Green Water Supply: The Key Element in World Food Security

During the past 50 years, world population increased from 3.1 to 7.0 billion, but grain production increased from 877 to 2459 Mt. Among the factors that made this remarkable achievement possible were improved plant breeding, chemical fertilizers, and an expansion of irrigation. Irrigated area increased from about 140 million to almost 300 million hectares. Although only about 20 percent of cropland is irrigated, it accounts for approximately 40 percent of crop production. Although irrigation water is considered blue water, estimates are that about 30% of the water by irrigated crops comes from precipitation that is green water. Only irrigated land in desert areas depend entirely on blue water. In many regions, food production depends entirely on green water. Approximately 95% of the farmed land in Sub-Saharan Africa, 87% in Latin America, 67% in the Near East and North Africa, 65% in East Asia, and 58% in South Asia depend on green water. Therefore, green water contributes much more water for crop production than blue water and all for grazing lands. Green water is the portion of precipitation that does not run off, or recharge the groundwater, but is stored in the soil, or temporarily stays on top of the soil or vegetation. 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 often less than 30% of the precipitation, and less than 50% of this may actually be used for transpiration. 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 from of maize grown in an area with 500 mm average precipitation will be only about 20% 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.