Rain, Rain, Go Away—But Where?

Suppose it rains one inch overnight. How much water does that produce, and where does it go? One inch (2.54 cm) of rainfall on a 1,600 square foot piece of land (40x40 feet, or 148.5 m²) equals about 1,000 gallons (3,785 liters)!

Rainwater that soaks into that ground may be used by plant roots, and whatever is left over percolates slowly down to the water table, below which the ground (whether soil, sediments, or bedrock) is completely saturated. Such groundwater, at a steady temperature of 55o F (13o C) in some regions, will discharge to a stream very gradually, over a period of years. This keeps some streams flowing during dry-weather periods (and helps regulate their temperature). Groundwater also seeps out of land surfaces elsewhere, supporting natural communities such as the Red Maple Seepage Swamp, and providing them with minerals dissolved out of the rocks underground.

Back to that 1,600 square feet of land: now imagine it covered by asphalt and curbs, or other impermeable surfaces that rain can’t soak through. That’s about a 50-foot stretch of a typical residential street (32 feet wide), or the amount of space that a house and driveway might occupy. With one inch of rain, that same 1,000 gallons of water will gush into storm drains and rush away to the nearest stream. The difference? That water creates erosion, pollution, and flash flooding during its hasty exit, and won’t be available to streams or upland natural communities during dry periods.

Impermeable surfaces can negatively impact the health of an entire watershed, if people do not take steps to reduce the stormwater flow off of them.