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14 Independent seepage studies of canals, laterals, and farm ditches were made by USBR, ARS, SCS, and Colorado State University(CSU). USBR installed and monitored about 278 wells in the area. USBR, USGS, EPA, CSU, ARS, state, and others collected salinity data on irrigation subsurface return flow and developed or helped develop water and salt budgets. USBR and USGS monitored about 14 major drains returning irrigation water to the Colorado River from the Grand Valley project. Records for about 15 years were compiled. Base flow concentrations collected during November, December, January, and February averaged consistently year after year about 4000 mg/L or 5 tons/acre foot. Flow weighted averages were published in the 1986 USBR Plan/EIS for Grand Valley of 4.25 tons per acre foot. An average 4.05 tons/acre foot of salt pickup by seepage and deep percolation was determined (1986 USBR EIS Report). This is a 20 percent reduction in the salt loading factor from that assumed in the 1977 and 1980 USDA Grand Valley planning reports. SCS in conjunction with ARS, USBR, USGS, Colorado State University, and Cooperative Extension has been monitoring onfarm irrigation improvements and return flows since 1979 for the salinity program. This information is being used to further refine and modify the USDA salt and water budgets and verify salt load reductions being obtained by the salinity control program. All data will be used in a hydrosalinity analysis and summarized in a USDA M&E report in 1993. USGS continues to monitor the Colorado River Basin and the Grand Valley river gaging stations. Water and salt budgets were developed for the Grand Valley unit using inflow and outflow analysis on the river system and by making irrigation water and salt budgets. Several hydrosalinity models have been developed by USBR (latest was the model results of 1985 published in the Stage Two 1986 EIS). The results have refined numbers and confirmed the validity of the basic overall salt loading published in the 1977 USDA Final Report of the Grand Valley Salinity Study. The 1977 published value for total salt loading was 600,000 tons per year and onfarm salt loading of 297,000 tons/year. This is within the range of accuracy (plus or minus lOt) desired in planning and operations. A 300,000 ton per year salt contribution (+ or - 10\) attributed to onfarm irrigation has been verified by USBR. A value computed by SCS in 199J, based on monitoring, results in 305,000 ton/year salt loading figure from onfarm irrigation for the future without project condition. However, SCS monitoring for the whole irrigation season during the last 10 years has changed the long time held idea that little onfarm deep percolation occurred in Grand Valley and in similar areas of Gunnison and McElmo Creek. Monitoring during the full irrigation season instead of only in mid-summer has revealed significant deep percolation; about 40 percent of the seasonal deep percolation occurs during the first two irrigations. The