introduction 



Impact of irrigation on U.S. agriculture 



Irrigation is an important factor in providing 

 food and fiber requirements of the Nation's popu- 

 lation. Irrigation farming not only increases pro- 

 ductivity of croplands, but also provides flexibility 

 which enables shifting from the relatively few 

 dryland crops that are grown without irrigation 

 to many other crops which may become greater 

 in demand. Irrigation also creates new employment 

 opportunities in processing and' marketing agri- 

 cultural products. 



Among the multipurposes for which water re- 

 sources are developed and used, irrigation is the 

 largest single-purpose beneficial consumptive use. 

 Therefore, water quality criteria for irrigation be- 

 come more and more significant as water resource 

 developments increase within each river basin. 

 Early irrigation developments in the arid and semi- 

 arid West were largely along streams where only a 

 small part of the total annual flow was put to use. 

 Such streams contained dissolved solids accumu- 

 lated through the normal leaching and weathering 

 processes with only slight additions or increases 

 in concentration resulting from man's activities. 

 Additional uses of the resource may have concen- 

 trated the existing dissolved solids, added new 

 salts, contributed toxic elements, microbiologically 

 polluted the streams, or in some other way de- 

 graded the quality of the water for irrigation and 

 most other consumptive uses. More intensive de- 

 velopment in recent years and the generally short 

 water supply in most western streams has ac- 

 centuated water quality deterioration in a down- 

 stream direction. The significance of establishing 

 water quality criteria for irrigation can be evalu- 

 ated best by examining: (1) the impact of irri- 

 gation on long-term food and fiber production in 

 the United States, and (2) the effect of water 

 quality deterioration on that production. 



An estimated total of 458 million acres of crop- 

 land in the United States during 1966 was utilized 

 for crop production, of which about 44 million 

 acres, located largely in the Western States, were 

 irrigated. This irrigated acreage, amounting to 

 about 10 percent of the total cropland, provides 

 about 25 percent of the total value of all crop 

 production. Value of production during the crop 

 year 1959, the latest year for which census data 

 are available, amounted to about $55 per acre 

 for all cropland in comparison to about $150 per 

 acre for irrigation land. 



For the most part, irrigated farms produce 

 crops that cannot be grown successfully in the West 

 under dryland conditions. 



From the value standpoint, irrigation's greatest 

 contribution is in the category of fruit, vegetable, 

 and other specialty crops. The environment of 

 the irrigated western areas is especially favorable 

 for these crops. Most of the commercial produc- 

 tion of apricots, artichokes, honeydew mellons, 

 hops, lemons, olives, dates, figs, garlic, nectarines, 

 prunes, English Walnuts, almonds, and filberts 

 come from the irrigated areas of the Western 

 States. During late fall, winter, and early spring, 

 the warm irrigated valleys of the Far West and 

 Southwest grow most of the Nation's supply of 

 fresh vegetables. The off-season production of 

 these fresh vegetables and fruits adds variety and 

 balance to the Nation's diet. 



Soil-Plant-Climate interrelationships 



Evaluation of water quality criteria for irri- 

 gation purposes must take into consideration the 

 interactive effects of soil, plant, and climate. Each 

 of these factors is highly variable. Yet, they are 

 important in determining the quality of water that 

 can be used for irrigation under a specific set 

 of conditions. 



Soil 



The physicochemical properties of a soil de- 

 termine the root environment that a plant en- 

 counters following an irrigation. The soil consists 

 of an organo-mineral complex which has the abil- 

 ity to react both physically and chemically with 

 constituents present in irrigation water. The degree 

 to which these added constituents will leach out 

 of a soil, remain available to plants in the soil, 

 or become fixed and unavailable to plants depends 

 largely on the soil characteristics. 



In irrigated areas, a water table frequently ex- 

 ists at some depth below the ground surface, with 



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