What are blue, green, and grey water?

One of the most precious resources on earth is dwindling, and its decline affects the lives of everyone on earth. Its scarcity is costly in economical, biological, and ecological terms. It’s not oil, but a resource even more precious – water.

Though we may see water frequently, not all of it is available for animal and plant life. If a beer barrel (13 gallons) represents all the water on earth, the total amount of fresh water is ten drops. Yes, only ten (10) drops. And out of those ten drops, only a fraction of one drop is in the surface water we can see and directly access.

Managing water in terms of access and freshness is one way to optimize the use of this resource. These are called blue, green, and grey water.

three vertical photos side by side showing examples of blue, green, and grey water: filling up glass cup with water, water droplets on leaf, and laundry machines
Blue water is clean enough for home use. Green water is what is available to our crops to grow. Grey water is water that has been used, but then cleaned – such as your laundry water going through a filter submerged in your yard. Grey water photo courtesy Grey Water Corps.

What is blue water?

Blue water is found in lakes, rivers, and reservoirs. It’s what you see in wetlands, or it can be pumped from below surface aquifers.* Available blue water is used for many purposes, including drinking water. It can be used in homes and businesses – like beverage manufacturers.

It is also used as irrigation water for agriculture. In fact, agriculture uses about 70% of the blue water – which is why researchers are finding ways to reuse water (which we cover below!)

Blue water is “recharged” by precipitation, either as rainfall or snowmelt. But as we build more cities, we put up pavement and buildings, meaning this water then flows into our stormwater systems. Permeable pavement, green roofs, and other solutions help capture and keep this water clean.

pull quote text: blue water is found in lakes, rivers, and reservoirs or pumped from aquifers

What is green water?

Green water is the water available in the soil for plants and soil microorganisms. It is the water absorbed by roots, used by plants, and released back to the atmosphere through the process of transpiration. Green water can also leave the soil through evaporation or subsurface runoff, but it is considered productive only when it is used for plant transpiration.

Various factors determine how much water is needed for a plant to grow: the temperature, amount of sun and wind, and how dry the air is. These factors change the amount of green water a crop needs, and therefore how much irrigation might be needed in addition to the rainfall in an area. The overall goal is to get more crop per drop of water and decrease the need for irrigation.

Some solutions to getting more from green water are reduced tillage and use of cover crops. Cover crops can shade the soil and decrease water loss from the soil surface. Likewise, no-till practices leave crop residue on the soil and prevent evaporation. Cover crops and no-till practices also prevent soil erosion and runoff by holding soils in place and encouraging water to soak into the soil instead of skimming off the surface.

pull quote text: green water is the water available in the soil for plants and soil microbes

What is grey water?

To supplement green water, scientists are looking to grey water. The term is typically applied to water that has been previously used and may contain some impurities. Grey water has been used by cities, households, and industries. It is the wastewater that is usually treated and discharged.

Nature makes blue and green water. Humans make – and have learned to reuse – grey water.

But how much grey water can there be? As it turns out, there is a lot of grey water out there. It comes from people washing their hands, cleaning their vegetables, or spraying down their driveways. In addition to household wastewater, industries produce a significant amount of grey water in their operations. A large vegetable-processing plant consumes as much water as a city of 100,000 people. And in many areas, power production will use as much water as irrigated agriculture.

Reusing grey water can decrease energy consumption—by up to 80%. Because the water has already been pumped from the ground, additional energy that would be used to pull out more blue water is saved. That reuse not only creates more green water for plants, but it also saves blue water while providing multiple other benefits.

Though grey water needs to be treated, it doesn’t have to be treated to a pristine level. Sufficiently treated water can be put on a field, and healthy soil will then finish the treatment by filtering it and removing chemicals, because soil is the world’s largest biofilter! As with green water, the quality of the soil is an important factor in grey water reuse.

pull quote text: grey water is water that has been previously used and may contain some impurities

Another benefit of grey water reuse is nutrient recycling. While the water is treated, it still contains significant amounts of nutrients that are needed by the crops. When water is put back into soil, the remaining nutrients go with it. Recycling those nutrients not only helps build crop yields, but it also means the nutrients stay out of surface waters and groundwater where they might otherwise affect water quality.

Water is a vital part of human life. Our food supply, among other necessities, depends on water security. Increasing the amount of green water available to crops and the efficiency with which the plants use it are necessary to secure food for a growing population. The reuse of grey water is essential too. Grey water reduces the use of blue water and provides additional green water while lowering energy costs and recycling nutrients.

2022 is the 50th Anniversary of the Clean Water Act – and water needs your help, too. We have also collected all our blogs, news stories and more onto the this Clean Water Act information page. You can help keep more rainwater and snowmelt in your yard and help the environment! Read this blog to learn more.

This blog is adapted by Susan V. Fisk from “Three Shades of Water,” originally published in CSA News doi:10.2134/csa2013-58-10-1.

*Aquifers are areas deep below the soil surface, in rocky formations, where water is stored. It can be accessed and transported for use, usually via pipes and pumps.

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