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esm223_17_Readings_Arizona_Salt_USGS_Fact_Sheet_170-98

esm223_17_Readings_Arizona_Salt_USGS_Fact_Sheet_170-98 -...

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Land in arid and semiarid regions of the world is irrigated to sustain agriculture, urban yards and lawns, and other vegetation. To prevent the accumulation of salts in the root zone, the quantity of water applied must be sufficient to flush the salts beyond the root zone as well as meet the plant requirements. Many factors determine the extent and severity of salt accumulation including chemical composition of the water supply; nature and composition of the soil and subsoil; topography of the land; quantity of water used and the methods of applying it; kinds of crops grown; climate of the region, especially the quantity and distribution of rainfall; and nature of ground- water and surface-water drainage systems (Hem, 1985). Irrigated agriculture can result in rising water tables, waterlogged soils, progressive mineralization of water and soils, briny wastewater-disposal problems and concerns, and contamination of ground water by fertilizers and pesticides applied to the land and by chemicals in treated sewage effluent when it is used for irrigation (Bouwer, 1990; Bouwer and others, 1998). In south-central Arizona, the conversion of desert and rangeland to irrigated agricultural and urban land has been possible because of the impoundment of rivers, the pumping of ground water, and the importation of water. The salts that remain in the soil when these waters are used for irrigation are of concern because they can adversely affect crop production; quality of the underlying ground water; and domestic, municipal, and industrial water uses. In order to understand the causes and effects of salt accumulation in water and soils and how to manage or mitigate those effects, we need to understand where the salts come from and where they go. What are salts? The terms “salt content” or “salinity” of water actually refer to the quantity of mineral constituents that are dissolved in the water. The dissolved minerals or salts in water typically are reported as the The accumulation of salts in soils and ground water in arid and semiarid regions as a result of agricultural and irrigation practices is as much a concern to modern civilizations as it was to ancient civilizations. For example, the flood plain of the Tigris and Euphrates Rivers, known as the “Fertile Crescent” in ancient Mesopotamia (present- day Iraq), was first irrigated more than 6,000 years ago. The resulting agricultural surplus provided the foundation upon which the civilization was built; however, canals built in 4000 B.C. did not sufficiently drain excess water from the agricultural areas, and salts accumulated in water and soils. Progressive waterlogging and salinization were evident from the historical succession of crops—a 50/50 split of wheat and barley was grown in about 3500 B.C.; by 2500 B.C., the more salt-tolerant barley represented 80 percent of the crop, and finally by 1700 B.C., wheat could not be grown because of the salts that had accumulated in the ground water and soil. Centuries of irrigating poorly drained soil with highly
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