Robert McDonald, The Nature Conservancy, Arlington, VA, USA

Presentation Title: The global sustainability of urban water sources




Urban growth is increasing the demand for freshwater resources; yet, surprisingly, the water sources of the world's large cities have never been globally assessed, hampering efforts to assess the distribution and causes of urban water stress.  This research team conducted the first global survey of large cities' water sources and then evaluated the sustainability of those cities' water supplies in terms of water quantity and water quality. It shows that previous global hydrologic models that ignored urban water infrastructure significantly overestimated urban water stress. Large cities obtain 78±3% of their water from surface sources, some of which are far away: cumulatively, large cities moved 504 billion liters a day a distance of 27,000±3800 km, and the upstream contributing area of urban water sources is 41% of the global land surface. Despite this infrastructure, one in four cities, containing $4.8±0.7 trillion in economic activity, remain water stressed due to geographical and financial limitations. The strategic management of these cities' water sources is therefore important for the future of the global economy.

 Policy/Practice Implications

The work on quantifying the dependence of cities on ecosystem services is motivated by a desire to increase investment by water utilities in source watershed conservation. However, even if there is a clear economic rationale for such an investment (i.e., green infrastructure is cheaper than the grey infrastructure alternative), there are numerous institutional barriers to such investment. Municipal utilities often do not have the jurisdiction to invest in conservation outside their boundaries; negotiating land transactions with multiple landowners may simply have too high a transaction cost; and in large basins with multiple water users, collective action is needed. One way out of these challenges is having a formal “watershed utility” that can serve to bring water users and landowners together and pool funds for wise stewardship of habitat that generates crucial ecosystem services.

Knowledge Gaps and Needs

While the biophysical data (slope, land cover, ecosystem function) is fairly well characterized, and the City Water Map begins to characterize the grey infrastructure (reservoirs, canals, etc.), it is still very difficult to know much about the social or institutional context for multiple cities. Ideally, this kind of information would be available so it would be possible to answer questions like:

  • Are certain institutional governance structures more likely to lead to wise watershed management?
  • Has investment in source watershed conservation been greater in some institutional contexts than others?
  • For trans-boundary urban source watersheds, has that lead to conflict or collaboration between cities?