Where Does the Rain Go? A Green Roof Study

The Roof Experiment

1Eastview Middle School sits in the middle of a busy city. When heavy rain falls, water rushes off roofs, streets and parking lots into storm drains, which empty into the Millbrook River a few blocks away. After big storms, some of the city's drains overflow and flood the streets. Last year, the school turned one part of its roof into a green roof: a layer of soil 10 cm deep, planted with low, tough plants. The school's environmental team, led by science teacher Ms. Greta Halvorsen, wanted to find out what the green roof does with rain.

2The team compared two roof sections of the same size, 100 m² each. One is the plain roof, covered with gravel. The other is the green roof. A pipe from each section drains into its own tank, where students can read how many liters have collected. A rain gauge on the roof measures the rainfall. Team member Ines Carvalho shared a useful shortcut: 1 mm of rain falling on 1 m² of roof adds up to 1 liter of water.

3On May 12, a storm dropped 25 mm of rain in about two hours. The team recorded the total in each tank and noted how long after the rain began water first flowed into it. By the next afternoon, the shallow puddles left on the plain roof were gone.

4No more rain fell for the next four days, and no more water flowed into either tank. Each afternoon, a sensor buried in the green roof estimated how much water the soil still held, and the students wrote down the weather. On Day 1, Kwame Asante tied clear plastic bags over a few plant shoots. Within an hour, tiny droplets had formed on the inside of the bags.

5Water from the school's tanks is later released into the storm drains, so it reaches the Millbrook River, which flows to the ocean. Every day, sunlight warms the surfaces of oceans, rivers, soil and plants, and gravity pulls on everything, water included.