This product is designed to assist individuals and agencies with water supply concerns in summarizing last year’s (1999) spring runoff and in planning for the coming year.
Please note that all observed values are provisional. Final values may differ from those listed herein. Many adjustments for diversions have been estimated from historical averages. In extreme years these average estimates may result in large discrepancies between provisional and final values. In addition, during hot, dry summers both unknown/unmeasured diversions and environmental losses due to evaporation and channel transmission tend to increase. Total abstractions engineered and environmentally induced, may cause natural flow calculations to yield a number less than zero, particularly at locations well downstream. At such locations, comparisons between forecast and observed flows become more difficult and less meaningful.
Included in this review is expanded treatment of the confidence intervals associated with forecasts. The reasonable maximum and minimum values, which form the boundaries of the confidence interval, are statistical measures reflecting both the accuracy of the regression equation used to produce the forecast and the natural variability of streamflow volume. As the forecast season progresses, confidence intervals should narrow as meteorological conditions become known. The most probable forecast, a 50% exceedance probability, is most often cited. However, the reasonable minimum, a 10% exceedance probability, and maximum, a 90% exceedance probability, are important indicators of the “confidence” of the most probable forecast. Under normal meteorological circumstances, observed flows will fall within the confidence interval 80% of the time; flows may occur outside interval boundaries in years exhibiting uncharacteristic conditions.