Boh Tea Plantation, Cameron Highlands, Malaysia
©EOP
Agriculture is by a considerable measure the largest single human use of water. Leaving animal husbandry aside for the moment, some of the water used by crop plants is taken up directly from soil moisture created by precipitation in what is often called rain fed agriculture. It has become conventional to call that water from precipitation green water (a somewhat unfortunate usage, for the term is also applied to water seriously contaminated with algae). In the humid zone countries of the northern Hemisphere, including the eastern parts of the United States, green water allows a variety of crops to be grown without supplemental irrigation, and precipitation supplies all of the water for the crop plants. Such is the case in the tea growing area in the tropical highlands of Malaysia where tea is but one of a variety of crops grown dependent on the ample rainfall.
Elsewhere agriculture as presently practiced requires the addition of water beyond that provided by precipitation. Water obtained from streams, lakes and underground aquifers is usually termed blue water (again an unfortunate usage, for blue water has a quite different meaning to sailors). In sub-humid zones irrigation may provide only small amounts of additional water applied at specific times during the growing season. In truly arid areas little or no crop production is possible without continual irrigation throughout the growing season. An area like the Salt River Valley of Arizona would not be useful for agriculture if not for irrigation. Many other areas in the western United States can be used for water dependent crops only because of irrigation. Without irrigation in those areas, rain-fed agriculture could only produce grains Like wheat and barley.
The map below illustrates blue water withdrawals from various sources like lakes and reservoirs and underground aquifers for agriculture. It illustrates the great importance of supplemental water in tropical and subtropical areas, particularly in the Middle East and Asia. A very large fraction of the world's population depends on food grown with at least some use of blue water in irrigation, including most of the populations of India and China. The data are averages for usage in whole countries from the Food and Agriculture Organization (FAO), a Rome based unit of the United Nations.
Not all water used in agriculture is consumed, that is lost to evapo-transpiration or incorporated in the crop, but a great deal is. The map below composed from remote sensing imagery at the Institut für Physische Geographie (Physical Geographical Institute) of the Johann Wolfgang Goethe University of Frankfurt am Main in Germany shows the consumptive use of blue water by agriculture across the globe. The map is part of a very large project on world irrigation at the institute.
