Abstract
Many urban indicators and functional citywide properties have been shown to scale with population due to agglomeration effects. We hypothesize that scaling relations may also exist for water-related urban indicators such as the water footprint. The water footprint is an indicator of water use that measures humans’ appropriation of freshwater resources. We analyze the scaling of the water footprint for 65 mid- to large-sized US cities using both empirical estimates and a social interaction network model of city functioning. The network model is used to explain the presence of any scaling exponent in the empirical estimates of the urban water footprint by linking to previous theories of urban scaling. We find that the urban water footprint tends to approximately show sublinear scaling behavior with both population and gross domestic product. Thus, large cities tend to be more water footprint efficient and productive than mid-sized cities, where efficiency and productivity are quantified, in a broad sense, as deviations from a linear scaling exponent. We find the sublinear scaling may be linked to changes in urban economic structure with city size, which lead to large cities shifting water intensive economic activities to less populated regions. In addition, we find that green water contributes to the scaling both positively by transferring the dependence of food consumption on population into the water footprint and negatively by increasing heterogeneity. Overall, the proposed scaling relations allow for the comparison of water footprint efficiency and productivity of cities. Comparing these properties and identifying deviations from the expected behavior has implications for water resources and urban sustainability.
Highlights
For the first time in human history, the 21th century has seen the advent of a city-dominated human settlement pattern where the majority of people live in cities [1]
The Freight Analysis Framework version 3 (FAF3) data represents, for the year 2007, the flow of economic commodities among 123 different origin-destination regions encompassing the entire geography of the US
metropolitan statistical areas (MSAs) are defined by the US Census Bureau as a geographical region containing at least one city core with population greater or equal 50,000, together with any adjacent counties that have a strong economic tie to the city core
Summary
For the first time in human history, the 21th century has seen the advent of a city-dominated human settlement pattern where the majority of people live in cities [1]. People are moving to cities because of opportunities, economic development, change in social structure and human behavior [2]. By 2030, cities in developing countries are expected to double in population and cities in developed countries are expected to increase by 20% [3]. It is increasingly recognized that cities are central to global sustainability because they can collectively create substantial stress on interconnected natural resources (e.g., food and water) [4] and have governance structures that allow for more flexible and independent decision making [5].
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