Irrigation is an ancient and important agricultural practice. Crop yields are higher under irrigation and less dependent on the effects of weather. While only 15% of the world's cultivated land is irrigated, it accounts for 35-40% of the global food harvest. Projected population growth rates for the next 30 years will require an increase in food production equal to 20% in developed countries and 60% in developing countries to maintain present levels of food consumption. Expansion of irrigated agriculture was in large part responsible for the "green revolution" in food production and will continue to play an essential role in providing the needed increases in food and fiber production, especially in developing countries.
Irrigation inevitably leads to the salinization of soils and waters. In the United States yield reductions due to salinity occur on an estimated 30% of all irrigated land. World wide, crop production is limited by the effects of salinity on about 50% of the irrigated land area. In many countries irrigated agriculture has caused environmental disturbances such as waterlogging, salinization, and depletion and pollution of water supplies. Concern is mounting about the sustainability of irrigated agriculture.
Application of irrigation water results in the addition of soluble salts such as sodium, calcium, magnesium, potassium, sulfate, and chloride dissolved from geologic materials with which the waters have been in contact. Evaporation and transpiration (plant uptake) of irrigation water eventually cause excessive amounts of salts to accumulate in soils unless adequate leaching and drainage are provided. Excessive soil salinity reduces yields by lowering plant stand and growth rate. Also, excess sodium under conditions of low salinity and especially high pH can promote slaking of aggregates, swelling and dispersion of soil clays, degrading soil structure and impeding water and root penetration. Some trace constituents, such as boron, are directly toxic to plants.
Over the course of history, thriving civilizations declined in part due to their inability to sustain food production on lands that had been salinized. It is estimated that 10 million hectares are now being lost every year as a result of salinity and/or waterlogging. Many of these problems are caused by excessive use of water for irrigation due to inefficient irrigation distribution systems, poor on-farm management practices, and inappropriate management of drainage water. Inefficient on-farm irrigation practices cause local salinity problems. Local problems increase as a result of poor on-farm drainage. Excessive irrigation increases salt loading in water tables and downstream aquifers which causes regional salinization. Lack of local and regional drainage systems results in lands being put out of agricultural production.
In the future, global food needs will continue to increase while the soil and water resources available for new crop production will be limited and of diminished quality. The need to protect soil resources as well as to conserve water will continue to increase. Water must be utilized more efficiently and its quality protected. World agriculture must expand its base of production and increase production on lands currently under cultivation. Appropriate management practices to control salinity must be implemented on irrigated fields, in irrigation projects, and for geohydrologic systems. In order to meet the ever increasing demands for food and utilizing ever decreasing and more marginal soil and water resources, the nation and much of the world community will continue to look to the U. S. Salinity Laboratory for expertise and leadership in salinity and water quality research and applications to solve these problems.