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

Cool Green Science
July 26th, 2016
By: Rob McDonald

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.

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