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What is blue, green, and grey water (example)? Is green water only rainwater? I came across words like "mostly rain", "predominantly rain", etc., so it's not only rain, is it? And if grey water is wastewater, doesn't it mean that it's double-counted (because it's counted as blue or green water too)?

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A document produced by the US Geological Survey, for what looks like a conference presentation, gives a good explanation.

Green and blue water are waters from two natural water systems.

Green Water is from unsaturated natural water sources, such as soils. One way to think of it is, green water is easily available to and is used green plants.

Blue Water is from saturated natural water sources, such as streams, lakes, ground water, wetlands, glaciers and snow. Another way to think of it is, blue is deeper than green and blue water is deeper than green water.

Grey Water is domestic waste water from the laundry, kitchen or bathroom which can be reused for irrigation.

Black Water is sewage.

Another document states,

Precipitation falling onto the land surface in terrestrial ecosystems is transformed into either “green water” or “blue water.” Green water is the portion stored in soil and potentially available for uptake by plants, whereas blue water either runs off into streams and rivers or percolates below the rooting zone into a groundwater aquifer. The principal flow of green water is by evapotranspiration from soil into the atmosphere, whereas blue water moves through the channel system at the land surface or through the pore space of an aquifer. Globally, the flow of green water accounts for about two-thirds of the global flow of all water, green or blue; thus the global flow of green water, most of which is by transpiration, dominates that of blue water. In fact, the global flow of green water by transpiration equals the flow of all the rivers on Earth into the oceans.

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