"Blue water" is the amount of rainfall that enters lakes, rivers and groundwater. This is the main source of water that we use and manage for industrial, domestic and irrigation purposes. Only 30-35% of all water within the hydrological cycle is blue water.
"Green water" is the amount of rainfall that is either intercepted by the vegetation,or enters the soil and is picked up by plants and evapotranspired back into the atmosphere. Some 65% of all rain water is cycled through the green water cycle and is the water source for rainfed agriculture.
Patitioning rainwater into blue and green, white and grey water:
This flowchart shows the % of rainfall that moves through the different hydrological systems and how much is then used for rainfed and irrigated agriculture.
What are Critical Limits for Blue and Green Water Availability:
Critical values for the availability of Blue and Green Water were determined by Rockstrom et al 2006 for a wide range of countries in m3/capita/year.. They showed that countries with < 1000 m3/capita/year of Blue water and <1300 m3/capita of Green water will have significant water challenges by 2050. In the three figures below it is shown how much of the global population will have blue water shotages, green water shotages or both. It also shows what each country can do to overcome these shortages and examples are shown how selective countries fit into this type of classification scheme.