Why can’t Lake Chelan always be higher in the spring?
Chelan PUD manages the drawdown and refill of Lake Chelan every year by scheduling and operating hydropower generation and spill throughout each year. Lake elevation target curves were developed by Chelan PUD staff, by modeling many years of historical inflows into the lake. The Lake elevation target curves guide us on cyclical inflows and weather effects. After all, forecasts for inflows, snowpack and weather are always changing and uncertain. The target curves help balance the PUD’s FERC License seven operating objectives that, together, provide recreation, protect fish, reduce erosion and restore year-round flows to the Chelan River.
So what’s going on with Lake Chelan so far in the 2016-2017 water year? Well, a little more normalcy than in the last couple of years from a historical lake level perspective, but more on that later.
The interactive graph on Chelan PUD’s website helps tell the story.
Below is a 2/21/2017 snapshot of the interactive lake level graph.
The graph is updated daily with the actual lake level and weekly with an updated Current Forecasted Operating Range. The Target Curve During Dry and Average Water Years is being used to guide our lake operations during the 2016-17 water year because the projected water supply forecast is currently 90% of normal. Once the lake level is drafted down to the target curve and the minimum tailrace flow obligations for fish are being met with a minimum level of hydropower generation through March, the timing of the runoff (i.e. daily inflows) will play the largest role in spring lake levels. Using the Northwest River Forecast Center’s current inflow forecasts, generation is expected to be zero for 1 to 2 months this spring.
So, if this year is more on the "normal" side of historical drawdown and refill years, what do not-so-normal years look like?
Let’s discuss our two most recent prior years. The graph at the end of this article representing the lake levels for these two years is very telling.
The 2014-15 and 2015-16 runoff seasons at Lake Chelan were anything but normal. On May 1, 2016, the lake level was more than eight feet higher than expected. In 2015, it was more than four and a half feet above the expected level for that date. These situations were for very different reasons. The lake was very high and high very early for both of those years. But why can’t it be that way every year? The short answer is: Mother Nature and managing risks – namely managing spill into the Chelan Gorge to an acceptable level. Let’s see what happened in each of those years.
Both 2014-15 and 2015-16 water years started out with something in common. Even with near maximum power generation by Chelan PUD’s two generators at the Chelan dam through the previous falls and winters, the lake still did not draft down to our target curve levels. Precipitation – rain primarily in 2014-15 and rain then snow in 2015-16 – was considerably above normal in the Lake Chelan basin.
In 2014-15, warmer temperatures during the fall/winter period meant higher than normal amounts of rain rather than snow. Because there was so little snowpack, the April - July runoff projection was very low at 66 percent of average – and the actual runoff ended up being 64 percent of average.
Even though the lake level was high in the spring from the rain, our concern was to be able to get the lake "full" before the already precious-little runoff ran out. Based on the modeling of some select, extremely low water years, it was decided to use a higher than normal target curve to bring the lake level up earlier than we normally would because there was no risk of large volumes of spill later in spring and early summer with the low snowpack. Hydropower generation was limited in the spring to make sure the lake would completely refill. The red line on the graph below represents the 2014-15 lake level.
In 2015-16, there was even greater precipitation during the fall/winter period than the previous year, but temperatures were lower and much of it fell as snow. Although the lake itself wasn’t as "full" coming into spring as it had been in 2015, it still was higher than ‘normal’ by a couple of feet.
Then came April! April of 2016 holds many records, including that it was nearly six degrees warmer than the monthly average. This warm weather translated into the highest inflows from early snowmelt into Lake Chelan than in any previous April in the 65-years of data. May 2016 was also unusually warm.
Additionally, one generating unit was scheduled out of service for an overhaul that was expected to take six weeks. The overhaul was planned several months in advance – before the early snowmelt. The maintenance turned out to be during this same period of record snowmelt and high inflows. Early spring, historically, is a good time for an outage because the lake inflow hasn’t increased substantially yet. The combination of high inflows and limited generating capability are the two primary reasons why the lake level shot up so quickly. The green line on the graph below represents the 2015-16 lake level.
Unlike 2015 where we deliberately worked to bring the lake up earlier than usual to help ensure refill, in 2016 Mother Nature commanded the lake elevations at a record pace. The significant amount of runoff still to come during the summer gave us a number of challenges, particularly how to manage the spill at the dam. You may recall seeing warnings about high spill amounts last spring in the Chelan Gorge. Significant, early snowmelt was the reason. Interestingly, our early runoff projection of 116 percent of average (April – July) ended up being what actually occurred – but the difference was when it occurred!
Although the two years started out in a similar fashion, the runoff periods and management of the runoffs were very different. Both unusual years in opposite ways. Most years lie somewhere in the middle as can be seen in the blue area of the graph below.
Questions about climate change and changing weather patterns do loom. Could we see more "unusual" years going forward? More precipitation falling as rain with warmer winter temperatures and earlier runoff? Higher spring temperatures with earlier runoff? Chelan PUD is watching the regional efforts studying these matters.
It’s unlikely we will face these exact circumstances every year as the water year 2016-17 is so far illustrating. The team at Chelan PUD continually updates its modeling efforts and uses the latest meteorological forecast and inflow information available to operate to the seven FERC license operating objectives while providing some certainty and predictability for lake levels.