Complexity, costs and services of urban ecohydrology: From urban footprints to regional climate mitigation

Background/Question/Methods Ensuring the sustainable supply of water for many of the world's cities has been and continues to be a substantial challenge.  How urban water is partitioned between the diverse users, the services provided by the water, and the costs associated with water delivery are not well understood.  An urban ecohydrological perspective may help to better understand the costs associated with the services provided by urban water allocation decisions.  To better understand the variation in area required to supply urban water needs we used an urban footprinting approach.  This approach was applied to a stratified selection of cities distributed throughout the United States.  We examined the role of geographic, social, and climate factors affecting the size of the urban footprint.  We further compared the footprint size to the municipal costs of water to develop an empirical price per footprint area.  We highlight one service provided by water redistribution, regional climate modification in the arid metropolis of Phoenix, AZ.  An initial study examined the relationships within this city between surface temperature, vegetation, and social segregation.  We are currently extending this analysis by directly measuring and modeling energy budgets for individual plots representative of urban patch types.

Results/Conclusions

Urban water footprint areas were differentially sensitive to changes in consumption and changes in water availability; our results suggest climate induced reductions in water availability may be equally important as population growth in affecting urban water footprints.  From the relationship between annual municipal utility expense and footprint area, the median monetary value for water footprint area was $88,808 km^2 yr^1 from all the cities we considered. The ratio between the footprint-estimated cost and the utility-observed cost was negatively related to the local availability of water, and was independent of population size.  Within a summer day in Phoenix, AZ we found substantial surface temperature differences within the city that correlate primarily vegetation cover, which is in this region is directly linked to irrigation activities. Furthermore, both of these patterns vary systematically with the social characteristics of neighborhoods through the region.  Overall, every 10,000 dollar increase in neighborhood annual median household income was associated with a 0.28 degree C decrease in surface temperature on an early summer day in Phoenix.  Linking ecosystem services provided by water with footprint derived estimates of the land area required to supply the water my help identify the trade-offs associated with urban water deliveries.