By Cynthia Houng
In November, after 8 months or so of dry weather, California's storm season begins. These winter rains provide the bulk of California's water supply. Though the state tries to capture and retain as much of the winter rainfall for the coming year, we lose a good percentage of our rainwater as runoff.
There are 2 reasons why California's winter rains, and especially the first heavy rains, have trouble making it into our reservoirs and water tables: (1) California's prolonged dry season, which creates dry, compacted soils that struggle to absorb rainfall; (2) the state's increasing urbanization, and the hard surfaces that accompany urban sprawl, do a poor job of absorbing and holding onto rainwater. As a result, the rainfall does not recharge the water table. Instead, the rainwater heads straight for the ocean.
With California's increasing urbanization, runoff has become a major environmental problem. The Environmental Protection Agency ranks runoff among the top 3 threats to American watersheds. Runoff collects chemicals, heavy metals, and other pollutants, and delivers it to vulnerable aquatic ecosystems. It also enters the watershed with incredible speed and force, literally scouring away sandbars, wetlands, and estuaries.
Hard surfaces, such as roofs, roads, driveways, parking lots (and even lawns) encourage rainwater to run quickly, and in large volumes, into storm drains and out to sea. Unlike meadows, marshes, or woodlands, whose soft surfaces soak up the rainwater and slow it down, these hard, man-made surfaces amplify the water's speed and force. Runoff also causes erosion, washing away valuable topsoil, and in some cases, runoff can trigger mudslides or landslides.
Rain gardens allow gardeners to recreate the natural hydrological cycle on a small scale. These gardens, which are designed to help slow down runoff and sponge up rainwater, can also help improve local water quality, filter out pollutants, decrease erosion, and even help replenish the local water table.