This case study describes current water management conditions and aquatic habitats in the Phoenix, Arizona, USA metropolitan area. Phoenix is a city of 3.8 million and growing rapidly in a hot, dry desert that has appropriated a substantial amount of surface water from the Salt, Verde, and Colorado Rivers. Current threats to aquatic habitats include agriculture, hard engineering solutions, loss of riparian areas, groundwater pollution, subsidence, recharging effluent water, and greening the desert with unfamiliar artificial habitats to the ecosystem and urbanisation. Currently, lack of ecological information, complex legal and institutional structures, and lack of extensive community interest impede maintenance and protection of native desert aquatic habitats.

The Salt River, Phoenix, USA.

Background

Phoenix is one of the fastest growing cities in the US, increasing from approximately 300,000 in 1950 to greater than 3.7 million in more than 20 municipalities in 2004, with a population density of 1074 people/km2. Models predict that in 2025, the population will be exceed 6 million, representing a 280% change since 1980 (Jacobs and Holway, 2004), and nearly all of the undisturbed and agricultural lands will have been developed to urban land uses (Jenerette and Wu, 2001). The growing population strains the existing water infrastructure and supply. Greater than half of the water supply comes from surface waters. With few geographical barriers to expansion, growth has been largely in an outward direction, estimated at almost one 0.8 km per year (Gober and Burns, 2002). Most new construction has been the result of conversion of agricultural to residential use, but increasingly, new areas of desert are being transformed into housing developments. This rapid expansion of the urban area has often resulted in the destruction of desert washes, but has also led to the creation of numerous artificial lakes. Thus, the demand for water stems from both necessity and strong aesthetic and recreational desires.

Located in the northern Sonoran Desert of the south-western USA, the Phoenix metropolitan area receives approximately 180 mm of precipitation a year, with an average January temperature of 12° C and an average July temperature of 34° C (Baker et al., 2004). Most rain is concentrated in two seasons: a summer "monsoon" season with short, intense, localised thunderstorms and a winter rainy season characterized by frontal storms of longer duration and lower intensity. Given its hot, dry climate, the area experiences an average of two metres potential evapotranspiration annually. The city is situated in an alluvial valley surrounded by rugged mountain ranges typical of Basin and Range topography (Jacobs and Holway, 2004).

Key aquatic habitat issues in urban water management

Very few locations throughout the urban area are typical desert aquatic habitats. Water, as a premium commodity, is moved miles to create desirable landscapes. What is desired is strongly influenced by the cultural backgrounds and experiences of stakeholders. Immigrants from more temperate climates, especially the US Midwest, represent a large proportion of Phoenix residents. With them they bring memories of lush grasses, abundant vegetation, small ponds and lakes. Two aspects of city life reinforce the perception that such landscapes are sustainable. First, regulation of water flows via damming and reservoirs has damped the strong "pulse" regime of desert hydrology, allowing available water to support vegetation year-round. Second, before the invention of air-conditioning, the cooling effect of increased evapotranspiration was an essential way to contend with extreme summertime temperatures. Thus, throughout Phoenix's history, new arrivals have seen a steady supply of water and abundant green growth, and many were attracted in the first place by promotions of the city as an idyllic place to retire, famous for its golf courses. There is no reason, given that such perceptions are actively encouraged, for newcomers to think of water as a limiting resource. Essentially, due to the creation of artificial aquatic habitats and lush gardens into the city landscape, many residents no longer perceive or appreciate that they are living in the desert; for them, the desert exists only outside of the city (Farley-Metzger, personal communication; Gober, 2006).

Cultural preferences, along with the relatively easy access to a variety of water resources, have drastically changed the ecology of the Phoenix valley. Demand for agriculture and later municipal uses have had a significant impact on contributing watersheds and downstream systems. Dams on all of the major rivers have eliminated pre-dam seasonal patterns of in-stream flow. Also, many flood mitigation efforts involved hard-engineering solutions including the lining of river channels. A significant amount of stormwater runoff is diverted to stormwater retention and detention basins associated with housing and commercial developments. These basins can serve several roles, providing flood mitigation, groundwater recharge, and recreational areas. The built environment has eliminated many natural flow paths.

Historical modifications have resulted in an overall loss of riparian areas in some places, and a general shift in riparian community composition via bank stabilisation, the introduction of non-native plant species, and the decrease in total woody plant volume (Green and Baker, 2003). Some river and riparian habitats exist downstream of wastewater treatment plants, in river washes receiving stormwater runoff, and sites designated for groundwater recharge, but it is only recently that agencies have begun to consider ecological factors in management of aquatic systems in the Phoenix area. For example, the Rio Salado Project, funded by the City of Phoenix, the Flood Control District of Maricopa County, the Arizona Water Protection Fund, and the US Army Core of Engineers (US ACE) began the "restoration" in 2001 of 240 hectares of riverbed and riparian areas in central Phoenix. However, because the river flow regime has not been restored and groundwater levels have been lowered, there is not enough water naturally available to support these communities. Therefore, the project will include groundwater pumps, canals, and reservoirs to ensure adequate supply (City of Phoenix 2005). Meanwhile, further downstream at the 91^st Avenue Wastewater Treatment Plant (WWTP), billions of litres of treated effluent are released into the Salt River annually. This nutrient-rich water supports an extensive riparian area, but little groundwater recharge is occurring because the area already has high groundwater levels. From the management perspective, this water is going to waste: a Bureau of Reclamation officer says "we just can't keep dumping it in the stream and letting it go downstream." So an $80 million project is in the works to pipe the water northwest and uphill to the dry Agua Fria riverbed to facilitate groundwater recharge. Officials note that, in addition to recharging the aquifer, the addition of this water will help restore native riparian habitat along the Agua Fria (Landers, 2004).
Thus, inevitably there are complex tradeoffs within the urban ecosystem between various water management and environmental objectives (Grimm et al., 2004). The culmination of historical decisions leads more and more frequently to the creation of "designed ecosystems" to satisfy particular goals: water recharge, habitat restoration, aesthetics, and recreation.

The Phoenix metropolitan area now has greater than 650 artificial lakes (E.K. Larson, unpublished data). In Gilbert there is a Riparian Institute and "water ranch": 18 recharge ponds for treated effluent and constructed "riparian" habitat (the area is not an historical wash or river) designed to attract birds and other wildlife (Edwards, 2001).

Objectives of the Case Study

Are the water supplies and management practices for the Phoenix metropolitan area sustainable? The crux of this question is that the answer depends on the interaction of a multitude of uncertain ecological, economic, social, and cultural variables, and the very question of whether sustainability includes the maintenance, restoration, or protection of aquatic habitats is as yet unanswered, as many people, including water managers, consider allowing water to flow downstream (as opposed to captured for human use) a "waste." Assessment of these variables is ongoing, but is hampered by significant technological, organisational and informational difficulties.

With respect to renewable (non-groundwater) sources, new analyses continue to emerge. Some authors, such as Gammage (2003), argue that populations as high as 7 million will be sustainable as long as there is a corresponding decrease in agriculture (a water-intensive land use). His view does not incorporate any climatic variability. Morehouse et al. (2002) conclude that the variety of water resources available to Phoenix provide more of a buffer to short-term drought conditions than Tucson, but that "even if agricultural demand were eliminated entirely, drought conditions would still force the AMA to rely on non-renewable supplies to meet 43% of its needs." However, one of the most basic underlying assumptions about the flexibility of Phoenix water resources, that Colorado River water will provide when the Salt and Verde are experiencing drought and vice versa, was recently challenged by a joint University of Arizona/Salt River Project (SRP) report. The report used tree-ring analysis to reconstruct drought cycle synchrony between the two basins, and found that only two events in the 443 years analysed showed asynchronous flow (Hirschboeck and Meko, 2005), leading the manager of water resources at SRP to opine, "our thought that the Colorado River would be able to bail us out is not a safe assumption anymore" (McKinnon, 2005).

Clearly, unlimited population growth rates are unsustainable due to accompanying environmental impacts and resource limitation. However, analysts predict the population of Phoenix to level off around 7 million (Gammage, 2003). Are there enough resources to support a population of this size without incurring serious environmental damage to groundwater and aquatic habitats, impairing resources for future generations? At the most basic level, research is still needed on human population trajectories, ecological impacts, climatic change, etc. There is a paucity of data on environmental outcomes of urban development, water use, landscaping practices, etc. at scales ranging from individual aquatic habitats to municipalities to watersheds to the Colorado River basin. As Gammage (2003) notes, "because water's absence is the defining characteristic of a desert, its management becomes the defining activity of living in the desert." Phoenix is not alone in addressing these questions; Fitzhugh and Richter (2004) estimate that "41% of the world's population lives in river basins where the per capita water supply is so low that disruptive shortages could occur frequently." Evaluation of Phoenix's sustainability, and implementation of steps to achieve it, will benefit not just Phoenix and the US Southwest but also rapidly growing cities throughout the world.

Stakeholders

From an academic and research perspective, there are numerous groups and institutions addressing water issues in the Phoenix Metropolitan area. The Central Arizona - Phoenix Long Term Ecological Research project (CAP LTER), a nationally funded program now in its eighth year, seeks to expand and develop the necessary research tools and data to understand the long term, regional dynamics of the urban ecosystem (Grimm and Redman, 2004). Additional vital insight will be provided by the newly funded Decision Center for a Desert City (DCDC), a research institute at Arizona State University focused on establishing relationships between climatic conditions and water decision making. The Arizona Water Institute, a joint collaboration with the University of Arizona and Northern Arizona University, focuses on water education, research, community assistance, and economic development in the State of Arizona as a whole. From a regulatory standpoint, the Arizona Department of Water Resources and the Arizona Department of Environmental Quality continue to assess Arizona water resources and ecosystems and implement regulatory plans and restoration activities. On a more local level, communities are beginning to work together to address management issues, as evidenced in the establishment of the East Valley Water Forum. However, integration of aquatic habitat concerns with water planning and management is still nascent.

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