Introduction

Although the concept of sustainability is widely touted as an ideal for urban growth policies, achieving sustainability can be difficult. Nowhere is this more evident than in the case of water and water uses. Sustainability in the realm of freshwater is generally defined as the provisioning of water for both people and the environment for generations to come [1]–[4]. Unfortunately, plans for urban growth rarely embrace this definition for water policy and instead focus almost entirely on the pursuit of new supplies to accommodate future growth without consideration of environmental water needs [5].

Conflict over water has become a hallmark of the southwestern United States, where large-scale water infrastructure projects in the 20^th Century facilitated rapid urban and economic expansion at the expense of the environment. As dams, diversions, and increased groundwater pumping modified hydrologic regimes throughout the western United States, surface flow diminished or disappeared altogether at some locations, and associated riparian and aquatic systems declined [6]–[8].

Populations and cities continue to expand in the western United States, which calls into question what might be done to promote sustainable water use moving forward. A crucible for these pressures is Arizona, where population is projected to double by 2050 [9] and streamflow depletion has been documented throughout the state [10], [11]. Seventeen of the state's 33 native fish species now have status under the U.S. Endangered Species Act [12]. Three fish species within the Colorado River Basin (Pahranagat spinedace, Las Vegas dace, Monkey Spring pupfish) have already been driven to extinction from human modification of aquatic habitats and introduction of non-native species [13]–[16]. Our analysis suggests there will be additional streamflow depletion and further species imperilment without actions to reverse current trends.

Using Arizona as a case study, we develop a scenario-based assessment approach as a tool for exploring how water management strategies could sustain water for both people and the environment. Our study area is experiencing some of the highest growth rates in the United States, and this growth is relying on the same groundwater that supplies year-round flow in our rivers. We present the first study in the southwestern United States that evaluates the potential impact of population growth and water demand on streamflow depletion across multiple watersheds.

Materials and Methods

Estimating River Base Flow

Base flow is the proportion of surface flow that comes from groundwater discharge and supports year-round streamflow. To estimate base flow, we first inventoried locations where groundwater discharge still supports perennial streamflow across the state. We intersected the USGS streamflow gage data layer (http://waterdata.usgs.gov/az/nwis/rt) with a GIS layer of perennial streamflow [17] and selected 18 gages on unregulated perennial streams (Figure 1). We focused on gages with at least a 20-year data record, although for four gages we accepted a record of 7 to 10 years. We digitized the watershed area reporting to each selected streamflow gage using groundwater basin boundaries delineated by the Arizona Dept of Water Resources [18] as a starting point (Text S1). We researched published base flow values and evaluated several methods of base flow separation. We found that accurately separating base flow is difficult in areas of low total runoff [19]. We examined flow duration curves and daily mean flow hydrographs, and, where available, we compared published base flow values to median flow. From our analysis, we concluded that median streamflow approximates base flow for our study streams (Text S1).

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Figure 1. Locations of study watersheds and USGS gages.

https://doi.org/10.1371/journal.pone.0011687.g001

Results

Our models indicate that in 2000, municipal water demand had already exceeded 100% of base flow in two watersheds, and was nearly 100% of base flow in one other (Figure 2). On the other end of the demand spectrum, there are seven watersheds where not only is current BDI low,demand is projected to remain in the low BDI range under any growth/water management scenario (Figure 2). The critical opportunities for management are found in the intermediate watersheds, where growth/water management scenarios determine whether or not watersheds fall into the dewatered status or remain a sustainable resource for biodiversity and people. In particular, implementing either of the two conservation scenarios would prevent four watersheds from transitioning from the low to stressed BDI range (Williamson Valley, Upper Cienega Creek, Verde Oak Creek, Big Sandy River; Figure 2). Three of these watersheds have relatively low annual discharge, so communities in these areas have little water to begin with. One watershed ranks within the top five in annual discharge; it has a relatively large volume of water, but also a population projected to more than double by 2050 (Table S1).

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Figure 2. Base flow demand index (BDI) for 18 watersheds for the year 2000 and across four future scenarios.

BDI is calculated as the percentage of municipal water demand to river base flow within a watershed. The solid horizontal line indicates the threshold where demand exceeds 100% of base flow; the dashed line delineates the transition from low BDI (≤50% of base flow) to stressed BDI (>50%–100% of base flow). See text for description of scenarios.

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In year 2000, demand in each of the five remaining watersheds was already approaching or within the stressed BDI range (Figure 2). However, implementing one of the conservation scenarios would prevent four of these systems from transitioning to the critical threshold BDI range (Arivaca Creek, Little Colorado River, Upper Verde River, San Pedro River; Figure 2).

The outcome of our modeled water demand/river flow relationships has important implications for plant and animal species that are dependent upon freshwater environments. While our study watersheds comprise only 11% of the area within the Colorado River basin (Figure 3), they host 45% of the aquatic, wetland and riparian species in the basin currently listed under the U.S. Endangered Species Act [30]. If current urban growth and water use trends follow our projections, more species will trend toward threatened or endangered status and, perhaps, extinction (Table S2).

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Figure 3. Regional significance of the study watersheds to species.

Nearly one-half (45%) of the aquatic, wetland and riparian species that are listed under the U.S. Endangered Species Act in the Colorado River Basin occur in the watershed basins of this study. Inset: Colorado River Basin within North America.

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Discussion

In Arizona, natural perennial streamflow has already declined or disappeared completely at a number of locations due to human groundwater use [7], [10], [11]. Our study demonstrates that if actions are not taken to reserve a portion of river base flows for the environment, then at least seven other river systems will be de-watered over time and an additional four will experience substantial degradation. Although coarse, our scenarios illustrate how future risks can be reduced by implementing alternative growth and water management strategies. The question is how to do so in practice.

Renewable water supplies are limited in the Southwest. Even though Arizona's alluvial basin aquifers contain substantial amounts of water, it takes only a small fraction of use for effects to show up as river depletion [31]. Time lags associated with groundwater systems can extend the time frame of full depletion out decades (Figure 4) [32].

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Figure 4. Cumulative effects of groundwater pumping on streamflow depletion even after pumping is stopped.

In this case, from Lower Clear Creek in Arizona, streamflow continues to decline for nearly 30 years after pumping is stopped in 2060. Adapted from Alley and Leake (2007).

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One challenge for any approach to sustainable water use in Arizona is that climate change will intensify existing difficulties of maintaining water supplies in an arid climate prone to drought. With climate change, surface flows in the southwestern U.S. will likely decline as temperatures continue to rise and evaporation rates increase. Colorado River flows are predicted to decline by 10–30% [33], and there is an 85% chance that flows in the Salt and Verde River basins will be reduced by 2050 [34]. Additionally, climate change may lead directly to reductions in base flows by reducing groundwater recharge [35], [36].

In addition to climate issues, there are regulatory and policy complications. The complexity of Arizona's laws and regulations guiding surface and groundwater use arose out of an attempt to bring equity to all water users. Water for the environment, however, was not considered in decisions regarding equity. In Arizona, surface water and groundwater are managed under different regulatory schemes and there is no legal recognition of the physical connection between the two. In the majority of our study area, groundwater use is unregulated and rivers can be de-watered through groundwater extraction.

Our current environmental policies and legal framework do not consider environmental water needs and, thus, are inadequate to protect flowing rivers. Planning is currently underway for water infrastructure projects to be implemented 20 or more years in the future. By the time those projects receive funding and are subjected to environmental compliance, further degradation and loss of river flows will already have occurred. With implementation of one or more conservation strategies now, impacts to streamflow can be reduced. As Figure 4 illustrates, if groundwater pumping continues, impacts to the river accumulate and cannot be immediately reversed if pumping is stopped altogether. Thus, mitigating project effects to endangered species 20 or more years in the future is not the same as acting now while we still have options in many watersheds to allocate water for the environment.

Fortunately, the discussion over water sustainability is already being reframed by communities that recognize the interdependence of the environment and economy. Many communities and regions around the world are implementing sustainable water management policies that address the needs of competing sectors while sustaining water for the environment. For example, Australia's constitution now mandates environmental flows as the first allocation [37]. Kansas [38], Michigan [39], and Massachusetts [40], among other states, have developed tools and regulatory mechanisms to limit new water uses such that existing users, including the environment, can maintain current condition. The Upper San Pedro River Partnership in Arizona has employed hydrologic and ecological models to support zoning overlays that encourage groundwater pumping more distant from the river [41] and recharge of treated municipal effluent near the river [42].

The underlying drivers in many of these examples are economic as much as environmental; in the era of sustainable development, would private investment flow to communities perceived as unsustainable? As our study shows, developing modest growth and water management strategies can ensure a continued water supply for people and rivers, and transform the rhetoric of sustainable water management into reality. The quicker we act, the more options we will preserve.

Supporting Information

Acknowledgments

We thank Saeid Tadayon and Linda Stitzer for providing data and helpful insights, and Stan Leake for insight on the study's conceptual underpinning. David Albert, Dave Anning, Tim Boucher, Laura Chartier, Peter Kareiva, Karen Lombard, Karen Poiani, John Sanderson, Stacey Solie, Robert Taylor, Rebecca Tharme, Dale Turner, and Kari Vigerstol, provided critical reviews of the manuscript.