The Water Resources Research Center at the University of Arizona published its Arroyo 2015 newsletter on “Closing the Water Demand-Supply Gap in Arizona.” The Arroyo’s executive publisher is Sharon Megdal, who spoke on sustainable water use in the Southwest at RNRF’s congress on December 1-2, 2015 in Washington, D.C. Authored by Susanna Eden, Madeline Ryder, and Mary Ann Capehart, this article provides an overview of Arizona’s current water sources, projected water demand, and strategies for supply and demand management into the future.
Arizona has an acknowledged gap between future water demand and available supply, although the nature and severity of this gap varies by region. Growing demand for water reflects population growth, the type of use, efficiency of use, and the location of use. Between 1980 and 2009, Arizona’s population grew from 2.7 million people to nearly 6.6 million people. Although growth has slowed since 2007, population is expected to continue increasing. Legal, political, economic, and financial factors affect the availability, distribution, and uses of water. As a result, no one-size-fits-all solution exists to closing the water demand-supply gap.
Water Sources and Their Challenges
In Arizona, unless additional supplies are developed and more stringent conservation measures are implemented, increased water demand could result in a statewide demand-supply gap between 1.8 million acre-feet (MAF) and 2.6 MAF in 2060.
Water demand has already surpassed the average annual Colorado River supply, and continued drought and climate change are likely to have additional impacts on this source. Water suppliers and users in Arizona have been proactively building resilience to Colorado River shortages through underground storage, conservation, and water recycling.
Most groundwater used in Arizona is nonrenewable, and continued withdrawal will eventually lead to depletion. The Groundwater Management Act of 1980 and subsequent legislation created a regulatory structure for reducing or eliminating groundwater depletion in areas where it was deemed to be severe.
Finally, other renewable supplies of water are provided by in-state river systems, which mostly come from precipitation in the form of snow and rain. The seasonal timing of snowpack and rainfall has been changing since the 1950s in the western U.S, frequently reducing the amount of water available for use.
Current and Future Water Demand
Despite dramatic population and economic growth, state-wide water use has either declined or remained constant at ~7 MAF during each decade since 1980. Water demand reductions resulted from retiring agricultural lands, increased use of reclaimed water, and widespread conservation efforts of farmers and municipalities. However, even with remarkable demand reductions, growing populations’ water demand will exceed existing supplies.
Agriculture is the biggest water user in the state, demanding between two-thirds and three-quarters of all water used in Arizona since 1980. Demand in this sector is declining in both water and land use.
Municipal and industrial demand uses about 25% and 6% of Arizona’s water supply, respectively, making municipal demand the second largest sector of water demand in the state. Municipal use was measured at 1.6 MAF in 2006, and is projected to increase to 2.7 MAF by 2035 and 3.4 MAF by 2060.
Most of the value from water for the environment is derived from instream flow and associated shallow groundwater, both of which support riparian and aquatic ecosystems. Including water allocations from natural areas in new water plans is difficult because there are few legal requirements to do so. In addition, scientific data on water needs for riparian and aquatic species and related habitats are either not available or are site-specific and complex. There are few instream water rights for natural areas in the West because they are the newest form of water rights. In general, water in streams has diminished due to an increase in surface water and groundwater demands by other sectors.
Supply and Demand Management Solutions
Potential solutions to the demand-supply in Arizona either decrease demand, increase supplies, adjust operations, or modify governance. There is a general consensus that diverse solution portfolios are needed to prepare for the projected gap, continued drought, and changing climate.
Conservation, which is applicable to any water use, relatively inexpensive, and relatively quick to implement, is generally considered the “no regrets” option for water management. Many local governments, water providers, and individuals already carry out mandatory and voluntary conservation practices. Several city water utilities provide incentives and rebates for residential and commercial users to remove turf and install water-efficient household fixtures. Water providers also offer educational programs for rainwater harvesting, water smart landscaping, and indoor conservation practices. Some utilities offer free and low-cost audit programs to detect leaks and test meter accuracy. Progressive or conservation tiered rate structures have kept water prices low for modest daily needs while providing disincentives for high water use. Large and medium-sized municipal water providers located within Active Management Areas (AMAs) are required to participate in a conservation program. Conservation in the municipal, industrial, and agricultural sectors is expected to continue to yield water savings, but other steps must be taken to fill the growing gap between supply and demand.
Planners in Arizona are increasingly looking to reuse treated wastewater to augment freshwater supplies. One study shows that 95% of reclaimed water generated in the Phoenix, Pinal, and Tucson AMAs is used for beneficial purposes, including agriculture, underground storage, power generation, industrial uses, turf irrigation, and riparian habitats. In addition to wastewater treatment plans, water may also be recycled through residential and commercial graywater systems, which is then typically used for irrigating landscaping. Although use is increasing, many municipalities fail to use all available reclaimed water supplies. Development of reclaimed sources if often limited by lack of available infrastructure; many municipal planners are looking for ways to expand distribution systems. Public support for reclaimed water is critical to increased use. Acceptance of non-potable reuse has increased, but potable use continues to generate negative reactions.
Water transfers can involve water from various sources and can be either temporary or permanent. Laws, policies, and procedures for transfers of groundwater, Colorado River water, and intrastate surface water are designed to protect the local area of origin and other water rights holders/users in the system, but also make transfers cumbersome to implement. In addition, water rights holders currently cannot sever and transfer a right to surface water saved through conservation. Even if laws and institutions were modified to better facilitate water transfers, the issue of physical transportation infrastructure remains. Transportation issues include obtaining permits for rights-of-way over federal land in addition to engineering and financing issues.
Available data and estimates indicate that Arizona contains a large amount of groundwater. Some portion of this in aquifer storage could potentially be developed to supply projected future demands. However, there are several barriers to using this supply. Importantly, large-scale withdrawals could have severe negative consequences, including subsidence and environmental impacts. Regional studies are needed to determine what level of groundwater usage is sustainable or optimal from a public policy perspective. Brackish groundwater represents an additional supply source. Reducing the salt concentration in brackish groundwater could make it suitable for potable use. The salt concentration is much lower than in seawater, which means desalination of brackish water is less expensive. However, in landlocked Arizona, brine disposal is an issue.
Augmentation refers to acquiring new water through any of several methods. Augmentation will likely be part of the portfolio of future water supply strategies, since efforts to stretch existing supplies are unlikely to be sufficient in the future.
Importation of significant freshwater supplies is unlikely to be developed, given that the source of potential supplies (Mississippi, Missouri, Green Snake, and Columbia Rivers) already faces needs from local communities. The costs would also be relatively high.
Seawater desalination is attractive because of its potential for supplying substantial quantities of new water. However, acquiring desalinated sea water would likely require substantial investment in treatment and transportation infrastructure, and arrangements would take a great deal of lead time for multiparty negotiations, regulatory hurdles, and large financial commitments. Potential sources for Arizona would be California or Mexico. The United States and Mexico have agreed to further study of desalination projects that would benefit both nations.
Watershed management could increase runoff by decreasing vegetative water consumption. For example, removal of tamarisk, an invasive, high-water use species, would help restore native habitat while increasing available water. Forest management through thinning of unnaturally dense forests can also increase runoff by reducing vegetative consumption and allowing more snow and snowmelt to reach the forest floor. Forest management can also reduce the potential for intense and large-scale fires, which leave watersheds vulnerable to flooding and erosion.
Weather modification often refers to the practice of cloud seeding. In 1874, the Bureau of Reclamation released a study on weather modification for the southwest region that indicated 300,000 acre-feet of potential additional supplies could be created in the Lower Basin States. However, the effectiveness of weather modification has yet to be quantified. Individual projects have experienced varying success with increasing precipitation through cloud seeding.
Rainwater harvesting is the capture and storage or use of precipitation runoff. As practiced in Arizona, rainwater harvesting is typically employed on a small scale for single houses, commercial properties, industrial lots, or parks. Given the low price of water, the payback period for investing in water harvesting at small scales can be very long. Large-scale rainwater harvesting and storm-water capture could have benefits beyond augmenting non-potable water supplies, such as controlling stormwater runoff, peak flooding, and pollutant loading from urban areas. This water could also be used to replenish local aquifers.
To read the 2015 issue of Arroyo in full, visit https://wrrc.arizona.edu/publications/arroyo-newsletter/arroyo-2015-Closing-Demand-Supply-Gap.
For more information on RNRF’s 2015 Congress on Sustaining Water and to register, visit http://www.rnrf.org/2015cong/.