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Estimating watershed degradation over the last century and its impact on water-treatment costs for the world’s large cities

  1. Daniel Shemief
  1. aGlobal Cities Program, The Nature Conservancy, Arlington, VA 22203;
  2. bYale School of Forestry & Environmental Studies, Yale University, New Haven, CT 06511;
  3. cCenter for Environmental Research, Education and Outreach, Washington State University, Pullman, WA 99164;
  4. dState of Washington Water Research Center, Washington State University , Pullman, WA 99164;
  5. eOffice of the Chief Scientist, The Nature Conservancy, Arlington, VA 22203;
  6. fGlobal Freshwater Program, New York, NY 10001

  1. Edited by B. L. Turner, Arizona State University, Tempe, AZ, and approved June 14, 2016 (received for review April 5, 2016)

Significance

Urban water-treatment costs depend on the water quality at the city’s source, which in turn depends on the land use in the source watersheds. Here, we show that globally urban source watershed degradation is widespread, with 9 in 10 cities losing significant amounts of natural land cover in their source watersheds to agriculture and development. This watershed degradation has impacted the cost of water treatment for about one in three large cities globally, increasing those costs by about half. This increase in cost matters because increases in water-treatment costs are paid for by those living in cities, so watershed degradation has had a real quantitative cost to hundreds of millions of urbanites.

Abstract

Urban water systems are impacted by land use within their source watersheds, as it affects raw water quality and thus the costs of water treatment. However, global estimates of the effect of land cover change on urban water-treatment costs have been hampered by a lack of global information on urban source watersheds. Here, we use a unique map of the urban source watersheds for 309 large cities (population > 750,000), combined with long-term data on anthropogenic land-use change in their source watersheds and data on water-treatment costs. We show that anthropogenic activity is highly correlated with sediment and nutrient pollution levels, which is in turn highly correlated with treatment costs. Over our study period (1900–2005), median population density has increased by a factor of 5.4 in urban source watersheds, whereas ranching and cropland use have increased by a factor of 3.4 and 2.0, respectively. Nearly all (90%) of urban source watersheds have had some level of watershed degradation, with the average pollutant yield of urban source watersheds increasing by 40% for sediment, 47% for phosphorus, and 119% for nitrogen. We estimate the degradation of watersheds over our study period has impacted treatment costs for 29% of cities globally, with operation and maintenance costs for impacted cities increasing on average by 53 ± 5% and replacement capital costs increasing by 44 ± 14%. We discuss why this widespread degradation might be occurring, and strategies cities have used to slow natural land cover loss.

Footnotes

  • ↵1To whom correspondence should be addressed. Email: rob_mcdonald{at}TNC.org.

  • Author contributions: R.I.M., K.F.W., J.P., T.B., and D.S. designed research; K.F.W., J.P., and T.B. performed research; R.I.M., K.F.W., J.P., and D.S. analyzed data; and R.I.M. wrote the paper.

  • The authors declare no conflict of interest.

  • This article is a PNAS Direct Submission.

  • This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1605354113/-/DCSupplemental.

Freely available online through the PNAS open access option.

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