A probabilistic assessment of reservoir fill under a range of winter flow regimes
Date
1996-12
Authors
Benjamin, Lyn, author
U.S. Committee on Irrigation and Drainage, publisher
Journal Title
Journal ISSN
Volume Title
Abstract
Regulated flow regimes below irrigation reservoirs frequently create undesirable conditions for downstream biota. In order to meet reservoir fill deadlines, winter discharge below Island Park reservoir on the Henry's Fork of the Snake River, eastern Idaho, has been dramatically reduced from pre-dam flows of approximately 400 cfs, affecting trout and trumpeter swan populations. The purpose of this study was to model the probability of meeting storage deadlines while providing minimum instream flows during the winter months. Five different winter release scenarios were simulated using actual outflow and reservoir storage data for each water year from 1940 to 1995, and the probability of reservoir fill was calculated for each of these scenarios. The sensitivities of reservoir fill to reservoir starting levels and fill deadlines were also compared by running the simulations with different reservoir starting levels and fill deadlines. Results indicate that the probabilities of meeting the April 1st fill deadline with winter flows of 200 and 300 cfs are 55% and 42%, respectively. Bureau of Reclamation operating procedures that link all reservoirs within the Minidoka system mandate filling Island Park by April 1st, despite the observations that irrigation water is rarely needed from Island Park before July 1st and spring runoff occurs in April and May. When later fill dates were modeled, probabilities of reservoir fill became greater. Reservoir fill is very sensitive to reservoir levels at the start of storage season; fill occurs 100% of the time by May 1st with winter outflows of 200 cfs when starting contents exceed 65,000 acre feet. These results suggest that in order to provide for both instream flow and irrigation needs, water managers consider the moving the mandated fill date for Island Park Reservoir later in the spring and implementing water conservation measures that will maximize reservoir contents at the end of irrigation season.
Description
Presented at Competing interests in water resources - searching for consensus: proceedings from the USCID water management conference held on December 5-7, 1996 in Las Vegas, Nevada.