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In the Southwest, where water is the controlling factor in agricultural development, intensive efforts have been made to conserve and regulate stream-flow by means of storage-reservoirs. An example is provided by the chain of reservoirs on the Salt River in central Arizona. The regulation of these reservoirs could be enhanced considerably by some means of stream-flow forecasting. Consequently, a method has been developed whereby the spring-flow of the Salt River can be forecast with a reasonable degree of accuracy from the flow of a small tributary, Parker Creek, located in the Sierra Ancha Experimental Forest. Correlating the flow of Parker Creek from October through February with that of the Salt River from March through May, an equation has been developed which forecasts the March–May flow of the Salt River. In addition, an equation is given which expresses the error associated with any future forecast. The practical application of expressing the error is to give the possible minimum or maximum flow that might occur under selected odds. Using the Parker Creek data for the October–February period of 1944, a forecast was made for the spring-flow of the Salt River. The estimated or forecast-flow came within 2.2 percent of the actual flow.

Matthew I. Rorabaugh (U.S. Geological Survey, Louisville, Kentucky, August 27, 1945)—In regions where snow is not a factor there is some advantage in making forecasts based on the effects of travel time and base flow. However, the runoff which results from precipitation during the period of forecast usually constitutes a substantial portion of the flow in many streams, and accordingly these predictions may be highly speculative. The authors state that on March 1 an average of 52 per cent of the winter flow of Parker Creek has passed the gaging station on that creek while only 36 per cent of the winter flow of the Salt River has reached Roosevelt Reservoir. The forecast is based on this lag, which would approximate 16 per cent of the winter flow of the Salt River or about 25 per cent of the flow during March to May. In giving reasons for the lag the authors minimize the effect of accumulated snow, because less than 15 per cent of the Salt River Basin is thought to have accumulated snow. It may be, however, that the snow on this portion of the basin is the principal reason for the lag. The snow cover accumulates at high altitudes, where precipitation is heaviest and temperatures lowest. As losses will be proportionally less in these low-temperature areas than in the remainder of the basin, it follows that storage of 25 per cent of the flow could result from snow accumulation on less than 15 per cent of the basin.