Desert urbanization and the challenges of water sustainability

Arid regions and their cities are vulnerable to future water scarcity because climate change threatens to reduce supply and rapid growth increases demand. The study of water sustainability in these regions and cities transcends concern about climate change and growth, however, and includes the dynamics of water–energy relationships, tradeoffs involved in the use of irrigated landscaping for temperature amelioration, and feedbacks between urban growth, the economy, and the environment. It requires analysis of how complex human and biophysical systems function at a range of scales and calls for new tools for risk assessment and decision support incorporating decision making under uncertainty.

Introduction

Water scarcity is the defining physical characteristic of arid regions, which the Intergovernmental Panel on Climate Change (IPCC) has highlighted as especially vulnerable to impending climate change (Figure 1). Large-scale development in these regions (either urban or agricultural) depends upon the ability to pump fossil groundwater, convey freshwater over long distances from natural sources, and desalinate brackish water and seawater. Desert cities from Riyadh to Phoenix rely on high-energy-use infrastructures to supply water from large hydraulic hinterlands [1]. Within these hinterlands, they often compete for scarce water with other cities, the environment, and agriculture.

Many arid-region cities lie in highly urbanized countries, including the USA (79% urban), Israel (92%), Australia (83%), Saudi Arabia (81%), Bahrain (100%), United Arab Emirates (83%), and Qatar (100%). Although they will experience continued urban growth, the rate of growth is limited by the fact that they have completed the cycle of urbanization — the transition from rural to urban. In other countries where agriculture still dominates, such as China (46%), Pakistan (35%), Uzbekistan (36%), Somalia (37%), and Niger (17%), the cycle of urbanization is in an accelerated phase, and the potential for future urban growth is high [2^••]. Moreover, the impact of rapid urbanization on water demand is sizable because the water consumption patterns of newly urbanized households in developing countries converge with those of the more developed world and diverge from rural counterparts [3]. Thus, urbanization causes a nation's water demand to increase much faster than it would merely as a function of population growth. Global urban water use increased by a factor of 20 between 1900 and 2000 as the world's population tripled [4]. Continuing this trajectory would leave 55% of the world's population in water crisis by 2050 [5].

Coincident with rapid growth in urban demand is the potential for substantial reductions in water supplies of arid regions. The IPCC Assessment Report 4 predicts with high confidence that ‘semi-arid and arid areas are particularly exposed to the impacts of climate change on freshwater’ [6, p. 175]. Exposure stems from two sets of factors: (1) increased temperature and evaporation, which will reduce rainfall and the loss of soil moisture in the arid zone itself, and (2) reduced runoff from remote headwaters, which supply water to points of human settlement and economic activity [7, 8, 9, 10, 11••, 12••, 13••, 14••, 15]. Milly et al. estimate decreases in runoff in the range of 10–40% in southern Africa, southern Europe, the Middle East, and mid-latitude western North America by 2050. Efforts to offset these declines in surface water supplies are complicated by the fact that groundwater recharge also will fall due to increased aridity [6, p. 175].

The traditional approach to integrated water modeling focuses on the water budget and asks whether there will be adequate water to support urban growth under particular climatic conditions. This type of modeling dominates studies of arid-region water systems in the western United States [16••, 17••], southern Europe [18, 19], Middle East [20^••], and Australia [21••, 22••, 23••]. Increasing interest in sustainability science challenges the water sciences to dig deeper into the interconnections between water and other urban resources, critical feedbacks that may alter growth trajectories and cause unintended consequences, scale effects — how system dynamics function across varying spatiotemporal scales [24], and new paradigms for decision making under uncertainty. In a call to arms, Alcamo et al. [25^••] contend that the water science and policy communities have overemphasized local and regional studies at the expense of research into systemic change and interconnectivities in the global water system. Focus on small-scale processes increases the risk of missing critical vulnerabilities in aquatic ecosystems that protect species and provide ecosystem services to human populations.