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Green water is the portion of precipitation that is stored in the soil, or temporarily stays on top of the soil or vegetation during the growing season. Eventually, part of it is used by plants as transpiration and the amount of water transpired is directly related to biomass production. For grain crops, a portion of the biomass is grain, and the ratio of grain to biomass is the harvest index. The portion of precipitation that becomes green water generally increases with increasing precipitation. In arid regions, green water is oft en <30% of the precipitation, and <50% of this may actually be used for transpiration resulting in evaporation losses of 85% or more. In more favorable areas, 65% or more of the precipitation may be green water, and as much as 70% or more used for transpiration. Also, the units of water as transpiration required to produce a unit of biomass increase as aridity increases while the harvest index generally decreases. As a result of these interactions, grain yield decreases at a faster rate than precipitation. By the use of generalized relationships based on past studies, it is estimated that the grain yield of corn (Zea mays L.) grown in an area with 500 mm average precipitation will be only about 25% of that from an area with 1000 mm precipitation. Therefore, while there is great potential for increasing the capture, storage, and use of green water, realizing this potential increases almost exponentially with increasing aridity.