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Cross-posted from Cool Green Science, a Nature Conservancy blog.

You’re probably doing your part to conserve water, especially if you live in a drought-stricken area. But water is in short supply across the globe because of people’s increasing demands for it — a huge problem for cities, agriculture, and industry that will only get worse with climate change.

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Getting an accurate handle on what’s causing the problem has been missing — until now. A new study in the journal PLoS ONE, coauthored by Nature Conservancy scientist Brian Richter, provides fresh insight into the factors behind water shortages in the world’s most important river basins.

The study provides the most comprehensive picture of the global water problem to date, looking at monthly rather than annual changes and digging into the actual causes of water depletion — agricultural, industrial, and domestic — in our ecosystems. While the findings aren’t rosy — more than 2 billion people are affected by water shortages each year — coauthor Richter says there are still reasons to be hopeful … read on to the end of this Q&A with him to find out what they are.

Q. Why is this study so important? Your numbers seem to corroborate previous findings — water is scarce and getting more so, affecting billions of people worldwide.

A. Previous studies only looked at the amount of water withdrawn from freshwater sources, not taking into account the fact that much water is returned after use. For example, cities use a lot of water, but 80-90 percent of that water is returned to the original water source after use. If it’s returned in good quality, it doesn’t deplete the water source and the water is available for others to use and to sustain aquatic life.

Even with this distinction between water consumption and depletion, our study shows that it’s getting very difficult to meet water needs in more than half of the river basins in the world — potentially affecting some 2.7 billion people.

And we found that 92 percent of this water depletion globally is tied to agriculture.

Q. So it sounds like ag is the place to start in order to solve the water problem. Can we change our farming practices?

A. Unlike cities, most water used to irrigate farms is not returned to the ecosystem. On average, more than half of that water is lost to the atmosphere — it either passes through the plant during growth or evaporates from the soil. So cities use more water than crops on a per-area basis, but it’s important to note that irrigated agriculture occupies four times as much land as cities do.

We need to help farmers implement state-of-the-science irrigation methods and improve the productivity of rain-fed farms as soon as possible. We are going to have to produce more food with less water. If we can’t do that, we can’t add another 3 billion people to our planet. No way. And we will kill our rivers and lakes in the process.

Q. Tell us more about the impacts of water shortages — who or what will be hit the hardest?

A. We would expect that places with scarcity at some time during the year are already experiencing ecological degradation in rivers or lakes, perhaps with recurrent fish die-offs. In places with multiple months of scarcity, they are likely experiencing serious competition for water, and during droughts they’ll have economic impacts in agriculture, power production, or other industries.

For example, during the 2007-8 drought in Georgia, a severe reduction in hydropower generation cost the Southern Company electricity provider more than $33 million. Water shortages in California in 2009 devastated the state’s agricultural industry, leading to an estimated loss of 21,000 jobs and more than $1 billion in revenue.

The competition for water between cities and farms is getting rather intense. If you’re living in an area where the available water is already being heavily depleted, it makes it very difficult and expensive for a city to access additional water needed for its growth. That’s why cities are spending billions to import water from far-distant rivers or remove the salt from seawater.

Q. There’s been lots of talk about companies labeling their products — from food to wine to clothing — with information about the “water footprints” of those products. Do you see this as a beneficial step? How close are we to it?

A. Labels could be very misleading and not very useful if they aren’t done right. The reality is that it takes different amounts of water to produce the same consumer item in different parts of the world, so just telling the consumer how much water was used in the product is not very useful.

What is far more important is whether the company producing that good is using water as efficiently as possible, and whether they are contributing to sustainable management of the shared water resources wherever they operate. That’s what the Alliance for Water Stewardship (AWS) will be seeking to certify beginning in 2013; if they decide to put out a label, it would be extremely useful because consumers could have confidence that the certified company they’re buying from is a good water steward.

Q. You also suggest that trade for agricultural products can help by importing water-intensive products from more water-rich areas — are locavores going to be up in arms over this?

A. When you know where your food is being grown and who’s growing it, you have a better chance of understanding any problems that might be caused by agriculture and you can help resolve those problems. So it’s good to try to buy our food from the closest-possible sources.

But buying everything locally simply isn’t possible for everyone. When you’re buying a food product from some distant part of the world, you want to ensure that the water used to produce it is being used sustainably — that’s what an AWS label could communicate.

Q. In your study you note that an increasing demand for biofuels is a factor worsening water scarcity — how big of a deal are biofuels?

A. It takes 10,000 gallons of water to produce one gallon of biofuel. That’s the most water-intensive way to produce energy. In the process, you’re trading off the use of a lot of water and land that could be used for producing food or supplying water to cities. We need to take a much harder look at those trade-offs.

Q. Tell us about some of the river basins seeing the highest amount of scarcity — what impacts are already being felt by people and ecosystems? How will those impacts intensify in the future?

A. Texas is the place to watch at the moment. That state’s rivers and aquifers are among the most heavily depleted in the world. When you put a drought on top of already-stressed water sources you can expect catastrophes.

Last year, the state lost more than $5 billion in agricultural production, and now they’re facing serious electricity shortages if they don’t get a lot of rain soon. Water depletion is seriously impacting tourism, aquatic life, and coastal fisheries.

To Texas’ credit, they have some of the best water policies and institutions found anywhere in the world. For example, the state updates its 50-year water plan every five years, and it does so with a great deal of citizen input. They have invested heavily in computer models that track how much water is being used, and for what purpose. They have policies that require that adequate water remain in their rivers to support aquatic life.

Let’s hope those plans and policies can get them through this crisis.

Q. What about climate change — how will variations in rainfall, flood, and drought patterns hurt the water situation?

A. We’re quite certain that things are going to look considerably worse in coming decades because of multiple factors (population growth, changing diets in India and China, etc.), including climate change.

Take rain-fed farmland, for example — 60 percent of food is grown with “green water” or direct rainfall. Even if the average annual rainfall stays the same, we expect that it will be distributed across the year in different ways, coming in different seasons or in more intense rainstorms. That could be catastrophic for agricultural production. I expect to see a lot of adjustments in the types of crops being grown in many regions.

Q. Speaking of the future: As an expert on global water issues, the future must seem pretty bleak to you. What keeps you hopeful that we can get this water problem under control?

A. Ironically, I’m optimistic because we waste so much! In the near term, we could make huge progress in reducing water stress by using the technology and knowledge we already have to reduce our water use. We have to start investing deeply in improving water use in agriculture or we’re going to see cultures, economies, and national security come apart in many regions of our world.

The work that The Nature Conservancy is doing with farmers on the Flint River in Georgia or along Silver Creek in Idaho are great examples of how to reduce agricultural water consumption while still producing the same amount of crops.