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The Bureau of Reclamation has released a report that identifies innovative approaches to improve drought resiliency within the Washita Basin in Oklahoma. The study specifically looked at Foss and Fort Cobb Reservoirs, and the approach can be applied anywhere across the Western United States.
The study showed that a repeat of paleo droughts could have far greater impacts on reservoir yield than the observed drought of record. The study also predicted the probability of paleo droughts being worse than the observed drought of record, and hence quantified the risks of a reservoir not fulfilling its intended purposes under different drought scenarios.
For Foss Reservoir, the firm yield, as determined by the observed 1970s drought of record, is 19,700 acre-feet per year. Through the Enhanced Drought Response Reservoir Operations model, it is predicted that the reservoir firm yield under five paleo drought scenarios would range from 14,000 to 7,400 acre-feet per year. If the maximum demand were to be placed on the reservoir, the demands would need to be reduced between 32 and 66 percent in order to prevent the reservoir from going dry. The probability of a paleo drought being worse than the observed drought of record was found to be about 30 percent.
For Fort Cobb Reservoir, the firm yield as determined by the observed 1950s drought of record, is 19,200 acre-feet per year. Through the model, it is predicted that the reservoir yield during the five paleo droughts will range between 18,700 to 15,300 acre-feet per year. If the maximum demand were to be placed on the reservoir, the demands would need to be reduced between 36 and 53 percent to prevent the reservoir from going dry. The probability of a paleo drought being worse than the observed drought of record was found to be about 10 percent.
This information will better inform long-term planning efforts and preparation for the next drought. The new modeling tools developed for this study also can be used in real-time during a drought to inform decisions on how much demands need to be curtailed to prevent a reservoir from going dry.
Firm yield is the amount of water a reservoir can reliably deliver to customers. The challenge to water managers is that the observed hydrologic record encompasses a narrow period of time. This study combined existing tree ring data and reconstructed the annual Palmer Drought Severity Index over a 600-year historical period to generate new inflow sequences that were used to calculate reservoir firm yield. By having the longer historical record, a more robust calculation of a range of reservoir firm yields that can be used to assist water managers to inform decisions that affect water supply reliability.
Foss and Fort Cobb Reservoirs are located in central Oklahoma and are responsible for 90 percent of the surface water supply in the area, providing municipal water for about 40,000 people. Although Reclamation maintains ownership of the dam and conveyance facilities, operations and maintenance responsibilities have been transferred to the Foss and Fort Cobb Reservoir Master Conservancy Districts.