Inter-City Virtual Water Transfers Within a Large Metropolitan Area: A Case Study of the Phoenix Metropolitan Area in the United States

Water footprints have been proposed as potential sustainability indicators, but these analyses have thus far focused at the country-level or regional scale. However, for many countries, especially the United States, the most relevant level of water decision-making is the city. For water footprinting to inform urban sustainability, the boundaries for analysis must match the relevant boundaries for decision-making and economic development.

Initial studies into city-level water footprints have provided insight into how large cities across the globe—Delhi, Lagos, Berlin, Beijing, York—create virtual water trade linkages with distant hinterlands. This study hypothesizes that for large cities the most direct and manageable virtual water flows exist at the metropolitan area scale and thus should provide the most policy-relevant information.

This study represents an initial attempt at quantifying intra-metropolitan area virtual water flows. A modified commodity-by-industry input-output model was used to determine virtual water flows destined to, occurring within, and emanating from the Phoenix metropolitan area (PMA). Virtual water flows to and from the PMA were calculated for each PMA city using water consumption data as well as economic and industry statistics. Intra-PMA virtual water trade was determined using county-level traffic flow data, water consumption data, and economic and industry statistics. The findings show that there are archetypal cities within metropolitan areas and that each type of city has a distinct water footprint profile that is related to the value added economic processes occuring within their boundaries. These findings can be used to inform local water managers about the resilience of outsourced water supplies.