Annual inflow and range of levels

Until the diversion from the Pembina River was restored in 1991 and the outlet was improved in 1992 the lake level was based on the inflow and evaporation. The lake rarely got high enough to cause the outlet to run but since 1995 the outlet has been used frequently.

The chart shows the inflow volume record since 1962. An acre foot is equivalent to one acre covered by a foot of water. It takes about 7,700 acre feet to raise Pelican Lake (and Grassie Lake)  one foot. So in a year like 2011 the inflow was equivalent to 12 feet on the lake.

It's not hard to see that the last 30 years have been very unusual compared to the years before that. Interesting to note that for example 2013 and 2022 would have been record years before 1995 but with the outlet they were uneventful years on the lake.

Net evaporation on the lake is typically about 10 to 12 inches but the inflow before 1995 was typically much less, some years had  near zero inflow. Between 1962 and 2010 (excluding 1999 and 2005) the average net drop from the peak to the lowest level was 1.15 feet with the least net evaporation of 0.43 feet in 1993 and the most, 1.84 feet in 1988.    

Three of the four largest runoff years; 1999, 2005 and 2011 were primarily rainfall events. In 1999 and 2005 the operations had ceased when rain required that the gates be opened wide and the outlet was run to capacity. 

In 2011 the rains were an extension of the high spring runoff. In May, 1999 201 mm fell, another 200 mm fell in the next three months. In 2005, 246 mm fell in June and July most of it in one storm at the end of June. In 2011, 320 mm fell in May, June and July. May, 1999 was the wettest since 1940 and June 2005 was the wettest June on record. Whereas 2011, although unusually wet, was well below the record amounts.  Another significant rainfall event occurred in 2010 with 152 mm in May and 114 mm in June, requiring the equivalent of more than two feet to be released from the lake ending in the middle of July. In the 37 years before 1999 rainfall did not cause one noteworthy problem on the lake and in the 24 years since there have been four major events and four minor ones. 

Using data for annual inflow since 1962 the probability of a given inflow is as show in the following table. The 2011 flood has the largest inflow at about 93,000 acre feet. This is expected to be equaled or exceeded about once in 160 years. In any given year there is about a 20% probability that there will be enough inflow to raise the lake two feet.

The chart below shows the range of levels on Pelican Lake since 1962.