Washita Pilot Study
Scientists can use tree rings to understand past drought severity and plan for future water demand and availability.
In 2011-2015, parts of western Oklahoma experienced the most severe drought in the past 100 years (“drought of record”).
The ongoing drought eventually brought many reservoirs to record low levels. In this example showing Foss Reservoir, the water level was less than a foot above the lowest operating intake. If levels had dropped further, communities would have been unable to access the water.
The Washita Basin Project, which includes Foss and Fort Cobb Reservoirs, provides numerous benefits, including drinking water and power generation to several communities throughout west-central Oklahoma.
The Washita Reservoir Operations Pilot Study sought to answer the question “How much water can a reservoir provide to the cities that depend on it during different critical drought scenarios?”
In the study, Reclamation used tree ring data to expand the period for analyzing water availability from the 100-year historical record to a period of over 600 years.
This pilot study provides a successful example of the methodology for developing paleohydrology drought scenarios (i.e. estimates of past streamflow) using the Palmer Drought Severity Index (PDSI) and using those scenarios to evaluate supply in a reservoir.
The analysis for long-range planning showed that the drought scenarios based on tree ring reconstructions of the PDSI (black) can result in reservoir supplies much lower than those based on the observed droughts of record (red and blue).
In other words, under a risk management approach, the actual risks of drought are greater than previously understood.
The work is important because when a wider range of demand curtailments are expected, steps may be taken earlier to identify demand management strategies to prepare for the next drought.
For more information about this case study and others, access the Collaborative Conservation and Adaptation Strategy Toolbox (CCAST) Case Study Dashboard in the adjacent panel.
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