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Blue Fresh surface and ground water, for example, the water in aquifers, streams and rivers
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Green The precipitation on land that does not run off or recharge the groundwater but is stored in the soil or temporarily stays on top of the soil or vegetation.

Eventually, this part of precipitation evaporates or transpires through plants. Green water can be made productive for crop growth.
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Grey Waste water from bathroom sinks, showers, tubs, and washing machines. It is not water that has come into contact with feces, either from the toilet or from washing diapers.

Greywater may contain traces of dirt, food, grease, hair, and certain household cleaning products. While greywater may look "dirty," it is a safe and even beneficial source of irrigation water in a yard.

If released into rivers, lakes, or estuaries, the nutrients in greywater become pollutants, but to plants, they are valuable fertilizer.
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Black Waste water than has come into contact with fecal matter. It includes sewage and other contaminated water sources, including all forms of flooding from seawater, ground surface water, and rising water from rivers and streamsthey are valuable fertilizer.
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GLOBAL AND ECONOMIC WATER SCARCITY

• Little or no water scarcity. Abundant water resources relative to use, with less that 25% of water from rivers withdrawn for human purposes
• Physical water scarcity (water resources development is approaching or has exeeded sustainable limits). More that 75% of river flows are withdrawn for agriculture, industry, and domestic purposes (accounting for recycling of return flows). This definition - relating water availability to water demand - implies that dry areas are not necessarily water scarce.
• Approaching physical water scarcity. More than 60% of river flows are withdrawn. These basins will experience physical water scarcity in the near future.
• Economic water scarcity (human, institutional, and financial capital limit access to water even though water in nature is available locally to meet human demands). water resources are abundant relative to water use, with less than 25% of water from rivers withdrawn for human purposes, but malnutrition exists.


Source: International Water management Institute analysis done for the Comprehensive Assessment of Water Management in Agriculture using the Watersim model; chapter 2.