“Green” water??

Q: I recently heard the term “green water.” I know that waste-water is sometimes referred to as “gray water,” but what does green water mean?

A: Green water is a picturesque name for the water available in the soil for plants and the soil biota (microbes, earthworms, soil insects, etc.) to use. It’s available because it did not run off into streams and rivers or percolate down into an aquifer to replenish groundwater.

Conservation practices, such as this ryegrass cover crop in Wright County, Iowa, can increase the amount of “green water” available to plants in soil. Photo: NRCS

It’s the water absorbed by roots, used by plants, and released to the atmosphere through a process called transpiration. Green water can also leave the soil through evaporation, but it’s considered “productive” only when it’s used for transpiration, which provides the plant with water and nutrients and, in agricultural settings, increases crop yield.

Conservation practices such as growing cover crops and practicing no-till agriculture can increase the amount of green water available to plants by limiting the amount of water lost through evaporation and runoff. These practices also increase soil quality–-a factor that greatly affects water storage and use. It works like this: Cover crops and no-till practices increase the amount of organic matter present in the soil. Organic matter promotes a healthier soil with more soil aggregates and larger pore spaces. This structure in turn helps soil hold onto and distribute water more effectively.

Also, earthworms like soils with higher organic matter content, and their burrowing can increase the number of larger pores in the soil where water can go and be stored.

Researchers and farmers want to find ways to conserve green water and make more of it available for crops to use. There is a lot of green water out there, even though we can’t see it as easily as we can the water in streams or rivers. In fact, the amount of green water that flows through plants via transpiration is greater than the flow of water in all the rivers on Earth going into the ocean. By conserving green water and improving soil quality so that more green water is used for crop production and less is lost to evaporation, areas that currently have low yields could produce more with the water they already have.

Answered by Gary Sposito, UC, Berkeley. Read more on this topic in the open access paper, Green Water and Global Food Security, in the Vadose Zone Journal.