Reexamination of Critical Period for Reservoir Design and Operation

Reservoir operating rules are often based on deterministic models using critical period (CP) analysis, which results in very conservative decisions. CP is defined as the historical hydrologic period that includes the lowest flow or the most severe drought, which represents functionally as the full-empty cycle of reservoir storage under a given firm yield. This paper compares CP with another time scale, the forecast horizon (FH) underlying hedging rule policies for reservoir operation. When the decisions in the initial few periods are not affected by forecast data beyond a certain period, the period is known as FH, and the number of the initial periods is known as the decision horizon. When a reservoir operation policy follows the concept of CP, CP is used as FH and the optimal yield generated within the FH is the firm yield. FH as long as the CP is not realistic because in the real world data for a such a long future period is unavailable; and using CP as FH is not effective either, because CP—the longest FH-involving the most severe drought leads to very conservative release decisions, i.e., the “near-zero-risk” decision and the time preference of utility diminishes the influence of hedging for over long periods.