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Rain’s Effect <br />A single day’s rain increases lake levels by: <br />1.The inches of rain that fall directly on the lake (this chart for when the lake is 19.5 acres) <br />2.The rain that runs through the storm sewer system into the lake from 25 acres of impermeable surfaces <br />3.Run-off from other surfaces in the watershed. For a daily rain in isolation, the first 2.25 inches of rain will be absorbed, then start to run off into the lake. For multiple days of rain, the wetter soil causes a single day’s run-off to begin sooner. <br />=> It would take an impervious area of under 5 acres to <br />achieve a stable lake level. Typical factors are: <br />•annual evaporation of 32 inches <br />•annual rain of 26 inches (assume daily rain < 2.25 inches, so <br />no run-off) <br />•the beginning and ending lake level would be the same if the <br />impervious area is about 5 acres <br />•Since Karth Lake has 25 impervious acres, pumping is <br />necessary to prevent long term rises in lake levels.