The three main components of rainwater at basin level (Falkenmark,2003)
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.
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.