The Green Water concept

From the perspective of crop production, rainwater may be split in green, blue and white components (Figures 1 and 2):

• Green Water is the water infiltrating into the soil, taken up by roots, used in photosynthesis and transpired by the crop;
•  White Water is intercepted and directly evaporated by the crop canopy and the ground surface;
• Blue Water is made up from run-off to rivers and deep percolation to aquifers that finds its way to rivers indirectly.

From the perspective of dry land cropping, green water is the productive component.

Figure 1 – The three components of rainwater at field level

 

Figure 2 – The three main components of rainwater at basin level

In addition to direct input from rain, the amount of green water may be increased by irrigation and run-on (Figure 1). The infiltration and storage of run-on water can be improved by in-field water harvesting techniques, which brings up the question of definition: where does green water start, and when does it become blue water? For sake of simplicity and clarity, Ringers (2003) includes run-off that is harvested by in-field techniques as green water, and excludes water from the larger off-farm water collecting works such as reservoirs. In other words, green water applies to dry land farming and blue water to irrigated farming.