Raingardens—Recreating the Small Water Cycle
by Janet Richards
Here on Whidbey Island, the native landscape is mostly forest with some naturalized prairies or meadows. When it rains in the forest, rain filters through branches, drips from bushes and runs down trunks, slowly making its way to the forest floor, which is covered with all kinds of organic material—leaf litter, fir needles and dead wood. Rain soaks into the soil slowly, where micro-organisms filter and purify the water as it works its way to underground aquifers.
Some of the rainwater is used by plants. Plants transpire, meaning they sweat to keep themselves cool as they photosynthesize, and this creates a humid local environment. Sometimes forests transpire so much, they create their own rain. Ecologists call this the small water cycle. It can have a large impact on the local ecosystem. You may have noticed that during a heat wave, it’s much cooler among the trees and planted areas. This is not just because of the shade. The air is more humid and cools the earth.
When we urbanize our environment, we take away the surfaces that can absorb water. The rain falls onto roofs, driveways, roads and degraded or compacted fields, where it can’t be absorbed and creates runoff. This runoff is full of mud, debris, and contaminants from roof shingles, asphalt pavement and agricultural chemicals. Because of the amount of urbanization around Puget Sound, the large amount of runoff changes the water quality of the sea.
That’s where raingardens come in. Raingardens are places to direct runoff and allow water to collect, planted so that rainwater soaks slowly into the soil, is purified, and refills the water table, thereby mimicking the small water cycle of a forest or native meadow. The perfect places to create raingardens are where there is runoff from buildings or paved areas, or perhaps at the foot of a slope with degraded soil.
A raingarden is an indentation in the ground, like a bowl, though it can have many shapes: oblong, kidney bean, s-curve or long and narrow. Water is directed into the indentation using pipes or swales.
A swale is designed to slow water down as it moves downhill. Swales are long and ditch-like, often filled with rock, and sometimes planted, so, as runoff slows, some water can soak in and be used by plants.
Raingardens, on the other hand, are designed to collect water. The water should stop at the rain garden, where it can be absorbed. Because standing water is undesirable, raingarden design includes an outlet in cases where there is too much water for the raingarden to handle.
Water flows to the deepest area in the center of the raingarden, which is planted with plants that don’t mind their roots being wet all the time. The next zone up, a ring around that central space, is planted with species that can tolerate wet and dry. Finally, the outer edge is planted with plants that like a normal amount of rainfall.
When we create a raingarden, we create beauty, and habitat for insects, birds, small mammals and soil life. Creating healthy soil, soil able to absorb water that would otherwise run off, helps restore the natural water cycle, keeps toxins out of our sea and refills our aquifers with good water.
Resources: