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Lee Vining Creek Peak Flow Predictive Model

Peter Vorster and Greg Reis, Mono Lake Committee

The Los Angeles Department of Water and Power (LADWP) is required to allow the peak flow on Lee Vining, Parker, and Walker Creeks to pass undiverted. Parker and Walker Creeks are only diverted in dry years, therefore in most years the focus is on predicting Lee Vining Creek's peak. For several years the Mono Lake Committee has been tracking temperatures, snowmelt, and creek flows during the peak flow period in an attempt to help LADWP determine when to stop diversions and allow the peak to pass, and when to resume diversions following the peak.

The "model" began as just a spreadsheet tracking various temperature stations and snow pillows in the area. When nighttime low temperatures reached certain thresholds and snowmelt was at least 1 inch per day, we predicted flows would increase until a peak when the temperatures began to drop or until the snowpack diminished. Temperatures below a certain threshold would "shut down" the snowmelt and delay a peak. Tuolumne Meadows and Dana Meadows temperatures generally correlate well with Lee Vining Creek peaks.

The model was refined in 2005, during a wet-normal year that provided a good dataset of peaks, and a snowmelt index was added combining all the thresholds into one number. That index was correlated with the change in Lee Vining Creek flow. Another correlation between forecast temperatures and the index allows a 6-day flow forecast. The index does not take into account rain-on-snow events or powerplant flow changes.

There are many variables involved, including power plant flows, reservoir levels, snowpack characteristics, rain-on-snow events, and difficulties in forecasting temperature changes. We plan to continue refining the model, however in its current relatively informal state it is very functional and useful and with some estimation and interpretation, it is able to predict peaks.

 

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