From: Amman, Dave
Sent: Tuesday, December 04, 2012
Updated April 12, 2012
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.
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.
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)
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