The Science Behind Mapping Our Water

Condensation, evaporation, precipitation… yes, all these “-ations” have something to do with where your water comes from.

But even if you understand the basic science behind the water cycle, do you really know the source of the water that flows from your tap? According to a 2011 Nature Conservancy poll, 77% of Americans don’t know where their water comes from.

Now there’s a remedy for that: The Nature Conservancy has created an interactive map of the drinking water sources for 493 cities across the globe, including 27 of the largest cities in the United States. If you are one of the 414 million people around the world living within these areas, you can now hop online and click around to find which rivers, lakes and streams supply water to your tap. Check it out.

To learn more about the science behind a map like this, I spoke with the Conservancy’s Kirk Klausmeyer. A conservation planner in California, Kirk worked closely with a small team—including geographers, marketers, water experts and an external researcher—to identify water sources and create an online map that is both easy to use and accurate with the most recent data available.


Explain why this was needed: Don’t maps of rivers and lakes exist already? Water utilities must know where their water is coming from.

Yes, water utilities have a good idea of what the watershed of their water sources look like, but most don’t have any maps. Some do—for example, New York City and California’s East Bay MUD—but many either don’t have maps at all or don’t have maps that the public can readily access and use.

What makes the situation complicated is that, in many cases, multiple rivers and lakes supply water to a utility, and sometimes one utility supplies water to many others. In some cases, water is diverted from one watershed to another miles away, stored there, and then pumped back. So tracing the water from a user to its source is like putting together a puzzle, and now we are finally starting to see what the final picture looks like.

What’s really cool about this online map is, the data gives people direct access to the latest science. I can go in and update the data in real time—change things, add a city or remove one—and it’s instantly reflected in what the public sees.

What kind of data and technology did you need to create this map?

We relied primarily on the Consumer Confidence Reports that each utility is required to create by the EPA. These reports list (but do not map) the water sources for each utility. Unfortunately, there is no central database of these reports, so our ace researcher and lead author Katie Fitzgerald had to look at each city one at a time. It took months.

Once we knew the names of the water sources, we dug a little deeper for a subset of 27 U.S. cities. We mapped each water source and point of diversion using the National Hydrology Dataset and identified the local sub-watershed using the USGS Watershed Boundary Dataset. Then we built a computer algorithm that would link each water source to all of the sub-watersheds that flow to it—essentially building a tree of sub-watersheds.

The cool thing about the computer code we built is that others can take it and use it to connect the dots for their water sources.

Once we identified the watershed sources, we then wanted to investigate the condition of the watershed. So we used National Land Cover Database and U.S. Protected Areas Database and layered those with the watershed maps. This way we could calculate the level of protection and the amount of development in the watersheds that supply drinking water to each of the 27 U.S. cities.

What did you find out about the condition of these watersheds, and how can this information help point conservationists in the right direction for future protection efforts?

We found that if you look at the condition of the land that supplies water to 27 of the largest cities in the U.S., these lands are made up of:

  • 8% urban/suburban lands;
  • 15% agricultural lands;
  • 37% protected lands;
  • 41% private, undeveloped lands.

For the largest category—the 41% that are in private, undeveloped lands—it could be a variety of land types, such as: military lands, timber lands, water utility owned, forest or grasslands privately owned for ranching (which is not considered agriculture, just cultivated crops are). These are basically lands that are not formally protected but often do have some land-use restrictions. While some of these watersheds are in forested areas that are often too steep to cultivate for agriculture or urbanization, others could be developed. These are areas where a greater conservation effort will help protect people’s water sources from development and degradation.

The next largest category is the protected lands, at 37%. These protected lands prevent development, but they are not always managed with a focus on water quality. For example, some of the multiple-use protected areas allow clear-cutting, mining and off-road vehicle parks. Improving the management of our public lands is still an important conservation priority.

For agricultural lands, the focus should be on best management practices that minimize the amount of fertilizers and pesticides that reach streams and lakes. For urban lands, improvements can be made to storm water systems to use plants to filter the runoff before it hits to the streams.

Some areas have very high rates of watershed protection already. For instance, Portland, OR, has the highest percentage of protected areas (90%) in watershed and the remaining area is owned by the water utility. Other cities have more work to do, like Atlanta, with 32% of their watershed in urban/suburban development.

What about water quality? Can people look at this map and assume that if their water source is largely agricultural, then their water isn’t as clean?

All U.S. tap water is heavily monitored and treated. So you can have confidence that your water is safe to drink, there’s no public health risk.

But we’re exploring how the cost of treatment is tied to the different land uses found within watersheds. How much does the cost to treat water from agricultural areas vary from protected areas? Interestingly, some of the more expensive treatment needs come from natural sources, such as rock formations leaching uranium and arsenic into water supplies.

We have found some anecdotal evidence so far. For instance, in California the treatment costs for water coming through the Central Valley, which is an agricultural area, are 3 times higher than water coming from Hetch Hetchy reservoir, which is in a protected area (Yosemite National Park).

What was the most interesting finding from this map?

When I first started looking at the maps we were generating, I thought something was wrong—I couldn’t believe so much land area goes to filtering water. Over 140 million acres for Los Angeles alone—that’s one-and-a-half times the size of California.

But the more we worked with the data, the more we found that we’re not talking about tiny, fenced-off watersheds—we’re talking about huge areas with diverse land use. For instance, New Orleans (which doesn’t have its own city map yet) gets its water from the mouth of the Mississippi River—which has an enormous watershed covering 736 million acres and parts of 31 U.S. states and 2 Canadian provinces.

It was also interesting to untangle the engineered aqueducts, pipelines and tunnels that move water around, especially in the West. In Colorado, for example, 80% of the state’s  rain falls on one side of the mountains and 80% of the people live on the other side—there are multiple aqueducts that crisscross the Continental Divide. There’s huge complexity in the engineering of our water and who has water rights and how far we need to go to get our water.

What areas are the most vulnerable?

The main area to look at is urban development in watersheds. Urban landscapes send water not through ground soils but over streets, pavement, storm drains, etc. Which means that water can pick up everything on the ground—like chemicals applied to lawns, antifreeze, etc.—and put it into our streams and water sources.

Think about what you might find in a Walmart parking lot—do you really want that in your water?

I’m confident in our water treatment plants. But this is a precautionary note for the future. And when you add the cost of treating water and the multiple benefits of intact watersheds, does it make more sense to ensure protection of our natural filtration systems?

What do you hope people will get out of this map?

My goal for this is for people to be inspired by this map and to go visit their watershed, to connect to the land. For instance, there’s a lake near where I live that I go bike and run around, and I found out it’s a backup water supply for my house. Now I look at it in a different way, because it’s directly connected to my drinking water.

Knowing where your water comes from gives you a connection to the land and helps you get more acquainted with nature.

Hopefully, this leads people to understand how important it is that our government fund watershed protection efforts. Preserving and strengthening the Clean Water Act and other laws that address water pollution is critical.

Read the full report and explore the interactive map.

If you believe in the work we’re doing, please lend a hand.

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