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~ ~ ~ ~ total adjustments and consumptive use. When total adjustments exceeded consumptive use, the difference was considered to be a net diversion for other purposes, such as municipal and industrial use, transbasin diversion, or an increase in reservoir storage, When consumptive use exceeded total adjust- ments, the difference was considered to be a net release from storage, In the fitted models, the coefficient for consumptive use was expected to be nega- tive, because it is an indicator of agricultural diversions that would remove dissolved-solids from the stream, Likewise, the coefficient for net other diversions was expected to be negative, Net release was expected to have a positive coefficient because additional dissolved solids would be included in the supplemental streamflow, The coefficient for total adjustments could be either positive or negative, but normally was expected to be negative because annual diversions were larger than releases for most sites, The only variable available as an indicator of salt pickup was upstream irrigated area. Because upstream irrigated area was reported as an annual value, it provided no information on monthly fluctuations, To achieve better monthly resolution, consumptive-use values from each of the previous 4 months were included as independent variables. Salt pickup .by irrigation return flow during a specific month then could be indicated as a linear combination of consumptive use during the previous 4 months, Optimally, the resulting combi- nation would indicate the monthly and yearly distributions of return flows that occurred in the basin. Such a relation between consumptive use and return flow assumes: (1) Return flow lags irrigation application by 1 to 4 months, and (2) consumptive use is an indicator of the total irrigation appli- cation during a specific month. The first assumption depends on the hydraulic conductivity in the soil-aquifer system underlying the irrigated area and the distance from the application site to a surface drain, A 1- to 4-month lag indicates that for an irrigation season from April through October and maximum consumptive use during June and July, return flow could occur from May through February and be maximum between July and November. Such a return-flow pattern seems reasonable for irrigated areas in the Upper Colorado River Basin. Data from sites downstream from large irrigated areas usually indicate that a sub- stantial volume of return flow occurs during September and October (Moody and Mueller, 1984). The second assumption, that consumptive use is an indicator of irrigation application, also is reasonable, Irrigation efficiency, the ratio of consumptive use to applied water, remains fai,rly constant from year to year when evaluated for large areas, During a 3-year study of the Grand Valley, near Grand Junction, Colo., Loftis (1983) reported no significant differences in mean annual efficiency, Long-term changes in irrigation efficiency generally occur only in areas where farmers have substantially changed their irrigation practices. In the fitted models, irrigated area and consumptive use in preceding months were expected to have positive coef- ficients, because they were indicators of return flow that would add dissolved solids to the stream, The final set of development variables was: Indicators of diversion or release: 1. Total adjustments to streamflow. 2. Consumptive use. 3, Net other diversions, 4. Net release, 14