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dating in semi-arid western America is a climatic one. This controlling factor turns out to be precipitation -- soil moisture -- water. In essence then, although I admit this is an over-simplification, the tree will put down a thick ring in a year of ample moisture and a thin ring in a year of drought. ******* A PHYSICAL BASIS FOR DEPENDABLE ANNUAL STREAMFLOW FORECASTS Dr. Irving P. Krick, President Irving P. Krick Associates, Inc. Denver, Colorado The large variations observed in seasonal and monthly distribution of the annual runoff in most river basinll poses a major problem in the operation of hydroelectric power systems, flood control, and irrigation. For example, in projecting firm power capabilities for a year ahead, most power producers consider rule curves based upon a low water year and upon the median water year. Some improvement on this procedure has recently been achieved by the use of a combination of historical streamflow correl- ations and antecedent conditions for the determination of an initial point on the "dependable" streamflow projection at the time of each forecast. Then the critical water year curve is used to project from this point. Unfortunately, such a procedure still tends to minimize the total inflow forecast at the outset of each water year and to fix the shape of the streamflow curve from month to month. In the Columbia River Basin, for example, it invariably leads to a forecast of sharp streamflow reductions in winter to conform to the critical year (1936-37) characteristics. The low flows during the winter period in that year were related to a rare weath- er sequence and therefore one of infrequent occurrence. Thus the estima- tion of firm power capabilities, based upon this unusual characteristic in the 1937 winter, greatly minimizes the flexibility of power system operation in the area during most years. As the season progresses streamflow estimates for various river basins are issued monthly by a number of agencies. Current flow data are projected on the basis of snow survey information and the use of normal precipitation for the balance of the season. All manner of correlations have been developed in order to produce streamflow probabilities within various ranges, but all of them suffer from the same limitation--an inad- equate knowledge of the actual sequence of precipitation and temperatures which will be responsible for streamflow during the course of a water year. This paper outlines procedures for forecasting the Ilequence of pre- - 14 -