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Runoff Report 1999..... Mild Summer

Mild summer simply means the opposite of mild winter. Temperatures are relatively low, warm periods are short, rain and rain showers are moderate. Mild summers lengthen the runoff time span as explained earlier in this report. The result is a lower and later peak of the lake, therefore reducing the potential of flooding. A mild summer effects the second and third stage of the runoff.

The summer of 1999 was mild in every aspect. There were only a few warm and sunny periods, which lasted not more than 3 or 4 days. The only exceptions were a 12-day period in mid September with temperatures not exceeding 25 C and a 12-day period in mid August. The last was interrupted two times by rain showers but maintained daily highs of about 30 C. For the rest of the summer day temperatures constantly went up and down. The weather charts in the earlier section show this effect very clearly.

The mild summer had a large impact on the 1999 runoff. In general, the low mean temperature lengthened the runoff time span and prevented the lake to reach a fast and even higher peak. Especially during the second runoff stage, warm temperatures did not last more than 2 consequent days before cooling down considerable. Night temperatures fell as low as 7 C. With the weather the water mass caused by the snowmelt also changed periodically. Short warm periods increased the flow of snow water. By the time this water reached the lake, temperatures were low again and reduced the supply for the lake inflow. The level increase slowed down, two times even causing the lake to fall temporarily before its peak.

The following graph compares the 1999 daily lake level changes with previous years.

This graph clearly shows this effect on the daily lake levels. A summer like 1997 definitely would have caused massive flooding for 1999.

The next section links the effect of the mild summer to the runoff stages.

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Copyright (C) 2003 Bernhard Kramer, Sicamous, BC - Canada