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Runoff Report 1999..... The Rain Factor

Heavy rain showers or long rain periods are adding additional water to the runoff. This can lead to temporary overloads of rivers and streams as well as to increased inflow into the lake. Rain also effects the snowpack itself. A faster meltdown or washout of the snowpack produces an increased water mass besides the rainwater itself. The time span gets shorter.

Rain during the runoff always carries a very high flood potential. Rain saturates the soil and increases the subsurface flow, quickly reaching the limits. Additional rain and snow water is forced to a surface flow, getting much faster into the watershed. The watershed can get overloaded in a very short time leading to spilling and flooding. High stream levels also mean that a maximum amount of water is transported into the lake. The lake inflow / outflow balance shifts significantly causing a much faster lake level increase.

The remaining snowpack during a rain period is capable of storing a certain amount of rainwater. The water equivalent and consistency of the snowpack is altered. The total water potential is increased and the snow melting process will be faster for some time. This effect doesn't shorten the time span, it just produces more water at lower temperatures.

The 1999 runoff had 66 days of rain or rain showers (see table at the top of this report). Luckily, rain showers were short and relatively mild. Rain periods also were mild and didn't produce much additional water. The effect of the rain on lake and streams was minimal. Day and night temperatures remain low during the rain periods, which temporarily slowed down or even interrupted the snow melting process. Rain periods were always accompanied by a reduced lake inflow. The remaining snowpack was limited to higher altitudes, which would have required higher temperatures to continue its melting process.

Normal rain periods carry the highest flood potential early in the runoff, where rain water adds to the continuing melting process in lower altitudes. Basically, the higher the altitude of the remaining snowpack, the lower the effects of moderate rain. Low temperatures and mostly dry weather during the first runoff stage prevented major problems in this respect.

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