Considering diversity in precipitation variability when updating seasonal flow forecasts

The Israel Hydrological Service forecasts the flow to Lake Kinneret by use of a hydrometeorological model which relates seasonal flow volumes to predicted annual depths of precipitation at representative stations. Issued forecasts include an array of 10 values assigned with the exceedance probabilities of 5–95%, as well as their average, which is an estimate of the expected value. A naive forecast is issued at the beginning of the precipitation season, and subsequent updating forecasts are issued during the season. Predicted annual depths of precipitation in an updating forecast are composed of the observed depth until the updating date and an array of predicted depths for the subsequent sub-season. No explicit relation was found between corresponding prior and subsequent depths concerning any updating date. But, the distribution of subsequent depths was found related to the category (i.e. low, medium, or high) of the corresponding prior depth. Empirical, parametric and non-parametric tests revealed a number of significant diversities in the distributions of later depths with respect to the corresponding categories of earlier depths. This diversity could result in considerable differences in the forecasted flow volumes, and is particularly important in cases of extreme situations, when management decisions are tied to revisions before the end of the precipitation season.