WSP00515 - Laserfiche WebLink

Home Browse Search

. . ~:SI cO t,.;, w As salinity concentrations increase, the damages and impacts become more severe. For irrigated agriculture, salinity levels of 500 to 700 mg/l begin to have a detrimental impact on net returns. Higher salinity levels create increased operating costs, supressed crop growth, and ultimatley adversely effects yields. As salinity concentrations exceed 700 mg/l, the cross-section of salt tolerant crops becomes more restricted and limiting. Normally salt tolerant crops do not provide as favorable an economic return that the more salt sensitive crops do. Salinity levels in excess of 1000 to 2000 create severe crop production problems because of the need for specialized and extremely costly irrigation management practices. With the maximum safe drinking standard set at 500 mg/l, salinity concentrations of over 500 mg/l also becomes costly for municipal, industrial, and residential homeowners to treat. The increased salinity concentration also has a corrosion and deteriorating effect on pipelines and home appliances. Within the Colorado River, increased salinity levels are caused by two different processes: 1) ~ concentrating; and 2) salt loading. The "salt concentrating process" essentially involves the loss of waters by reservoir evaporation, export, and by evapo-transpiration of irrigated crops. As excess waters are evaporated-and used by the plants, residual salts are left behind to concentrate in the soil and/or remaining waters. The "salt loading process" occurs aa seepage and deep percolation dissolves mineral salts-in the surface soils and highly saline geologic formations as it returns to the river system. As additional salts are picked up, the total salt burden or load carried by the river increases. Figure II-2 provides a good example of the salt loading process from over-irrigation in the Big Sandy irrigated area. In 1962, the salinity of the water delivered to Mexico increased from an annual average of about 800 mg/l to nearly 1,500 mg/l. This was primarily attributed to the highly saline drainage return flows from the Wellton-Mohawk Irrigation and Drainage District area, which empties into the Colorado River below Imperial Dam, and partially to the concentrating effects and salt loadings from upstream water development. The total salt load in the river entering Lake Mead above Hoover Dam is estimated to average 9 million tons per year. To meet the salinity control objective of the Colorado River Basin Salinity Control Act, it is necessary to remove some 2.8 million tons of this salt load per year. The present average annual salinity concentration of the river varies from about 50 mg/l in the headwaters to about 820 mg/l at Imperial Dam. The USDI projects a future salinity level of 1,141 mg/l at Imperial Dam for the year 2000 as additional upstream development takes place, assuming no corrective action is taken. The long-term average annual salinity concentration at Imperial Dam is 875 mg/l under current development conditions. Each mg/l increase in Salinity concentration at Imperial Dam causes approximately $513,000 per year (1982 dollars) in economic damages to downstream agricultural, municipal, and industrial water users within the United States. It has been estimated that irrigation contributes about 37 percent of the total salt load to the river above Hoover Dam. Natural sources contribute 47 percent with reservoir evaporation (12 percent), exports (3 percent), and municipal and industrial use (1 percent) contributing the balance. Average onfarm irrigation and distribution system efficiencies, especially in the Upper Basin, are generally low. Low irrigation efficiencies generally indicate high surface runoff and/or over-irrigation. Over-irrigation can result in excessive deep percolation which leaches salts from the soil into the river. This greatly contributes to the salinity problem. S ;1-