importance of flexibility in groundwater allocation and demonstrates Tucson Water’s ability to develop a portfolio of alternative sources of supply (see Health Check #3, Part I). The financing and implementation of dedicated infrastructure was crucial for the success of the allocation of CAP water through an S&R approach (see Health Check #9, Part I).

By 2015, there was no mined groundwater use in Tucson. Though cities will not be affected in the short term by the shortage conditions on the Colorado River (due in large part to the establishment of the AWBA) the potential for some curtailment of surface water deliveries has underscored the importance of a diversified water resources portfolio. Demand management and water reuse are significant elements of this approach (Megdal, 2014). Tucson Water’s experiences have also demonstrated the importance of effective stakeholder engagement (Megdal, 2014) (see Health Check #1, Part I).


Megdal, S. (2014), “Managing water for future cities: Case study of the city of Tucson, Arizona USA”, Background Paper prepared for OECD (2015), Water and Cities: Ensuring Sustainable Futures, Studies on Water, OECD Publishing, Paris.

OECD (2015), Water and Cities: Ensuring Sustainable Futures, OECD Studies onWater, OECD Publishing, Paris,

Ponce, V. (2006), “Groundwater utilization and sustainability”, (accessed 22 June 2016).

Towne, D. and J. Jones (2011), “Groundwater quality in Arizona: A 15-year overview of the ADEQ ambient monitoring programme (1995-2009)”, Arizona Department of Environmental Quality, (accessed 14 August 2016).

Tucson Water and City of Tucson (TW & CoT) (2015), “Water production (1940-2015)”, figure prepared for the “Tucson Water Clearwater Renewable Resources Facility 15 Years” event, 25 May 2016.

Tucson Water (2015), “Total water demand”, figure prepared for the “Tucson Water Clearwater Renewable Resources Facility 15 Years” event, 25 May 2016.

Wilson, W. (2016), Chief Hydrologist at Tucson Water, personal correspondence.


← 1. This document and any map included herein are without prejudice to the status of or sovereignty over any territory, to the delimitation of international frontiers and boundaries and to the name of any territory, city or area.