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ASA members talk water security, part 2

In 2011, the American Society of Agronomy (ASA) charged a task force of its members with answering a critical question: As competition for water intensifies and the global population soars, how can agriculture secure enough water to feed a hungry world?

Image of Earth taken from space

In part one of a three-part interview, ASA Water Security for Agriculture Task Force leaders Fred Vocasek (Servi-Tech, Inc.), Rattan Lal (Ohio State University), and Gary “Pete” Peterson (Colorado State University) defined the issues and explained why ASA has made water security such a priority.

In part two below, the trio outlined what it will take to achieve water security: Conserving "blue water,” maximizing "green water,” and reusing the "gray.” [See also the task force’s symposium at the American Association for the Advancement of Science (AAAS) meeting in mid-February: Green Dreams, Blue Waves, and Shades of Gray—The Reality of Water.]

These colors of water: blue, green, and gray. What do you mean by them?

Rattan: Functionally, there are several types of water. “Blue water” is the water that’s stored in rivers, lakes, streams, oceans, and in the ground. “Green water” is the component of blue water that’s actually used by plants for their growth—the water that enters the plant, and then is used and transpired. So, our objective is to maximize the green. [See part one of this interview for a discussion.]

Then there’s another type called gray water; it is the water that’s used by humans for industrial use, urban use, and contains some kind of contamination. So this means that if we want to use gray water for agriculture, some of the pollutants in gray water, especially heavy metals or other chemicals—asbestos, drugs, and herbicides—must be removed from the water first. So that’s another question: Can we perhaps convert gray water for reuse as green water?

So, the goal with green water is to maximize its use, and the goal with gray water is to recycle it as green water. What’s the goal with blue water in relation to water security?

Map of Dodge City, KS, showing area where wastewater is being used for irrigation

Fred: With respect to the hydrologic cycle and the recharge of water supplies, we need to look at timeframes of days, weeks, months, and centuries. For example, one of our consultants at Servi-Tech is working with a producer who needs to look ahead over the next few weeks or few months at how he’s going to use his water, in this case groundwater, to irrigate his crops. But at the same time he’s asking: Will the water be available for my son in the future if he takes over the farm? Or, will blue water still be available for crop production at 2050—decades in the future?

In other words, blue water is not limitless and we need to think about protecting it both now and into the future?

Fred: Yes, of all the blue water on the planet, only a very small fraction can be used as green water for producing food. So how do we deal with that?

Pete: Here’s an example. In Colorado, we have wastewater from cities that’s being used for irrigation on golf courses. So instead of using blue water, we’re substituting gray. Now, that brings up problems, too, because in this case the gray water is high in salts, and so those golf courses have to manage very carefully to prevent salt accumulation in their soil. So, there are additional problems that you deal with by using gray water instead of blue.

Rattan, you mentioned there are several types of water. Are there more than three?

Rattan: There’s another term that we didn’t use in the AAAS symposium, and that’s black water. Black water is actually gray water, but it’s contaminated by human fecal material. In developing countries, black water is a major cause of diseases because it contains bacteria, pathogens… and whatever else. But the advantage of black water is that it also contains plant nutrients. So the question is: Can black water rich with nutrients also be used as green water?

Fred: I’ll chime in here. Gary talked about the golf courses. In Dodge City, I’ve worked on a project now for 26 years where the city uses gray water from one of our meat packing plants (hence it’s black water, you might say), combined with the municipal wastewater stream, to irrigate about 3,000 acres of cropland. And Pete’s right—one of the things we have to be cautious of is the salinity, and we monitor that.

Two men standing in a soil pit examining blue tracer dye in the soil

But that water also contains nutrients, and we see those nutrients being utilized by the crops. The challenge is that those nutrients have the great potential to become pollutants as well, so we have to track the groundwater below our project. Nitrates are a concern because groundwater is a source of drinking water and potentially we also have the issue of phosphorus runoff. Well, surface water runoff is not a big issue in southwest Kansas (laughs), but maybe that’s another point. From practical standpoint, it’s not an issue. But from a legal, regulatory standpoint it becomes an issue.

What will it take to reuse more gray and black water? Technology or …?

Rattan: Technology certainly from a heavy metal [standpoint] is one part. Black water also has health risks because of some microorganisms, which are dangerous to human health, so that part becomes important. So, safe use of that water will require a multi-disciplinary approach, and that’s why our definition of “water security” encompasses all those words, such as “safe” and in “adequate amount.”

I can add that as the population grows from seven to 9.5 or 10 billion at the end of the century, and as the urban population goes from 50 percent now to 70 percent by 2050, this gray and black water conversion into green water is going to be a very high priority. Large cities with a population of 10 million or so use 6,000 tons of food daily. That’s a lot of nutrients and eventually those nutrients get into the sewage water. They’ve got to be recycled somehow, so that rather than being a liability—as they are now—they are made into an asset. It’s that philosophy which we need to address.

How do we convince more people to adopt a “recycling philosophy” toward water and conserve this precious resource in other ways? Stay tuned for part three.

Read part one