Where did all the Sylvan Lake water go?

Bob Dylan must have been thinking about Sylvan Lake when he wrote the lyrics to the song “Blowin’ in the Wind”.

Because that line answers the question: “Where did all the Sylvan Lake water go?”

A lot of it, about 20 million tonnes per year, 4.8% of the lake volume, just evaporated and blew away in Alberta winds like those in May-to-September 2019:

 

Wind Rose 2019-May to Sept

See even more detail in this 2019 May to September hourly wind history measured at Hespero.

Here is the evidence from Environment Canada’s Sylvan Lake level monitoring combined with Alberta Agriculture weather history recorded at Hespero west of Sylvan Lake where records were acquired for each day over the last decade.

Cumulative Precipitation May to Oct by Year

While 4.08 m of precipitation arrived, during the same period, the lake dropped from 936.999 masl in 2008 to 936.947 masl (masl: metres above sea level) in 2018. That is, the lake level dropped by 0.447 m (447 mm) !!!

Over the decade, the lake water inventory has decreased by (0.447 m x 42,000,000 square metres of lake surface area =) 18.77 million cubic metres, or about 4.5% of all the water in the lake, about 420 million cubic metres.

Each year’s lake level record has a characteristic shape: An initial increase from May through June is followed by a steady decrease after an annual maximum. Increments of open-water-season rain (the little red S-curves) are displayed to illustrate the cumulative precipitation during the decade:

Lake Level vs Cum. Precip. 2008-2018.v5

The analysis for any year must wait until data from the air bubbler-type hydrograph are reported from the lake level sensor in the SylvansEdge Marina after it is started up and calibrated by an Environment Canada field tech from Calgary in early May.

Hydrology stationEnvironment Canada’s hydrograph transmits data in real time.

For any year, the precipitation and runoff into the lake before the lake level is measured and logged are given by the difference in two data points: The first level measurement in a new year, and the last one of the previous year, as summarized in this graph:

Lake Level Nov to Apr

High runoff explains the increase in lake level in 2011. The graph also destroys the popular myth that Sylvan Lake is mainly “fed by underground springs”. That inflow, estimated separately by groundwater modelers to be 1 to 3 million cubic metres in a whole year, can be no greater than the observed 10 cm increase (equivalent to 0.42 million cubic metres) under about six months of ice cover, minus the watershed snowmelt runoff volume into the lake in any Spring.

The increase occurs when the combined rates of surface runoff and rain exceed the evaporation rate. When evaporation exceeds the inputs, the lake level drops.

Here is what all the data mean quantitatively:

Lake Level table

The last column shows that the Sylvan Lake surface area of 42 square kilometres loses about 20 million cubic metres of water annually because the rate of evaporation exceeds the combined rate of incoming precipitation and land runoff.

Over the 2008-2018 decade the average evaporation rate of incoming precipitation plus loss of lake water inventory was 2.8 mm/day, similar to the Standard-Grass evapo-transpiration value modeled by Alberta Agriculture for the Hespero station of 3.0 mm/day over the decade. Here is the Hespero station record for 2019.

Hespero Evaporation Model 2019

The average open-water season quantity of water lost was 20.2 million cubic metres. Over a decade, that 200 million cubic metres was about half the volume of Sylvan Lake !! Fortunately for the watershed, precipitation has balanced those losses.

So now we know. As Bob Dylan wrote:

The answer, my friend, is blowin’ in the wind
The answer is blowin’ in the wind

But, but, but………

If the lake water inventory is slowly being depleted, what is happening to the critical but out-of-sight groundwater supply? Does it still recharge when surface water is evaporating and transpiring away? Stand by for the answers to those questions.

The Alberta groundwater monitoring wells around Sylvan Lake have not been maintained for 25 years. You can check the Environmental records at https://rivers.alberta.ca/