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The sum of the blue <br />and green squares <br />(streets, parking <br />lots, etc.) is 25 to 28 <br />acres. <br />The lower number <br />assumes that the <br />five green <br />highlighted squares <br />do not drain directly <br />into the lake. <br />The best fit <br />hydrology model <br />implies impervious <br />of 25 acres so <br />agrees to this <br />smaller estimate. <br />Estimating impervious <br />acreage: