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Grand Valley Project -5- Studies by the U.S. Geological Survey (1964) and the Environmental Protection Agency (1971) have set the stage for the present emphasis on salinity control in Grand Valley. The basis for evaluating the salt contribution (salt pickup) from Grand Valley to the Colorado River is an inflow-outflow analysis. Agricultural Research Service (ARS) studies will result in determina- tions of seepage losses from canal reaches, and determine deep percola- tion losses from on-farm water application. They are examining the mechanisms by which the salt load of return flow water is modified after it leaves the farm unit and moves toward the Colorado River. Also being studies by ARS are the mechanism of ground water flow to the river and usefulness of the ratio of calcium/magnesium as a tracer to apportion mixed waters as to their source, and will be used to evaluate the sources of surface drain effluent. The effectiveness of small, frequent irriga- tions to reduce the necessary leaching fraction (and therefore, reduce ground water flow) and to encourage harmless precipitation of salts is being studied by ARS near Fruita, Colorado. A study is being conducted by Colorado State University for Environmental Protection Agency to evaluate the effect of different water management practices on crop yields along with the chemical quality of the deep percolation losses after having moved through the soil profile. Besides the CSU-EPA studies defining the surface and subsurface hydrology in this study area, investigations have been completed regarding the value' of channe'l I ining, irrigation schedul ing, and drainage in reducing the salinity in the Colorado River. Present CSU-EPA studies are evaluating on-farm improvements and the benefits (both farmer benefits and reduced salinity) of combining channel, on-farm, and drainage improvements. The effect of water management on fertilizer efficiency and crop yield is being demonstrated. CO,lorado Water Conservation Board activities include demonstrations of the use of time-operated automatic gates on border and furrow irrigated fields, and the effectiveness of a pump back system and deep percolation loss determinations. Also being demonstrated are two drip irrigation systems for orchards, and seepage losses from a canal are being monitored. In 1968 the Colorado Water Conservation Board organized the Grand Valley Purification Project, a group of irrigation companies that obtained an EPA grant to line canals and laterals in the Clifton area. CSU was contracted to do the study. These studies defined both the surface and subsurface hydrology, including the water contributions from channel seepage, channel spills, field tailwater runoff, deep percolation losses, drain outflows, and ground water flows. 163'5