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CU0176 . streambed level, thus creating ground-water flow toward the stream and causing a gaining or effluent stream condition, During dry periods when the draft upon the ground water is high (from both wells and phreato- phyte growth) this situation is probably reversed in portions of the reach, causing a losing or influent stream condition. Selection of study period A IS-year study period for the model analysis was chosen to begin January 1947 and run through December 1961. This time period was chosen principally for two reasons: (1) Data for the study period, such as estimates of amount of ground water pumped under each ditch system, were previously assembled by the U. S. Bureau of Reclamation studies made in connection with the Narrows Reservoir project. (2) The time period includes the major drought period of 19S4 through 19S6. . Water budget The annual irrigation water supply for the study reach is highly dependent upon the return-flow phenomena discussed earlier. Except for the heavy mountain snowmelt runoff times in May and June, and the occa- sional flood runoff due to summer thunderstorms, the water used in the study reach is return flow from irrigation activities upstream. This is not only true for the direct-flow rights but also for the storage rights in that the stream flow during the fall, winter, and early spring months is essentially all derived from irrigation return flow. Tables 1 and 2 show the estimated average water budget for the stream and the stream-aquifer system for the IS-year study period of 1947 through 1961. The importance of ground-water return flow and deep perco- lation of irrigation water can be seen in these budgets, . -19-