Big City, Small Water Footprint | Fluence

Researchers have found that in general, larger cities have smaller per capita water footprints.

Research team hopes to produce tool to predict water use

A 2018 study by a team of Penn State researchers has discovered that large cities are generally more water efficient and productive than mid-sized cities. Large cities have a smaller water footprint for their size, but rather than being a case of economy of scale, the relationship may be linked to factors that push water-intensive industries to areas with smaller populations.

The team — Tasnuva Mahjabin, Susana Garcia, Caitlin Grady, and Alfonso Mejia — proceeded from recent demonstrations in diverse fields that show that multiple urban indicators and metropolitan functional properties can be described mathematically in terms of scaling relationships. These fields included engineering, economics, complex systems, and geography.

Differences in Efficiency

With partial backing from the National Science Foundation (NSF), the team set out to determine whether relationship of scale exists in the water footprints of cities of different sizes, and if so, what accounts for the differences in efficiency. Many studies have been conducted at the national or international level, with only a few conducted at the city level.

They found that if city water footprints conform to scaling theory, they will vary in predictable ways according to size. Ultimately, the team hopes to create a new predictive tool to inform urban water sustainability strategies and policies.

Penn State has been aiming to increase its leadership in water-energy-food nexus research.

Calculating Water Footprints

The Penn State research significantly focused on comparing water footprint scaling behavior across 65 small- to medium-sized cities in the United States, looking at direct water flow and virtual water. Caitlin Grady, assistant professor of civil engineering and co-author of the study, explained that the team examined the big picture:

[It’s] not just the water that comes out of your tap but also the water that goes into the food that each city produces and consumes, so it’s both the direct water use and indirect water use, which we call your water footprint.

The team analyzed data from the U.S. Department of Agriculture on livestock, agriculture, and industrial commodities and analyzed the virtual water they represent, using data from the U.S. Geological Survey and U.S. Department of Transportation. The study did ultimately demonstrate a sublinear relationship, meaning that as city size grows, the water footprint of each resident shrinks. This sublinear scaling is, therefore, considered “efficient.”

But large cities may attain this efficiency by externalization. Co-author Tasnuva Mahjabin explained:

[Large cities are] service-oriented with less prevalence to secondary sector industries. This allows large cities to have reduced water footprints by shifting water-intensive economic activities to less populated regions.

The relationship did not hold in all cases. Water from surface or groundwater generally tracked precipitation patterns with minimal correlation to city size. Water from precipitation, however, had mixed effects.

Exceptions to the Rule

Bucking the trend, Las Vegas, located in a basin on the floor of the Mojave Desert, fell well below the average water footprint production for a city its size. And New Orleans, which lies directly on the largest river in the nation, the Mississippi, had a much larger footprint than expected for a city its size. The river carries 4.4 million GPS of water through the city.

But water footprints are complex. A city may have ideal water conservation, yet be a major food producer and use more water than another city in its class. Caitlin Grady explained:

[If] you look at these things together you have a more complete picture of how you can manage the limited resources you have, and how you can prioritize the use of those resources.

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