Green and Blue Water

Green water is rainfall stored as moisture in the unsaturated soil and used by plants. Blue water is water in rivers, lakes, reservoirs and aquifers that can be withdrawn for irrigation or other uses. The distinction concerns the water's source and storage before crop use, not its physical colour.

How the two water sources reach crops

• Green water enters the soil directly through precipitation and becomes available in the crop root zone; its agricultural use is measured through evapotranspiration originating from this soil moisture.
• Blue water reaches fields mainly through canals, wells or other irrigation systems drawing on surface water or groundwater.
• A rain-fed crop may rely almost entirely on green water, while an irrigated crop commonly uses a combination of green and blue water.

Treatment in agricultural water accounting

Accounting separates crop water consumption according to whether the consumed water originated as rainfall-held soil moisture or as an abstracted blue-water source.

• The green water footprint is the volume of rainwater consumed during crop production, commonly expressed per unit of output.
• The blue water footprint is the volume of surface water or groundwater consumed, including water evaporated, incorporated into produce or not returned to the same catchment within the relevant period.
• Gross irrigation withdrawal is not identical to blue-water consumption because part of the withdrawn water may return through drainage or seepage.

Why the distinction matters

• Green-water availability depends strongly on rainfall timing, soil characteristics and the soil's capacity to retain moisture.
• Blue water is an allocable resource that faces competing demands from agriculture, households, industry and ecosystems.
• Separating the two prevents all crop water use from being treated as irrigation demand and improves water budgeting, crop planning and assessment of scarcity.