Total Precipitation and Sylvan Lake Level

Sylvan Lake watchers have conflicting personal opinions about lake level. This post presents some facts about the one factor that causes the lake level to rise: the Total Precipitation that falls on the watershed area each year. Other factors cause the lake level to drop, particularly seasonal evaporation and transpiration from land, crops and foliage that is measurable after about July 1 each year.

An understanding of these historical data is important to the debate about control of the level of Sylvan Lake for the convenience of lake users. The simplest fact to know is that the watershed receives about 500 mm (half a metre) of precipitation annually. Statistically, the yearly values are typically within +/- 10 percent of 500 mm about 70 percent of the time. In the other more extreme years the total precipitation can differ as much as +/- 30 percent of the average.

The fate of that water determines if the lake level goes up or down. To calculate how much water is in play multiply the area of the watershed (106 million square metres) by the 0.5 metre of annual total precipitation. Of the 53 million cubic metres of water that is typically delivered by weather systems, most evaporates, or else it infiltrates and is stored as groundwater.

The Red Deer Airport has collected and recorded those weather data for many decades and provides quality-controlled history that is representative of conditions here in Central Alberta. Those data are used to create the following graphs.

As there is no weather station in the Sylvan Lake watershed we do not have data for the 106 km2 of the area. Other weather stations have reported data at locations within 50 km of Sylvan Lake however records have been neither continuous nor quality controlled to national meteorological standards.

For a more comprehensive regional analysis of precipitation and evaporation see the Edmonton-Calgary Corridor Groundwater Atlas.

Annual Precip-RDA-1940 to 2011

Total precipitation recorded at the Red Deer Airport since 1940. Source: Environment Canada’s climate data.

Precipitation Anomaly-RDA-1940-2012

This graph shows how annual amounts of Total Precipitation have varied when compared to the 71-year average.

Change in Total Precip Y-to-Y 1940-2011

From year to year Total Precipitation has behaved unpredictably. One year it can be very high, then the next year we can have drought-like conditions. Trends are not evident in the 71-year record.

Cum Precip vs Cum Time 1971-2000

In any year the Total Precipitation has an “S-Shaped” characteristic as this graph shows. The middle 40% of the year delivers about 70% of the precipitation. The points in this graph are average values calculated from 31-years of data.

Alberta Environment and Sustainable Resource Development provides continuous reports on the level of Sylvan Lake.

Since the beginning of July the level had dropped about 150 mm over 60 days. That’s a rate of 2.5 mm per day, and it means that more than 100,000 cubic metres a day evaporate! That is typical of the post-July 1 lake level response during the last decade.

Evaporation is driven by high Temperature (T) and low Relative Humidity (RH) conditions, and wind velocity. Sylvan Lake does not have a dedicated weather station, however hourly data logged at Hespero on Highway 11, just west of the watershed. You can retrieve and view the records on this Alberta Agriculture website. The T and RH data for each of the four 2013 months from May to August are shown in this graphic.

Regional precipitation recorded at the Red Deer Airport and other Environment Canada weather stations has been reasonably consistent in the last decade. The S-curve form of the cumulative precipitation curve through a year is presented in the linked graph. Note that about half of the annual precipitation occurs before July 1. Direct rain onto the lake surface plus runoff from the land and groundwater inflow cause the lake level to rise. After July 1, at 50% of the year, the rate of evaporation exceeds the incoming precipitation and the lake level traditionally drops.

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