The Blue vs. Green Water Calculator separates crop water use into green water (evapotranspired rainfall) and blue water (irrigation). Green water cycles locally; blue water competes with drinking water and ecosystems — that distinction determines whether your irrigation is sustainable.
500,000
m³
200,000
m³
700,000
m³
71.4
%
28.6
%
600,000
m³
0
m³
100,000
m³
500,000
m³
200,000
m³
700,000
m³
71.4
%
28.6
%
600,000
m³
0
m³
100,000
m³
All water isn't equal when it comes to agricultural impact. When rain falls on a field, gets stored in the soil, and evaporates through crop leaves — that's green water. It was going to evapotranspire from the landscape anyway; the crop just channeled it. When you pump from an aquifer or divert from a river to supplement rainfall — that's blue water, and it directly competes with drinking water, aquatic ecosystems, and downstream agriculture. The water footprint calculator separates these two fundamentally different resource uses for any irrigated crop scenario.
Green Water Footprint (the rain-fed component):
Green WF (m³) = min(ETcrop, effective_rainfall) × area_ha × 10
(multiply by 10 to convert mm × ha to m³)
Blue Water Footprint (the irrigation component):
Blue WF (m³) = (ETcrop − effective_rainfall) × area_ha × 10, if ETcrop > effective_rainfall; else 0
In other words: blue water fills the gap between what the crop needs (crop evapotranspiration, ETcrop) and what the rain provides. If rainfall meets the entire crop demand, blue water = 0 regardless of how much you irrigate — excess irrigation that doesn't contribute to evapotranspiration runs off or percolates and isn't counted as blue water consumed (though it may contribute to grey water through nutrient leaching). Use this online calculator for any crop and climate combination. The water footprint calculators cover virtual water in food production.
Global averages (m³ per tonne of crop) illustrate the wide variation:
A crop with a high green water footprint grown in a rainfall-abundant region may be completely sustainable. The same crop with a substantial blue water footprint in an arid region with overdrawn aquifers is not. Water scarcity indices like the Water Stress Index and Falkenmark Indicator quantify how much of available freshwater is already consumed by human uses — when blue water withdrawal exceeds 20% of available flow, the water body is considered moderately stressed; above 40%, severely stressed. The carbon sequestration calculator and environmental impact calculators provide complementary sustainability analysis tools.
Grey water footprint is the volume of freshwater required to dilute pollutants from agricultural runoff to acceptable concentration levels. Grey WF = (leaching rate × fertilizer application) / (Cmax − Cnat), where Cmax is the maximum acceptable concentration of the pollutant and Cnat is the natural background concentration. Nitrate from nitrogen fertilizers is the primary driver of grey water footprint in intensive agriculture. Grey water footprint represents downstream water quality degradation — a completely different impact pathway from volume extraction. All three components together — green + blue + grey — constitute the complete water footprint as defined by the Water Footprint Network (Hoekstra et al., 2011).
A blue water percentage below 20% indicates primarily rain-fed agriculture (sustainable). Above 50% suggests heavy reliance on irrigation. Regions with high blue water dependency are vulnerable to drought and groundwater depletion.
Inputs
Results
Primarily rain-fed.
Inputs
Results
Heavy irrigation dependency.
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