1. Virtual water transfers in U.S. cities from domestic commodity flows
Ikechukwu Chris Ahams 1 , Willa Paterson 1, and Alfonso Mejia 1
OBJECTIVE/APPROACH
STUDY AREA
For this study, we considered 65 major US cities based on the available commodity flow data
from the US Federal Highway Administration3. The city boundaries were delineated according to
the Metropolitan Statistical Areas and Combined Statistical Areas definitions of the US Office of
Management and Budget. Fig. 1 shows the spatial distribution of the 65 selected cities, together
with their boundaries and approximate population.
Urban water sustainability requires that cities improve their water-use efficiency to reduce or
shift water requirements. To attain this goal, cities need to track their water use and the impact
their consumption patterns have on freshwater resources. We implement here the water
footprint (WF) concept1 to quantify the domestic or internal water use of consumption and
production for 65 major US cities. We use available commodity flow, water-use, and virtual
water content data to estimate the WF2,3.
The commodities analyzed make up ~51% of the total domestic freight flow (Fig.
4). For cities, the industrial commodities make up ~90% by weight, leaving the
agricultural commodities with ~10% (Fig. 4).
On average, the analyzed cities produce ~30% of their own domestic WF of consumption while
the remaining 70% is imported from other locations, mostly non-city sources. This dependency is
illustrated in Fig. 2 for the 65 selected cities. Fig. 2 shows that there are large variations in the
level of virtual water dependency of cities to domestic locations outside the city.
In terms of the net per capita WF, Fig. 3 shows that some cities
are net consumers while others are net producers. To compute
the net per capita WF, we used the WF of production minus the
WF of consumption. However, the majority of the cities
considered here are net consumers, as may be expected given
the strong dominance of the agricultural sector in WF estimates
and the limited availability of productive land areas within cities.
1. Hoekstra, A. Y. and M. M. Mekonnen. 2012. The water footprint of humanity. Proceedings of the National Academy of Sciences.
2. Dang, Q., X. Lin, and M. Konar. 2015. Agricultural virtual water flows within the United States. Water Resources Research.
3. Paterson, W., R. Rushforth, B. Ruddell, M. Konar, I. Ahams, J. Gironás, A. Mijic, and A. Mejia. 2015. Water Footprint of Cities: A Review and Suggestions for Future Research. Sustainability.
Fig. 1
Animalfeed,12.60%
Cerealgrains,58.09%
Liveanimals,2.16%
Meat,12.14%
Milledgrainprods.,15.01%
Otherindustrialcommodities,30.35%
Basemetals,5.03%
Basicchemicals,5.72%
Coal-n.e.c.,17.80%
Nonmetalmin.prods.,21.15
Waste/scrap,19.95%
Proportion of individual industrial
commodities consumed
Proportion of individual agricultural
commodities consumed
Otherindustrialcommodities,29.58
Basemetals,5.83%
Basicchemicals,6.41%
Coal-n.e.c.,15.94%
Nonmetalmin.prods.,21.66%
Waste/scrap,20.58
Animalfeed,15.82%
Cerealgrains,48.43%
Liveanimals,2.98%
Meat,12.36%
Milledgrainprods.,20.41%
Proportion of individual industrial
commodities produced
Proportion of individual
agricultural commodities produced
Fig. 2
Fig. 3
Fig. 6Fig. 5
Fig. 4
REFERENCES
CONCLUSIONS
Water footprint of consumption Water footprint of production
RESULTS
1The Pennsylvania State University, Department of Civil and Environmental Engineering, 212 Sackett Building, University Park, PA 16802-1408
Corresponding author: Ikechukwu C. Ahams, 1ca102@psu.edu
Fig. 7 Fig. 8
- The urban per capita WF varies greatly from city to city, thus indicating
that cities are heterogeneous in terms of their agricultural and
industrial production and consumption.
- City resiliency is likely to exhibit large variations given that some cities
are mostly self-reliant while other depend strongly on locations outside
their boundaries.
Both Fig. 5 and Fig. 6 highlight the spatial heterogeneity of WF estimates
across cities. The WF of consumption is dominated by agricultural
commodities, making up ~ 99% of the total WF of consumption (Fig. 5). The
10 largest cities account for ~34% of the total WF of consumption (Fig. 5).
The WF of production is also dominated by agricultural commodities,
making up ~ 99% of the total WF of production (Fig. 6). The 10 largest cities
account for ~32% of the total WF of production (Fig. 6).
The WF of consumption (Fig. 7) shows more reliable (significantly higher R2
values) scaling relationships, both in terms of population and GDP, than the
WF of production (Fig. 8), except for the industrial sector.
Overall, the scaling tends to be sublinear (i.e., the WF decreases faster with
increasing size), suggesting that large cities place less stress on domestic
water resources than small cities.
- The scaling of the urban WF of consumption with population and GDP,
together with trade information, could be used to explore how cities may
shift stress from domestic to external freshwater sources as they grow.
- A few areas of improvement that we will investigate in the future are:
linking key industrial commodities to indirect sources and determining
more localized industrial water use coefficients.
H13D-1569