The impact of urbanization on water vulnerability: A coupled human–environment system approach for Chennai, India

Abstract

While there is consensus that urbanization is one of the major trends of the 21^st century in developing countries, there is debate as to whether urbanization will increase or decrease vulnerability to droughts. Here we examine the relationship between urbanization and water vulnerability for a fast-growing city, Chennai, India, using a coupled human–environment systems (CHES) modeling approach. Although the link between urbanization and water vulnerability is highly site-specific, our results show some generalizable factors exist. First, the urban transformation of the water system is decentralized as irrigation wells are converted to domestic wells by private individuals, and not by the municipal authority. Second, urban vulnerability to water shortages depends on a combination of several factors: the formal water infrastructure, the rate and spatial pattern of land use change, adaptation by households and the characteristics of the ground and surface water system. Third, vulnerability is dynamic, spatially variable and scale dependent. Even as household investments in private wells make individual households less vulnerable, over time and cumulatively, they make the entire region more vulnerable. Taken together, the results suggest that in order to reduce vulnerability to water shortages, there is a need for new forms of urban governance and planning institutions that are capable of managing both centralized actions by utilities and decentralized actions by millions of households.

Introduction

The developing world is undergoing a major demographic transition from a rural, agrarian society to an urban, industrial one. By 2050, 70% of the global population will inhabit urban areas, up from about half today (United Nations, 2001). Almost all of this increase in urban population will occur in the developing world and more than half the growth will occur in just two countries, India and China (Cohen, 2004). The urbanization transition in developing countries today is fundamentally different from historical patterns in terms of the scale and rate of change (Seto et al., 2010). One of the challenges associated with the magnitude and speed of urban change will be to supply water to urban areas. With growing urban population size and density, additional water supply must be arranged from sources located outside the boundaries of the cities (Lundqvist et al., 2003) and more wastewater is collected, treated and released safely into the environment at a pace and scale unprecedented in history. Climate change is likely to further impact water supply by changing the frequency and severity of droughts. An estimated 3.1 billion urban dwellers will experience seasonal water shortages by 2050; almost a billion of these will experience perpetual shortages within their urban areas (McDonald et al., 2011).

There is emerging consensus that the relationship between urbanization and environmental change is bi-directional (Seto and Satterthwaite, 2010, Seto et al., 2010). However, the relationship between urbanization and water vulnerability is highly debated. An optimistic view, usually supported by engineers and hydrologists (Lundqvist et al., 2003, Meinzen-Dick and Appasamy, 2002), argues that urban water supply is rarely constrained by lack of sufficient water resources in the developing world, and that freshwater availability to cities can be increased by reallocating water from agricultural to urban uses (Rogers et al., 2000). Because urban uses currently account for, on average, 10–20% of the total water withdrawals in developing world basins (Gleick et al., 2002), modest improvements in agricultural water-use efficiency and storage could yield sufficient quantities of water to serve urban areas. It is also economically efficient to transfer water from low-value agricultural uses to high-value urban uses and many governments explicitly give high priority to drinking water provision (Meinzen-Dick and Appasamy, 2002). Urbanization may actually play a positive role in lessening inter-sectoral competition and reversing groundwater declines because of the conversion of agricultural land to less water-intensive urban-related uses (Kendy et al., 2007) and urban growth also is not generally constrained by competition with agriculture (Molle and Berkoff, 2006).

A more pessimistic view, usually taken by geographers and urban planners, argues that many urban centers will be unable to expand supply to meet the demand because of poor governance or inadequate co-ordination among relevant agencies (Vo, 2007a, Vo, 2007b). As cities grow without adequate supply infrastructure, they may become reliant on unsustainable extraction of groundwater or face frequent water shortages stifling further growth (Güneralp and Seto, 2008, Vo, 2007a, Vo, 2007b). Beyond a certain level of urban growth, a lack of water resources could slow down development and constrain further urbanization, a carrying-capacity based threshold which some call a “water resources constraint” (Bao and Fang, 2007).

These two perspectives have developed in parallel but distinct academic communities, and the contrast stems in part from disciplinary differences in framing the issue. By relying primarily on water-balances, water resources researchers overlook the coupling between water and urban systems and the problem of path-dependence: different human adaptations lead to different patterns of urban growth. By viewing urban water supply independently of the larger hydrologic system, urban planners and geographers often overlook the relatively small footprint of urban water supply on basin water balances (recent work on Phoenix's water supply linking governance and decision-making to land cover and water resources is a notable exception e.g. Gober and Kirkwood, 2010). Moreover, focusing only on average supply and demand neglects the variable nature of hydrologic systems. In reality, most water “crises” occur during droughts – when resource availability drops sharply albeit for a short period. Understanding the bi-directional links between urbanization and water resources requires examining the underlying nature of the relationship. Does urbanization result in long-term unsustainability of the resource base (e.g. via groundwater depletion)? Does urbanization mainly impact short-term vulnerability to water shortages during droughts?

This study contributes to the understanding of dynamic water vulnerability by addressing the following research question: Does urbanization increase or decrease a region's vulnerability to water shortages? We focus on vulnerability caused by water shortages during multi-year droughts under changing environmental conditions; no long-term trends in water resources availability were discernible in our study site. Long-term unsustainability in water resources occurs when a stored stock of water (aquifers, lakes, or wetlands) is gradually depleted over time. In places where the aquifer has limited storage and there is no surface freshwater body, the problem is not one of depletion of a non-renewable resource. Rather the problem is one of managing a renewable, but temporally variable, resource under an increasing baseline demand. Quantitative assessments of dynamic vulnerability remain rare and none have considered the impacts of large-scale urbanization in the developing world in a dynamic manner.

The article is organized as follows: Section 2 describes the conceptual framework used to evaluate the relationship between urbanization and water vulnerability. Section 3 describes the model including the assumptions and feedbacks between urbanization, supply and demand for water, and vulnerability. Section 4 presents results of the simulation model for the study site, Chennai, India and present vulnerability assessments in two different urbanization states. Section 5 discusses the results, followed by conclusions and directions for future research in Section 6.