Georgetown Lake Update 10-3-2018

Heading into Autumn, the lake level today (October 3) sits at 6428.71 feet, and outflow is at about 42.3 cfs.  All conditions are good and there is even snow forecast at the lake level for the next several days.

The water surface elevation for August 31 was 6429.21 feet and for September 30 it was 6428.69 feet.  So the lake lost 1,547 acre feet during September, which is about 26 cfs.  The average measured outflow was 47.75 cfs, therefore inflows/inputs averaged 21.75 cfs, which is 99% of  normal for September, basically right at the long-term average.  Some of the input was from surface inflows, but almost none from precipitation as Peterson Meadows logged only 0.3 inches during the month.  The majority of inputs would be from groundwater.  The groundwater combined with shorter day length and cooler air temperatures significantly decrease evaporative lake losses.

At this time, the Climate forecast from CPC is for increased chances for warmer than normal air and below normal precipitation for the coming winter months.  The forecast will be updated around October 20.

I ran the model using 100% of normal inputs and using 80% of normal inputs (see attached JPG files).  For October, outflows are held at 42.3 cfs for the first half of the month, then decreased to 25 cfs (for the 100% of normal run), and 18 cfs (for the 80% of normal run).  Both model runs indicate an abundance of lake water for the winter, the main difference being winter outflows.  The lake typically freezes up in the third week of November and at that time, the target water surface elevation should be around 6428.50 feet, plus or minus 0.10 feet. 

I will reevaluate the lake level, inputs, outflows and climate forecast later this month or in early November.

Thanks, Dave

D. Amman, Montana DNRC, 444-6648


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