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2 The new awareness of downstream salinity damages derived from excessive leaching or deep percolation adds a new challenge to improving irrigation efficiency, particularly by reducing excessive deep percolation and seepage. A consortium of seven universities derived $50 as the annual damages to downstream water users per ton of salt in the Colorado River-1991 prices. Basin-wide and detailed irrigation salt and water budgets are needed to define the magnitude of salt loading by irrigation subsurface return flow. According to EPA and USBR studies in the Colorado River about 37 percent of the salt load to the lower basin is due to irrigation in the upper basin. SALT AND SALINITY This discussion will focus on that portion of the water and salt budget associated with irrigation. Some information is included on controlling watershed erosion to reduce salt loading. Salts are among the most common compounds found on earth and are among the most common pollutants of our water supplies. "Salinity" is the concentration of dissolved mineral salts in water(Tanji,1990 p.1). More than 85 percent of the world's water supplies, other than the oceans, are unfit for most consumptive uses because of salinity. Only one percent of the earth's water is fit to drink. Salinity presents an ominous threat to irrigated agriculture throughout the world. Salinity reduces crop yields and can turn fields into salt flats. Salinity adds significant economic cost to non agricultural uses. It impairs water for municipal and industrial users. It clogs hot water heaters and pipelines, adds to the cost of municipal water treatment, and accelerates the corrosion of boilers, pipes, and car radiators. Salts can be grouped by their water solubility and composition. All natural water supplies contain some amount of dissolved solids or salts: Table 1. COMMON SALT COMPOUNDS Cations(+) Anions(-) Common Name Solubility product-Ksp 101. 57 10-0.26 10-0.55 10-4.61 10-8. compounds NaCI Na2S04 MgS04 NaHC03 Na2C03 CaS04 CaC03 Sodium Sodium Magnesium Sodium Sodium Calcium Calcium Chloride Table salt(Halite) Sulfate Glauber's Salt Sulfate Epsom Salts Bicarbonate Baking Soda Carbonate Sal Soda Sulfate Gypsum Carbonate Lime(calcite)