Cities: Investing in conserving your watershed just might help secure water for your city.
That’s one of the key findings from a just-released report from The Nature Conservancy on the potential for nature to help protect the water supplies of hundreds of the world’s largest cities — supplies that will face mounting demands and threats from growing urban populations, agriculture and climate change.
Our report — the Urban Water Blueprint— quantifies the potential for conservation activities to help protect these cities’ water supply by mapping where nature could most help protect urban water supplies, and how much conservation investment would be needed to achieve impact.
It finds that, for about one in four cities around the world, investment in source watershed conservation could pay for itself in terms of avoided water treatment cost.
Why Conservation Can Be Critical to Securing Water for Cities
The report combines hydrological models and data from the City Water Map, which scientists at the Conservancy and others created to capture where 534 large cities get their water from.
The information the report provides is critical for urban planners across the globe. That’s because the condition of a city’s watershed (the areas upstream of a city’s water source, like a lake or reservoir) affects the quantity and quality of the water that eventually reaches the city’s water source.
For instance, forest and other natural habitats help prevent soil erosion and maintain water quality. On the other hand, if a city’s source watershed contains a lot of cropland or developed areas, it can be difficult to prevent pollutants from reaching a city’s water source.
For each of the 534 cities in the City Water Map, we estimate the water quantity and quality risks that the cities’ water supplies faced. More importantly, we assessed the opportunity for five common conservation strategies to reduce sediment and nutrient pollution for urban water supplies:
- Forest protection, which reduces the risk of future deforestation increasing pollution
- Reforestation of pastureland to reduce pollution
- Agricultural best management practices, to prevent erosion and nutrient runoff
- Riparian restoration to prevent pollution from reaching streams
- Forest fuel reduction, which reduces the future risk of catastrophic wildfire and massive erosion
Why Small is Beautiful and Other Factors When It Comes to Watershed Conservation
Our approach moves past platitudes about source watershed conservation to specific predictions about how it might work for specific cities. And it reveals that, for effective conservation interventions, hydrology, geography and size matter.
Source watershed conservation, the report finds, makes the most sense where there’s a small source watershed that serves a big population, such as New York City.
Indeed, New York City is the poster child for source watershed conservation, which has clearly helped the city save hundreds of millions per year. But the strategy may not be as helpful in those other three out of four cities.
For instance, the five conservation strategies we considered are helpful primarily for maintaining the water quality of surface water sources, such as reservoirs, lakes, and streams.
Thus, cities that depend primarily on groundwater — like Berlin and Mexico City — have relatively low potential to be helped by these specific strategies. (Other strategies, like protecting aquifer recharge areas, are possible for these cities).
Even for surface sources, there is enormous complexity in how cities obtain water. Sao Paulo, for instance, draws from twelve different surface water sources, some of them interlinked with others. Some cities draw water from a river immediately adjacent to them, while some cities go hundreds of miles away for water.
This geography ends up mattering a lot when you evaluate the potential for nature to help urban water supplies.
And as for New York, we discovered that for source watershed conservation efforts, small is beautiful.
The size of the source watershed from which a city draws varies enormously, from a few thousand hectares up to millions of hectares. New York City draws most of its water from a watershed around 84,000 ha in size, while Bangkok draws its water from a watershed around 14 million ha in size.
Perhaps unsurprisingly, the conservation action required to meaningfully change the water quality in these two reservoirs varies accordingly.
In general, in small watersheds conservation action is needed on fewer hectares to change the concentration of pollutants an equivalent amount. This makes running conservation projects much more feasible in New York’s watershed than in Bangkok’s.
Small watersheds also seem to offer a greater return on investment, all else being equal. It costs less to work on fewer hectares. This in turn means it will be more likely to be cost effective for the water utility.
For instance, if a conservation action reduces the costs of treating water at a facility, then the utility will be more likely to embrace conservation. On the other hand, the size of the city’s population also matters. All else being equal, a big city is able to spend more money to protect its water supply than a small one.
The bottom line: While there is no universal strategy for solving conservation challenges, our report shows that source watershed conservation is an important tool in the toolbox for urban water utilities, to be used where the conditions are right.