Georgetown Lake Update 5-17-2018

Current Lake Elevation 6429.06 (May 17) (PROVISIONAL DATA from USGS gage 12325000)

Reservoir outflow has been around 65 cfs, increasing through the month. Winter outflow was generally around 30 cfs and increased in March and April.

Lake Inputs

On April 30, the lake water surface elevation was 6428.25 feet, and on May 15 it was 6429.00 feet. For those 15 days, the lake gained about 2,231 acre-feet of stored water even while outflowing 58 cfs average. That means that inflows and inputs averaged 133 cfs over those 15 days.  That amounts to 199% (almost double) the historic rate of normal inputs for May. 

Historically, the North Fork of Flint Creek peak inflow occurs in early June.  Air temperatures dictate how the snow melt progresses and the peak could be a bit later than usual. Follow Boulder Creek hydrograph behavior online to see how the North Fork is flowing; they have the same behavior but the North Fork is about 20 to 25% of Boulder Creek.   

Precipitation and Snowpack

After receiving zero precipitation last August, Peterson Meadows SNOTEL has logged above normal precipitation for all months between September and April, and is at about 133% of normal for the water year.  December received 167% of normal precip, and February 144%. So far in May, Peterson Meadows has received 2.7 inches.  

What a winter!  Snow Water Equivalent for Peterson Meadows peaked at 18.2 inches (186% of normal) on April 20; SWE for Warm Springs peaked at 40.0 inches (172% of normal) on May 4.  That is a new record snowpack for the Warm Springs SNOTEL site.  Snow depth actually peaked on April 13 at 111 inches.  In early May last year, the Flint Creek snow pack overall was at 121% of normal; this year it was 171% of normal. 

Graphed below, the blue lines show current SWE relative to the long-term normal (red line). The black line shows total precipitation since October 1, relative to the long-term normal (gray line).  Today, Peterson Meadows is at 120% of normal SWE, and Warm Springs is still way up at 162%.   

Climate Forecast

The CPC climate forecast (May 17) is for above normal air temperatures for the entire summer through November. Precipitation is forecast to be below normal for June and July, then normal for the rest of the season. We’ll see, those forecasts are updated around the 20th of each month. 

The current Weather Service forecast from Missoula is indicating strong chances of precipitation beginning today and for the next week. Cool air temperatures may slow the snow melt, but rain can contribute yet more moisture to the basin. 

NRCS Flow Volume Outlook

The NRCS May 2018 Water Supply Outlook was released last week and it details an outstanding winter and snowpack.  Many sites in Montana set new snow pack records.  For Flint Creek near Southern Cross, the best current estimate (50% chance of exceeding) is for 157% of normal flow volume for May through September. With such a large surplus, it’s very important to consider the more aggressive 30% chance of exceedance, which is 181% of normal flow volume.  There is also a 10% chance of exceeding 213% of normal flow volume. 

 Model Forecast

Given that the first two weeks of May had 199% of normal inputs, I first ran the model using 199% as the input and 65 cfs for the current outflow, just to get a May 31 elevation.  If outflow is held at 65 cfs for May AND if inputs continue at 199% of normal, the reservoir will fill by the end of May.  I suggest increasing outflows in anticipation of extended snow melt and possible rain-on-snow events. If outflow is immediately increased to 90 cfs, and inputs continue at 199% of normal, then the average May outflow would be about 77 cfs and the May 31 lake level would be 6429.36 feet. 

I then ran the model for the rest of the year, assuming a May 31 level of 6429.36 feet, and inputs at 157% of normal (see below). Outflow would need to be held very high, 131 cfs, through June and quite high for the rest of the summer.  An alternative would be to increase May outflow to more than 90 cfs, which could reduce the high June outflow somewhat. At this time, it’s most important to focus on May and June, given the amount of water in the system.  

Given the snow pack status and water year precipitation, we also need to look at the 30% chance of exceeding 181% of normal flow volume.  Again, I ran the model using a May 31 level of 6429.36 feet (see below). Note the very high 152 cfs average outflow for June.  Although the NRCS 50% exceedance forecast is for 157% of normal, it is not unreasonable at all to think that actual volume could be more like the 30% exceedance of 181% of normal in the next few months. 

Keep in mind these are forecasts averaged through a season, i.e. May through September. Individual months will have different “percent of normal” flow volume runoff figures.  For example, two months could see flow volumes of 200% of normal, then the next two months could see flows volumes of 100% of normal, which would make an average for the season of 150% of normal.

D. Amman, Montana DNRC, 444-6648


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