Rainwater harvesting is going on all around us. Rain drops collect on leaves and are channeled to the roots of plants. Fresh water that settles in a depression of the landscape supports wildlife, and slowly infiltrates the soil until it reaches an impermeable layer of rock. Eventually groundwater makes its way back to the surface through seeps, springs, and the capillary action of soil, then is transpired through leaves and returned to the atmosphere.
This Earth's water cycle has sustained terrestrial life for millions of years. The introduction of sidewalks, roads, buildings, and impervious surfaces now convey rainwater to our combined sewer system, and after costly treatment, discharge it into the Pacific Ocean. Runoff falling on these impermeable surfaces is often warmed and may be contaminated. If allowed to enter the watersheds, warm water lowers the oxygen level and harms aquatic organisms. If we don't recharge our groundwater supplies, our local water table becomes lower, and forests and their ecology, which depend on the natural cycle of water, will suffer.
Water that falls on rooftops, sidewalks, and roadways can be channeled into water tanks, to be used for irrigation during our dry months, or into rain gardens (planted depressions in the landscape) for passive groundwater infiltration. SF State's "From Sky to Soil" Project provides two examples for our community of improved rainwater management.
Our flagship rainwater harvesting system collects 12,000 gallons of rainwater each year from the roof of the Recycling Center and can be observed from the northeast side of the parking garage. We are utilizing an adjacent 630 watt solar array to power a pump to irrigate over 6,000 square feet of native plants until they become established. We will then redirect the water to other new landscaping projects.
Our first rain garden installation collects about 60,000 gallons each year from the adjacent Corporation Yard warehouse roof which can be viewed from the northwest side of the garage, or up close from North St State or St. Drive. This rain garden was planted with native plants that are both drought tolerant, and can handle short periods of inundation of water in the winter.
These methods reduce, filter, and slow stormwater runoff, recharging the groundwater and lessening the burden on our sewer system. By mimicking natural watershed processes, we are helping to conserve a precious resource and mitigate some of the ecological damage done by our urbanized society.
These two projects were funded by the San Francisco Public Utilities Commission, and were installed by the SF State Grounds staff with help from a group of volunteers from the American Society of Civil Engineers.
The Science Building Bioswale replaces 1600 sq. ft. of resource-intensive lawn, that now captures 60,000 gallons of the rainwater per year that falls on the Science Building roof and channels it into a vegetated swale (aka a "rain garden"). A varieity of locally native plants now attracts birds, bees, and butterflies.
What is a bioswale?
A bioswale is a wide, shallow depression in the landscape that allows for the temporary retention and slow infiltration of fresh water, a critical resource for terrestrial ecosystems. Plants and microorganisms in an established bioswale trap pollutants, slow the flow of water and allow it to percolate into the soil. This recharges the groundwater and reduces the burden on stormwater treatment facilities.