This paper presents a method of forecasting the discharge of streams during periods of low flow by use of the normal base‐flow depletion curve and frequency curves of runoff from concurrent precipitation and snow melt. Base flow from antecedent precipitation, together with runoff from concurrent precipitation and snow melt, constitutes the total runoff. The base flow resulting from antecedent precipitation may be projected several weeks or, in some cases, months into the future. The added runoff from subsequent precipitation may be evaluated by a frequency curve of such runoff component from past records. If the latter component is not great in relation to base flow, reliable forecasts of total flow are possible. The method is illustrated by application to two streams in the Columbia River basin where conditions are favorable to its use.

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Mean flow of a stream is usually computed from. a continuous record of flow aT a gaging station. A less costly method consists of (1) estimating 12 individual monthly flows from one discharge measurement per month and a concurrent gaging station record on a nearby stream, using a different relation for each month, (2) computing the annual mean from the estimated monthly means, and (3) using a relation based on gaging station records to estimate the long-term mean from the one annual mean. An annual mean can be estimated within about 10 percent of its measured value even though both the gaged and ungaged streams are affected by diversions and have different runoff characteristics.The method has greatest utility in regions where mean runoff is not closely related to drainage area.