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. 000057 Governor's Colorado Agricultural Outlook Forum February 11,1999 Salinization ofIrrigated Soils and Associated Waters: A Major Colorado Problem J. D. Rhoades! Introduction Irrigated agriculture is a major contributor to crop production. It is presently practiced on about 15 percent of the nation's cropland but contributes about 40 percent of the crop value. The dependency on irrigation in this regard is expected to increase over the period of the next thirty years. The growth in the expansion of world-wide irrigation has dramatically slowed over the past decade or two to a present rate that is inadequate to keep up with the projected expanding food requirements. At the same time, many presently developed irrigated lands and associated water resources have become substantially degraded through salinization caused by irrigation and drainage activities. In Colorado, this degradation appears to be on the increase in the Lower South Platte River and Arkansas River Valleys. Soil and Water Degradation by Irrigation/Drainage Irrigation water infiltrated into the soil in excess of that used by the crops becomes drainage water once it passes through the rootzone. This drainage water becomes salinized as the salt in the applied water is concentrated in the reduced drainage volume. This drainage water, together with that percolating downward from canal seepage, often gains additional salts from the dissolution of mineral-salts and the weathering of silicate minerals present in the soil and underlying substrata (especially those of marine origin). This salt-laden drainage and seepage water generally flows into lower-lying landscapes or is transported into receiving waters. Lower-lying fields become waterlogged when the water tables become shallow in depth; they become salinized when the salt-laden ground waters are "wicked up" into the soils and the salts are accumulated in the topsoil by evaporation. Receiving waters, especially rivers, become salinized when salt-laden drainage "returns" are discharged into them. The above-described combined processes of salt mobilization, redistribution and localization associated with drainage/seepage flows are the most prevalent causes of waterlogging and of the degradation by salinization of irrigated lands and associated water supplies. Extensive areas of irrigated land in Colorado have become salinized in the above- described manner, and their associated water supplies have become similarly degraded. The Arkansas River is well known for the degree of its progressive downstream salinization; only in relatively recent few years has it been recognized that the Lower South Platte River and associated irrigated lands are becoming increasingly salt-affected. I Former Director, U S Salinity Laboratory; now at Agricultural Salinity Consulting, 17065 HarIa Hts., Riverside, CA 92503