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Watershed Degradation Costs Global Cities $5.4 Billion in Water Treatment Annually

July 26, 2016

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A glass of fresh drinking water. (studio photograph against a background photo of a West Virginia waterfall taken by Kent Mason) Photo © The Nature Conservancy and Kent Mason

There is a large body of science evidence from particular water utilities that 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. And there is a lot of anecdotal evidence from particular source watersheds that land-use has really degraded water quality.

But nobody had a good estimate of how significant this process was globally. How much has watershed degradation really increased water treatment costs for the world’s cities?

Our study, entitled “Estimating watershed degradation over the last century and its impact on water-treatment costs for the world’s large cities” basically focused on quantifying this one number.

In order to get at that number, you need first of all to know where cities get their water from. This sounds simple, but is actually quite complex, as many cities are served by multiple water utilities, and most water utilities draw water from multiple sources, utilizing a complex mix of surface, groundwater, and other sources. We have been mapping these source as part of an effort called the City Water Map, a map that has been a couple years in the making. Here we use extract information for the water sources of 309 large cities (population > 750,000).

You then need information on the long-term trend in land-use in these source watersheds. Here we built off of efforts of the History Database of the Global Environment (HYDE), which allowed us to reconstruct changes in land-use and population density over more than a century. From there, we then estimated how much these changes in watershed use are correlated with water quality. Finally, we needed to make the link to water treatment costs, so we assembled a database of the technologies cities use to treat their water, which we can statistically related to water quality.

After all that, what did we find? We found 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. Watershed degradation has impacted the cost of water treatment for about one in three large cities globally, increasing those costs by about half. If you add up the impact globally, that is around $5.4 billion a year in economic impact.

Trends over time in watershed degradation and water quality in urban source watersheds. Estimated pollutant yield over time, relative to the average pollutant yield in 1900. Figure 2C in Robert I. McDonald et al. PNAS doi:10.1073/pnas.1605354113. © 2016 by National Academy of Sciences
Trends over time in watershed degradation and water quality in urban source watersheds. Estimated pollutant yield over time, relative to the average pollutant yield in 1900. Figure 2C in Robert I. McDonald et al. PNAS doi:10.1073/pnas.1605354113. © 2016 by National Academy of Sciences

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. There is a certain power in being able to say precisely what that cost is, particularly dealing with policymakers.

On the other hand, as I have talked to reporters about this paper, I have been painfully reminded of how one number can never be the whole story. Reporters understandably want to know the whole story: the problem, why our results quantifying the problem matter, and what the solution is. And I think some of the Nature Conservancy’s conservation work can be part of the solution.

The Nature Conservancy works with cities globally to protect watersheds, both via traditional nature preserves and conservation easements; and with new funding strategies like water funds that connect urban residents with the protection of the sources of their drinking water upstream. These strategies all seek to connect the needs of water users and freshwater species, which both need clean water, with the landowners upstream whose actions control water quality. It won’t be all of the solution, but on a world that is getting increasingly crowded, I am convinced it is part of the solution to the $5.4 billion problem.

Rob McDonald

Dr. Robert McDonald is Lead Scientist for the Global Cities program at The Nature Conservancy. He researches the impact and dependences of cities on the natural world, and help direct the science behind much of the Conservancy’s urban conservation work. More from Rob

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2 comments

  1. This is an excellent article. I’m going to share it with my 1156 friends joined together in opposition to the construction of an industrial wind complex on the ridges of watershed in Penn Forest Township , Pa.
    This watershed has been classified as an “exceptional value” watershed. In testimony before the zoning board Dr. Pamela Dodds, Ph.D., Licensed Professional Geologist stated, “The damage to water resources resulting from construction of the proposed wind facility consists of 1) increased stormwater runoff; 2) decreased groundwater recharge; and 3) changes in the direction of groundwater flow or availability to exceptional value wetlands and water courses. Deforestation of approximately 292 acres of woodlands will result in increased stormwater runoff.” Dr. Dodds further testified,”Construction activities create impervious areas where forested areas previously existed. These impervious areas do not facilitate groundwater recharge, such that areas where water penetrated the ground as groundwater recharge can no longer provide avenues for surface water to reach shallow aquifers or to travel through rock fractures which intercept the ground surface in lower areas to form seeps and springs. This results in changes in the groundwater flow whereby groundwater does not maintain seeps and springs in the stream headwater areas. “
    With the possibility of such devastation to an “exceptional value” watershed , It seems unbelievable The Nature Conservancy could support this project!

    Jack Englehart
    Penn Forest Resident

  2. When discussing The Working Woodland program Stephan Repasch, Executive Director of the Bethlehem Authority stated, “ Our primary mission is to supply the highest quality drinking water to the 115,000 plus customers of the Bethlehem Water System as we possibly can. Preserving the pristine quality of our watershed properties supports that mission”.
    In a letter opposing the Penn East Pipeline, John Tallarico Bethlehem Authority Chairman urged the route be moved away from the watershed “such that there is no potential negative impact on the water supply for the over 115,000 customers in over ten Lehigh Valley communities.
    The Nature Conservancy designated the watershed as one of the world’s “Last Great Places.” It’s flora, fauna and spring fed creeks, which carry an exceptional value rating, have attracted scores of scientific studies on the habitat that took root there thousands of years ago when the glaciers receded.
    Regarding the watershed Mayor Robert Donchez said, “ It is a very important resource and we will make sure it’s protected.”
    Each of these gentlemen supports the wind farm project in the watershed. Surely they don’t believe the thirty seven – 525’ high wind turbines, access roads and crew quarters will magically appear in the watershed. Clear cutting of nearly 300 acres and bulldozing 16 miles of access roads and turbine sites will cause erosion and runoff full of debris from blasting for foundations in the ridges above the reservoirs.
    This watershed has been classified as an “exceptional value” watershed. In testimony before the zoning board Dr. Pamela Dodds, Ph.D., Licensed Professional Geologist stated, “The damage to water resources resulting from construction of the proposed wind facility consists of 1) increased stormwater runoff; 2) decreased groundwater recharge; and 3) changes in the direction of groundwater flow or availability to exceptional value wetlands and water courses. Deforestation of approximately 292 acres of woodlands will result in increased stormwater runoff.” Dr. Dodds further testified,”Construction activities create impervious areas where forested areas previously existed. These impervious areas do not facilitate groundwater recharge, such that areas where water penetrated the ground as groundwater recharge can no longer provide avenues for surface water to reach shallow aquifers or to travel through rock fractures which intercept the ground surface in lower areas to form seeps and springs. This results in changes in the groundwater flow whereby groundwater does not maintain seeps and springs in the stream headwater areas. “
    With the possibility of such devastation to an “exceptional value” watershed , It seems unbelievable The Nature Conservancy could support this project.The 115,000 plus customers must demand an impact study to protect the purity and quality of their water supply. Once the delicate ecosystem of the watershed is lost, it cannot be restored.

    Jack Englehart
    Penn Forest Resident