Actionable science, Climate adaptation, Water resources
Washita Basin Project: Using Paleohydrology from Tree Rings to Inform Long-Term Planning and Reservoir Operations During Drought
Case Study by the Conservation and Adaptation Resources Toolbox
Status
Completed

Location

States

Oklahoma

Subject

Adaptation
Climate change
Dams
Drought
Hydrology
Reservoirs
Water budget

Introduction

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Foss and Fort Cobb Reservoirs, located within the Washita Basin Project in Oklahoma, supply 90 percent of the surface water in the area, providing municipal water for about 40,000 people. 

The Bureau of Reclamation conducts Reservoir Operations Pilot Studies to identify approaches that can improve reservoir operations in response to future variability in water supplies, floods, and droughts. Reclamation’s Great Plains Region conducted the Washita Reservoir Operations Pilot Study to improve management during drought conditions at the Foss and Fort Cobb Reservoirs within the Washita Basin Project in Oklahoma. This reservoir system predominantly serves municipal and industrial (M&I) needs, including needs for domestic and residential purposes—i.e., water for human consumption—public health and sanitation, and commercial and industrial processes. Managing water during times of drought is particularly challenging in the Washita Basin because an interruption in water supply has detrimental impacts on critical water uses, such as public health and sanitation. 

Reservoirs in this system depend mostly on rainfall and streamflow for their supply and are, therefore, vulnerable to drought. Droughts vary in intensity, duration, and severity, which means that water managers face different water supply issues from one drought to the next. Given these uncertainties, preparing for drought requires identifying an acceptable level of risk for water shortages.

In managing Foss and Fort Cobb Reservoirs, risk is often quantified in terms of a reservoir’s “firm yield,” which is the amount of water each reservoir could reliably supply (in this case for M&I uses) during a repeat of the worst drought on record. Operating reservoirs based on firm yield gives managers an assumed level of certainty that a minimum water supply will be available to customers. 

However, a disadvantage of the firm yield approach is that record keeping for these reservoirs encompasses only 90 years of historical data on which to base assumptions about future hydrologic conditions, and the drought of record may not be the worst drought that could be experienced in the basin. Indeed, the 2011 drought exceeded the previous drought on record for much of the area. 

Key Issues Addressed

In 2011, parts of western Oklahoma experienced the most severe drought in the past 100 years (“drought of record”) and reservoir levels reached record lows. The drought revealed risks to the system that could be mitigated through new approaches to better understand how long-term variations in climate can affect reservoir yield. In the Washita Reservoir Operations Pilot Study (pilot 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.

Project Goal

  • Investigate how much water a reservoir can provide to the cities that depend on it during different critical drought scenarios

Project Highlights

  • Using Tools Driven by Paleohydrology: In the Washita Pilot Study, Reclamation used reconstructions of the Palmer Drought Severity Index (PDSI) based on tree rings, to develop reservoir inflow sequences (paleohydrology) that encompass about 600 years of past wet and dry cycles, including “mega droughts” for Foss and Fort Cobb Reservoirs. Reclamation then generated over 1,000 paleohydrology drought scenarios using statistical resampling approaches and evaluated their impacts on reservoir firm yield. The reconstructed data shows drought periods (noted in brown on the figure where PDSI is less than zero) that exceed the 1950’s drought of record for Foss Reservoir in magnitude (High Intensity Paleo Drought) and length (50- year Paleo Drought). 

    Using paleohydrology drought scenarios, Reclamation also developed a tool that can be used to plan for and respond to drought scenarios in real-time: the Enhanced Drought Response Reservoir Operations (EDRRO) Model. With stakeholder input, Reclamation selected several scenarios of drought to provide benchmarks for real-time delivery of M&I water supplies. With this range of scenarios, the model can be used to quantify the needed percentages of curtailment to avoid emergency drought response.

  • Key Results and Findings:  The tools and approaches developed in this pilot study can help inform long-range planning decisions or real-time drought response. 

    The analysis for long-range planning showed that the drought scenarios based on tree ring reconstructions of the PDSI result in a much lower reservoir firm yield than that based on the observed droughts of record. In other words, under a risk management approach, the actual risks of drought were greater than previously understood. 

    Using a 600-year paleohydrology record helps develop robust statistics to understand the likelihood of each reservoir going dry under different drought scenarios, along with the demand curtailments necessary to prevent the reservoir from going dry. As shown in Table 1, if maximum demands are placed on Foss Reservoir under high intensity drought conditions, they would need to be curtailed by 32 to 66 percent to prevent water shortages. However, the same conditions under observed droughts at Foss Reservoir would not have required any curtailment (0%) to prevent shortages— providing a false sense of security. 

    Similarly, at Fort Cobb Reservoir, maximum demands would need to be curtailed by 34 to 53 percent to prevent water shortages under the most severe paleo droughts modeled, compared to the observed 1950s and 2010s droughts that only required curtailment between 4 and 29 percent. 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. 

    Incorporating paleohydrology information captures more variation in wet and dry cycles (both in severity and length), which results in a range of firm yield values and provides information on risk exposure. Accounting for real-time water demand allows for a more informed calculation of available reservoir storage. This allows for a more robust assessment of risks to the system to better inform near and long-term water management decisions.

Lessons Learned

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Moving Forward: “They say that necessity is the mother of all invention. The 2011 drought proved that in western Oklahoma. With this new tool, we can feel more confident that when the next drought comes, we will be better prepared.” - Shawn Dewee, GM, Foss Reservoir Master Conservancy District

This pilot study provides a successful example of a methodology for developing paleohydrology drought scenarios (i.e. estimates of past streamflow) that can be used to develop a range of firm yield estimates beyond the observed drought of record. 

By having a quantifiable, risk-based assessment of supply, managers can make more informed decisions on long-term investments and drought contingency planning that can help water managers reduce the risk of water shortages. 

This study details on how this information may be used for long-term planning or to inform real-time operational decisions using a broader range of historical data than had been considered before. 

Tree ring data are available for most of North America, so these methods may be applied in other Reclamation basins.

Next Steps

  • In the case of Fort Cobb, local officials are using the results to inform the decisions related to the investment in a new multi-million dollar aqueduct. In showing that the reservoir is fairly reliable under even the most severe drought scenarios, the study may instill more confidence of a positive return on investment should customers be willing to invest in the project.
  • Reclamation’s project partners in Texas have asked Reclamation to perform a similar analysis on Reclamation’s three Texas reservoirs to enhance drought preparedness and response. In doing so, Reclamation may collaborate with the Texa Water Development Board, the state’s lead water planning and financing agency, which has an interest in potentially integrating a quantifiable, risk-based multi-scenario approach into the state’s regional water planning process. 

Funding Partners

Bureau of Reclamation

Resources

Contacts

Case Study Lead Author

Collins Balcombe, Bureau of Reclamation, cbalcombe@usbr.gov

Suggested Citation

Bureau of Reclamation. (2020). “Washita Basin Project: Using Paleohydrology from Tree Rings to Inform Long-Term Planning and Reservoir Operations During Drought.” CART.Retrieved from https://www.fws.gov/project/washita-basin-project-paleohydrology-drought.

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