UPDATED SEPTEMBER 3, 2010
There has been no lack of water at the lake this summer, as the August 31 lake level was at 6429.15 feet. That is almost exactly the same level as last year at this time, and well above the long-term median of 6428.70 feet. August inputs (precipitation, surface and ground water) averaged 36.2 cfs, which is 172% of the long-term input average.
Water Year precipitation at Peterson Meadows has totaled only 83% of normal, but last month accumulated 2.2 inches, or 116% of average. Other good months were June, with 144% of normal, and last October, with 147% of normal. Water Years 2007, 2008 and 2009 have all seen above-normal precipitation at Peterson Meadows.
Current lake outflow is set at 60 cfs and the August outflow averaged 53.7 cfs, which is 158% of the long-term norm.
For April through August, lake inputs have averaged about 120% of normal, with a high of 172% in August. I chose to use the average 120% figure to model a “low-input scenario”, and the 172% to represent the “high-input scenario” for the reservoir over the next few months.
Low Input Scenario (120% of normal):
Inputs Outflow Month-end El.
September 26.3 cfs 60.0 cfs 6428.45 feet
October 27.4 40.0 6428.20
November 30.2 30.0 6428.20
December 31.2 25.0 6428.32
High Input Scenario (172% of normal):
Inputs Outflow Month-end El.
September 37.7 cfs 60.0 cfs 6428.68 feet
October 39.2 55.0 6428.37
November 43.3 50.0 6428.23
December 31.2 25.0 6428.32
Either scenario shows that outflows must be kept relatively high in order to reach a suitable winter lake level. Precipitation and air temperatures over the next couple of months will dictate lake inputs (and evaporation rates) and the corresponding strategy for reducing the water level by freeze-up time in November.
According to climate and weather prediction sources at this time, this coming winter is supposed to be above-normal in precipitation.
Updated July 24, 2010
On June 30, the Georgetown Lake water surface elevation was 6429.43 feet, and the July 11 elevation was 6429.52 feet. The high inflows and outflows of June have subsided and outflow is currently set at 41 cfs. With this reduced outflow rate, the lake level is still rising. According to the most recent figures, inputs now are between 65 and 70 cfs. It is possible that the lake will rise again to the spillway crest by the end of July.
Calculations show that June inputs averaged 136% of the long-term norm. So far in July, inputs have averaged about 130% of the long-term norm. It is impossible to know if these generous inputs will continue through the summer, so I have modeled the reservoir using three different scenarios: 130% of normal inputs; 100% of normal inputs; 80% of normal inputs.
The attached graphs tell the story. If outflow is held at 40 cfs for the summer, there appears to be plenty of water even if inputs decline to 80% of normal. While that could happen, the current Climate Prediction Center forecast is for normal conditions through autumn, trending to a cooler winter with above-normal precipitation.
The lake has benefited from good spring precipitation and cool air temperatures, remaining very high. Even at 80% of normal inputs, relatively high autumn and winter outflows would still be possible. At the normal and above-normal input levels, even more water would need to be released to achieve a suitable November freeze-up elevation, and maintain that level through the winter.
Thanks, Dave
Updated July 2 2010-
On June 30, the water surface elevation was 6429.43 feet, about 1.5 inches below the spillway crest. Earlier in the month, water was spilling slightly over the spillway. Granite county increased outflows from 75 cfs to 170 cfs to accommodate the relatively heavy June precipitation and the snow melt inflows. June inflows averaged 120.9 cfs, which is 136% of the long-term norm; June precipitation at Peterson Meadows was 4.6 inches, which is 144% of normal; June outflows through the USGS gauging station (Flint Creek near Southern Cross) were 115 cfs, which is 198% of normal.
The lake level was 6429.46 feet on July 1, after outflows were reduced to about 80 cfs. Calculations indicate that inflows and inputs (including some rain on June 30) were about 126 cfs. Of that, the North Fork Flint Creek was contributing 57 cfs; Stuart Mill was very high at 23 cfs; observed miscellaneous surface flows were about 5 cfs; estimated groundwater inflows were 34 cfs. The high flow of Stuart Mill Spring is an excellent indicator of good groundwater conditions.
So right now (it’s always a state of flux at this time of year), if outflows were held around 90 to 100 cfs, inflows would approximate outflows and the water surface elevation would remain fairly steady.
The North Fork hydrograph is interesting this year, exhibiting two moderate peaks: 91.8 cfs on June 8, and 97.0 cfs on June 17. These peaks were only about 2 hours after the two peaks on Boulder Creek, which of course were much higher at 404 cfs and 396 cfs, respectively. The North Fork typically peaks during the first week of June at around 120 cfs. Later this season, it will be interesting (to me) to review the data and calculate the “center of mass” for the North Fork hydrograph. This calculation tells us when half of the total water volume has flowed past the gauge.
I will provide modeling output and projections in mid-July when flows begin to stabilize and we have a clearer picture of how water yield and water supply will look for the rest of the summer.
Updated May 11, 2010
The Georgetown Lake level was 6429.46 feet on April 30. That is a new record high for April 30 and about one-tenth of a foot below the spillway crest. Last year the month-end level was much lower at 6428.72 feet, but outflows had been increased to draw the lake down. Beginning May 2 this year, Granite County has wisely raised outflows, which are now set at 59 cfs.
Lake inputs for April were exactly at the long term normal 33.5 cfs. The lake gained 952 acre feet during April despite precipitation of only 77% of normal (at Peterson Meadows). April outflows averaged 17.5 cfs.
The May 7 NRCS flow volume projections for June through September are (50% - 70% - 30% exceedance):
Flint Creek near Southern Cross (below the dam): 61% - 39% - 83%
Flint Creek below Boulder Creek: 67% - 51% - 83%
The 50% chance of exceedance projection represents the best estimate at this time, so according to the NRCS it will be a drought summer with water in short supply. Note that the NRCS projections cover a four-month period. Fortunately, the lake level is high heading into the irrigation season.
Using the model right now is complicated since the snowpack which supplies the lake is in great shape. As a result, lake inputs should be near normal over the next two months, then diminishing through the summer according to the NRCS forecasts.
The following figures represent lake level estimates based on 83% of normal inputs for May and June, transitioning downward to 61% of normal for July through September. The goal at this time is to have a full reservoir on June 30.
May 31: 6429.27 feet; outflows average 65 cfs
June 30: 6429.46 feet; outflows average 65 cfs
July 31: 6429.40 feet; outflows average 30 cfs
August 31: 6429.04 feet; outflows average 30 cfs
September 30: 6428.70 feet; outflows average 30 cfs
The NRCS will formulate and publish one more projection, in early June. I will update lake level projections at that time. Let me know if you have questions.
Thanks, 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.
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)
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