EFFECT OF CANOPY COVER ON THE VOLUME OF RAIN THROUGHFALL

Stormwater runoff has become increasingly difficult for municipalities to manage as areas experience development. This increase in residential and commercial development has led to a substantial reduction in the natural canopy cover, and post-development plantings have not begun to replace the loss. This study was carried out to establish the extent to which post-development rain throughfall, and resulting runoff, could be reduced by leaving canopy cover in place. The study compared the volume and intensity of rain that reached the ground in an open area (no canopy cover) versus two areas with intact canopy covers. Rain gauges were placed in a parking lot, and in wooded areas in an environmentally-friendly development in Shelby County, Alabama. Rain was measured for a period of twelve consecutive months and rain throughfall was compared between the sites by season (Spring/Summer vs. Fall/Winter) and by rainfall depth. The sites were sufficiently close to each other to assume that the rainfall characteristics were the same between the sites in terms of the intensity and the variation of intensity and volume during the storm. Investigation of the relationship between the amount of throughfall on an area without canopy cover and with canopy showed that, for these sites, even in an area with high rainfall intensities, canopy cover could be expected to reduce the total throughfall by approximately 13.5% during spring and summer months. No significant results were seen for intensity but this evaluation was limited by the study techniques. The results of this project indicated that in areas where reducing stormwater runoff is desirable, there is a significant, measurable advantage to preserving the natural canopy on a site wherever possible. The reduction of the rainfall amount reaching the ground reduces the amount of runoff that has to be managed by the developer and the property owner. It also could be expected to reduce the amount of erosion being generated from a site.