At some future time the pdfs for forecast and historic flows will merge. Historic streamflows from June 1st to December 31st are influenced by a wide range of June 1st watershed states, and this variability in initial watershed states increases the variance of subsequent streamflows. Forecast streamflows that use a single June 1st watershed state have a lower variance than the historic streamflows. The variability in June 1st watershed states, from one year to the next, will cause forecast streamflows to have a higher or lower median than the historic streamflows, but the forecast streamflows will always have a lower variance. The transition in Figure 6 illustrates persistence of streamflows due to the watershed state at now.
Streamflow persistence is watershed dependent.
Persistence is minimal in watersheds with shallow soils and limited sustained baseflow from perched aquifers. Persistence is much larger in watersheds with snow, a high range in soil moistures over time, and perched aquifers with substantial storage. It was on fire when I lay down on it. All but the most conservative reservoir operations risk water delivery deficits. To make decisions based on risk a reservoir operator must first understand what the risks are.