Water availability and the vulnerability of large United States’ cities

April 16th, 2013

Dr. Julie Padowski & Dr. James Jawitz, University of Florida, United States

For the first time in human history, urban areas hold the majority of the world’s population. The accelerated growth of cities has been accompanied by drastic changes in the way natural resources are used and managed1,2,3 and a fundamental transformation of the relationship between society and the environment.4 One of the most profound changes brought about by urbanization has been the development of highly localized, intense demand for water in urban areas.

Over 60% of United States’ citizens live in cities with more than 100,000 inhabitants, and water demands in the public supply sector have increased steadily with population growth, more than tripling since the 1950s.5 This increased need for water has strained local and regional water supplies in many areas, and has led to growing anxiety over source water availability, i.e., maintenance of adequate volumes of treatable water for human consumption, in the US.6 These concerns are compounded by uncertainties associated with the impacts of climate change, environmental regulation and further population growth on existing water resources.7,8

Despite the national importance of maintaining adequate urban water supplies, there has been no comprehensive or uniform assessment of urban water scarcity in the US. The highly decentralized regulatory policies of the public supply sector provide little standardization across systems, and no means by which information about urban water availability or vulnerability can be easily consolidated or compared. National and global-level studies have developed different methods for measuring water scarcity, but from a natural, rather than urban, system perspective.9,10 Remarkably, although nearly all urban utilities rely heavily upon constructed infrastructure to extract, import and store water, such hydraulic components are typically excluded from these assessments, which measure scarcity only in terms of renewable water.

In response to this problem, Padowski and Jawitz11 conducted comprehensive, national assessments of water availability and vulnerability for 225 large cities in the US. In this study, water availability represented the mean annual volume of water available to cities, given a set of basic environmental limits on withdrawals. Urban water vulnerability was measured as the susceptibility of those urban supplies under low-flow (drought-like) conditions. To better understand the impact of urban hydraulic infrastructure on water scarcity assessments, water availability and urban water vulnerability were determined by two different approaches; a conventional method that estimates availability and vulnerability solely on local renewable flows, and a hydraulic-based approach which accounts for the impact of infrastructure on water supply.

Water availability measurements (Figure 1) based solely on renewable water supplies indicated that nearly half of the sampled urban population (47%) faced moderate (27%) or severe (20%) risk of water scarcity. Of those considered “at-risk”, 14 urban areas were identified as having availability levels below the national average of 600 liters per capita per day (lpcd). These results suggest that these cities suffer perpetual water shortages not from variability in supply, but rather from a perennial, systematic, lack of water. In contrast, water availability assessments that included supplies obtained using hydraulic infrastructure placed no urban areas below the critical 600 lpcd threshold. The hydraulic-based analysis resulted in a substantial decrease in the estimate of the “at-risk” population, with only 17% of the sampled urban population experiencing moderate (13%) or severe (4%) risk of water scarcity.

The two assessments produced significantly discordant conclusions about water scarcity risk for 12% of the sampled population (19 urban areas). These 19 areas were assessed to be at high risk of water scarcity based upon the analysis that included estimates of renewable water only. Accounting for hydraulic infrastructure (large groundwater storages and imported water from networked reservoir systems) resulted in assignment of 18 of these urban areas to a low-risk category. The only urban area to trend in the opposite direction was Miami, FL. Located in humid, tropical south Florida, Miami benefits from rapid regional aquifer recharge, although from a hydraulic perspective the city’s accessible aquifer storage volume is relatively small compared to the urban demand. This suggests that the city’s water source faces a reduced capacity to provide drought protection or bear increased withdrawals without potential environmental repercussions. Fig 1 shows five of the largest urban areas (populations > 106) with significant differences between the two methods (MIA- Miami, FL; DEN- DenverAurora, CO; LAS- Las Vegas, NV; DFW- DallasFort WorthArlington, TX; PHX- PhoenixMesa, AZ).

Estimates of water vulnerability, or the susceptibility of urban supplies to low-flow (drought-like) conditions, were also computed for each urban area and categorized as being at severe (III), moderate (II), or little (I) risk of facing vulnerability issues (Fig 2). While the differences between the percent of the population considered at-risk were less striking than those from the availability assessments, there was a noticeable redistribution of which cities were considered vulnerable. Variation is due in part to the assumptions required. Using the runoff-based method, water supplies are assumed to be only local renewable flows, where humid locations tend to have high availability and low vulnerability. However, results from the hydraulic-based method – based on cumulative flows, storages, and imports – are only weakly tied to regional conditions of humidity. These assessments must factor in assumed environmental constraints for storages, which limit withdrawals from large sources.

The differences between the two methods can be large enough that for some locations relying on natural lake (e.g. Buffalo, NY) or groundwater (e.g. Gainesville, FL) sources, tighter restrictions on the hydraulic sources make these areas more prone to vulnerability issues, despite the overall size of the source. With no comparable estimates of urban scarcity available, validation of the vulnerability results was achieved using a qualitative proxy based on national media reports related to urban water scarcity issues. This text analysis revealed a significant correlation between the mean number of water scarcity-related reports and ratings of increased vulnerability as determined by the hydraulic-based approach. No correlation was observed between reports of frequency and vulnerability determined by the runoff-based method, suggesting that inclusion of hydraulic features in vulnerability assessments may better reflect the reality of urban water scarcity as reported in the popular press.

When vulnerability estimates were combined with availability data, the two analysis methods differed most substantially in terms of their assessment of urban areas in the western US. In western cities, both availability and vulnerability improved when urban infrastructure was included in the analysis, further emphasizing the importance of groundwater and regional water management in these drier areas. Cities in the eastern half of the US evidenced a more limited effect of analysis method on water availability ratings. However, the cities considered vulnerable varied between the methods, depending on whether they were susceptible to low-flows from renewable sources (runoff method) or from fluctuations in availability due to limited hydraulic inputs.

When considering water scarcity at the national level, estimates of availability and vulnerability based on renewable flows may not adequately describe urban systems, but appear to be useful for identifying areas where demand outpaces renewable recharge (i.e. arid areas) and thus where storage depletion may be occurring. The disparity between hydraulically-determined availability and vulnerability may help to explain the apparent contradiction between natural water abundance and widespread water scarcity in the US. Most urban areas have acquired sufficient supplies to meet mean annual demands, however, the reliability of these sources under low-flow conditions belies this apparent security in many cases. Yet while scarcity assessments factoring in hydraulic infrastructure appear to provide better measures of urban vulnerability, this is not to say that mining storages or importing distant water supplies are necessarily sustainable strategies for meeting urban water demands. However, inclusion of hydraulic components in water sustainability assessments provides a better context for understanding the nature and severity of urban water scarcity issues.

References:

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2. Crane, P., Kinzig, A., 2005. Nature in the metropolis. Science 308 (5726), 1225.

3. Grimm, N.B., Faeth, S.H., Golubiewski, N.E., Redman, C.L., Wu, J., Bai, X., Briggs, J.M., 2008. Global change and the ecology of cities. science 319 (5864), 756.

4. Kates, R.W., Parris, T.M., 2003. Long-term trends and a sustainability transition. Proceedings of the National Academy of Sciences 100 (14), 8062-8067.

5. Huston, S., Barber, N.L., Kenny, J.F., Linsey, K.S., Lumia, D.L., Maupin, M.A., 2005. Estimated Use of Water in the United States in 2000. United States Geological Survey.

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7. Means, E.G., West, N., Patrick, R., 2005. Population growth and climate change will pose tough challenges for water utilities. Journal American Water Works Association 97 (8), 40-46.

8. Ginley, J., Ralston, S., 2010. A Conversation with Water Utility Managers. Journal American Water Works Association 102 (5), 117-122.

9. Hurd, B., Leary, N., Jones, R., Smith, J., 1999. Relative regional vulnerability of water resources to climate change. Journal of the American Water Resources Association 35 (6), 1399-1409.

10. Vörösmarty, C.J., Green, P., Salisbury, J., Lammers, R.B., 2000. Global Water Resources: Vulnerability from Climate Change and Population Growth. Science 289 (5477), 284-288.

11. Padowski, J.C., Jawitz, J.W., 2012. Water availability and vulnerability of 225 large cities in the United States. Water Resources Research 48 (12).

Julie Padowski is a postdoctoral fellow at Stanford University in Palo Alto, California working on the Global Freshwater Initiative. Her research focuses on water resource sustainability and adaptive water management. She holds a Ph.D. in Soil and Water Science from the University of Florida. The work presented here draws on research published in Water Resources Research, entitled: ‘Water availability and vulnerability of 225 large cities in the United States’. Julie can be reached at: padowski@stanford.edu. James Jawitz is a Professor and Associate Chair of the Soil and Water Science Department at the University of Florida. His research focuses on human impacts on natural hydrologic ecosystems and hydro-ecological modeling. He also develops techniques for remediation or contaminated groundwater and soils. He can be reached at: jawitz@ufl.edu.

The views expressed in this article belong to the individual authors and do not represent the views of the Global Water Forum, the UNESCO Chair in Water Economics and Transboundary Water Governance, UNESCO, the Australian National University, or any of the institutions to which the authors are associated. Please see the Global Water Forum terms and conditions here.