From: Amman, Dave

Sent: Tuesday, December 04, 2012

The lake water surface elevation was 6428.00 feet at the end of October and rose to 6428.26 feet by December 1. That translates into a gain of 774 acre feet, or an average gain of 12.6 cfs through November. Since outflows averaged 10.6 cfs for the month, total lake inputs (surface water, ground water, precipitation) averaged 23.2 cfs, which is 109% of normal for the month. That is great news given the extremely low inputs late last summer, on the order of 20% of normal. Late summer also saw severe lake evaporation due to the warm, dry weather.

The lake level is set right for winter, and the most recent short-term and long-term forecasts look promising for cold, snowy weather. Attached are the December precipitation and temperature forecasts by the CPC.

If lake inputs continue at 109% of normal for the winter, the model shows that outflows will need to be increased from the current 12 cfs to avoid filling the lake by the end of March. With the most recent precipitation forecasts, which are very different from those of a few months ago, concerns of the lake not filling in 2013 have subsided, and outflows can be increased (either now or later this winter) to 15 or 16 cfs.

Thanks, Dave

Updated April 12, 2012

Today the snow water equivalent for the Georgetown Lake watershed is at 105% of normal (see attached). The Weather Service has issued a Winter Storm Warning for the next few days, calling for 10 to 15 inches of snow at the lake, which will boost the snow pack if it pans out. At Peterson Meadows SNOTEL, water year precipitation is at 108% of normal.

The lake water surface elevation was 6428.58 feet at the end of March. That level is a couple of tenths higher than last year, but three tenths lower than the average March 31 level since 1999. The outflow has held steady all winter at 29 cfs, and the lake level has stayed steady even with such high outflow. Normal winter outflow is around 21 cfs. Back-calculated March inflows/inputs were 32 cfs, which is 117% of normal.

The NRCS published their latest flow volume forecast yesterday. The “best estimate” (50% chance of exceedance) for summer flow volume for Flint Creek is 101% of normal; a more conservative estimate (70% chance of exceedance) is 81% of normal. On the high side, there is a 30% chance of exceeding 120% of normal flow volume.

I ran the model using the conservative 81% of normal volume figure (see attached). At this relatively lower level there is still ample water to satisfy all reasonable needs, albeit with limited management options. In all likelihood, flow volume will be greater than 81% of normal.

The May forecasts are usually the most important and most accurate for the summer as they reflect peak snow pack conditions. I’ll have another update in about a month. Dave

Updated March 4, 2010

On February 28, the Georgetown Lake water surface elevation was 6428.82 feet. That elevation is right at the median level for the period since 1999. For the period up through 1998, the median February 28 elevation is 6427.83 feet.

On February 1, the elevation was 6428.78 feet, and the February gain of 0.04 feet represents an increase of 119 acre feet. Outflows have remained unchanged at around 16 cfs. Calculations tell us that overall lake inputs were 18 cfs, which is about 78% of the long-term average. Recall that January inputs averaged about 75% of normal, very similar to this month.

Precipitation at Peterson Meadows was a measly 0.3 inches. That is only about 18% of the normal February total of 1.7 inches, leaving Peterson Meadows at only 49% of normal so far in this water year. This morning, the Peterson Meadows + Warm Springs snow pack index is at 81% of the long term average. The typical snow water peak occurs in late April, and hopefully we will receive some more big storms over the next two months.

The CPC forecast as of February 16 indicates warmer than normal air temperatures for March through August; near-normal precipitation for March, April and May; below-normal precipitation for June, July and August.

The NRCS flow forecast (50% chance of exceedance) is for 71% of normal flow volume for Flint Creek below the dam. At the 70% chance of exceedance, the forecast is for only 49% of normal volume. These figures will be updated within the next week. I ran the model based on both of these scenarios. At 71% of normal volume, there should still be enough water to supply all needs, if managed astutely (see attached graph, GTLMarch71.jpg).

At 49% of normal volume, we can almost certainly expect shortages, even though the model shows otherwise. Keep in mind to focus on the next six months. This graph (GTL.March49.jpg) also shows that while we may skimp by this year at 49%, if this trend continues through next fall and winter, conditions will be severe in 2011.

The last scenario (GTLMarch49Reduced.jpg) again shows the supply at 49% of normal volume. However, I adjusted outflows downward to 12.5 cfs beginning immediately. This represents 78% of the current 16 cfs outflow; 78% being the current lake inputs percentage of normal as explained previously. This action would store additional water in the event that the 49% scenario begins soon, and would likely allow the lake to fill completely.

At this time, all indicators point towards a drought for this summer. Georgetown Lake is in better shape than just about any place in Montana right now, but as we have seen in several of the past 10 years, the water supply continues to shrink throughout the summer in these dry, hot conditions.

Lake Level (Updated 2/15/2010)

The December 31, 2009 lake level was about 6428.68 feet, compared to 6428.52 feet last year. As a reference, the long-term median lake level for December 31 is 6428.39 feet, and the near-term (1999 - 2010) median is 6428.50 feet, so the lake is above the normal level at years’ end. December lake inputs were 34 cfs, about 131% of normal. I am guessing that relatively large groundwater inflows, provided by the past two years of excellent overall moisture conditions, continue to increase the lake level even though precipitation has been very spotty for November through the present time.

The Georgetown Lake water surface elevation was 6428.78 feet on January 31. That is right at the normal (median) level, based on the last 12 years. For the entire period of record (since 1940), the lake is 0.78 feet higher than the median level at the end of January. During January, the lake gained about 0.10 feet of elevation, equivalent to 298 acre feet.

Precipitation for January, as measured at Peterson Meadows, was a scant 1.0 inch, which is 50% of normal. However, bolstered by good groundwater inflows, calculated lake inputs were about 21 cfs, or 75% of normal. Outflows from the lake remain at 16 to 17 cfs. Today, the combined snow pack for the lake (Peterson Meadows and Warm Springs) averages 85% of normal.

The February 5 NRCS forecast estimates flow volume for the period April through September at 71% of normal at the 50% chance of exceedance (best estimate at this time). The 70% chance of exceedance estimate is for only 48% of normal flow volume.

I have attached graphs of these scenarios. At 71% of normal flow volume (GTLFeb50Exc.jpg) there appears to be enough water to fill the reservoir and keep it at full pool through July. May outflows of 23 cfs reflect the average of half the month at 16 cfs and half the month at 30 cfs. Careful attention must be paid to reservoir operations during the runoff period in order to maximize contents while cautiously monitoring for potential high flows.

87201266250121GTLFeb71.jpg

At 48% of normal flow volume (GTLFeb48.jpg) we can expect water shortages. In this illustration, I reduced outflows to 10 cfs beginning in March. The model shows that a rigid outflow schedule of 30 cfs through the summer will still maintain a decent lake level. However, experience has shown us that below 60% of normal flow volume, the lake and Flint Creek begin to suffer. Also we have seen that problems with limited water supply are compounded by warm, windy summers.

The latest Climate Prediction Center forecast (January 21) is for warmer than normal air temperatures through August, and near-normal precipitation, except for July and August which would have below-normal precipitation. Right now, all indicators point towards a tight water supply for the summer.

The NRCS will update flow volume predictions in another few weeks and I will provide another update then.
Thanks, Dave

While it is important to be conservative in these water supply forecasts, it is also important to note that it is early in the winter and the snow pack accumulation season. It is also important to note that both of these graphs do not reflect the fact that December inputs to the lake were quite elevated at 131% of normal and that is likely due to groundwater inputs. I think we will see January through March end-of-month lake levels a bit higher than the model outputs show here.

Will groundwater flows remain high? Will el nino continue to strengthen and leave western Montana high and dry? Will precipitation be near-normal but come as rain, with warmer air temperatures and an early spring run-off? Only time will tell. Stay tuned and we will revisit the snow pack, flow volume forecasts, and model results monthly from now through the new year.
D. Amman, Hydrologist, Montana DNRC, 444-6648

Water Levels (August 2009)

Georgetown Lake and Flint Creek have held up extremely well this summer and the lake water surface elevation on August 31 was 6429.16 feet. The median August 31 elevation since 1999 is 6428.50 feet, and last year the elevation was 6429.43 feet, which lead to increased outflows through autumn in order to create freeboard prior to the winter freeze. Outflows for July averaged 62.2 cfs and for August averaged 61.8 cfs. That is the second-highest August average flow for the period of record (since 1940).

Also on August 31, the North Fork of Flint Creek was flowing 4.7 cfs, which is probably near the long-term average, and is above the short term (7-years) average. Stuart Mill was flowing 10.2 cfs yesterday and that appears to be well above average for this time of year. Total surface streamflow inputs are at about 16 cfs, and the area has received some heavy rain events in July and August. Overall, precipitation at Peterson Meadows sits at 117% of normal for the 2009 water year.