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Ensemble Streamflow Prediction (ESP) forecasting became available in the Truckee River Basin in 2008. These ESP forecasts are useful in many ways for seasonal reservoir operational forecasting such as: providing a temporal disaggregation method for seasonal volume forecasts, establishing uncertainty in seasonal forecasts, determining the risks of various outcomes, communication to the public, etc. However, when it comes to making operational decisions the practical use of ESP forecasts tends to be selecting a trace that is "expected" and making release decisions based on this expected inflow. These release decisions may be subjectively informed by the ESP forecasts, but how to objectively use these forecasts is not well defined.
Lake Tahoe is the primary water supply feature in the Truckee River Basin and basin policy requires that the maximum elevation be maintained "in-so-far as practicable" to protect interests surrounding the Lake. Whenever there is sufficient inflow to fill the lake, it is desired to fill to the legal maximum as closely as possible to maximize the water supply without damaging Lake shore properties. Given the high degree of uncertainty in seasonal inflow and the limited release capacity of the Lake, a method is desired to utilize ESP forecast data to reduce the risk that the maximum elevation is exceeded while filling the reservoir to the legal limit as closely as possible.
Such a method was developed in real time during 2017, 2018 and 2019 when inflow was sufficient to fill the lake (which had not occurred since 2006). This method will be discussed. |
