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Rain gardens are as old as agriculture in arid lands. For thousands of years, humans have harvested the water from rains too heavy to fully soak into the ground.
All over the world civilizations owe their foundations to rain gardens used to manage water scarcity in lands of droughts and floods. At Machu Picchu in Peru, the Incas carved mountains into terraced rain gardens. At Chaco Canyon in New Mexico, the Ansazi harvested rainwater off the surrounding mesas, the rooftops of the canyons, onto waffle rain gardens on the canyon floor.
This ancient idea has had a modern resurgence. Rain garden websites now flourish on the Internet. A water-harvesting manual can be found online at the Texas Water Development Board website, www.twdb.state.tx.us. TWDB projects the population of Texas will double by 2050, creating concerns about sufficient water supplies, even with conservation efforts and water development projects. On its website, the Environmental Protection Agency also promotes rain gardens to mitigate pollution.
How rain gardens work
A rain garden or catchment is a depression or basin on the surface of the soil where rainwater is collected from the surrounding landscape and can be used by plants in the basin. A depression may be man-made; however, many occur naturally. A man-made rain garden should be placed to optimize the capture of water from the immediate drainage area. Ditches or swales also may be used to divert water from other drainage areas. Water from a rooftop may be channeled through gutters and pipes to the rain garden. Other ways to harvest water include small dams or berms on the low side of sloping ground, crescent-shaped mounds around trees or plantings, and terraces built with rocks or soil to catch the flow of water running over the surface after a good rain.
Rain gardens are right for public and private landscapes. They may be built in back yards, front yards or even side yards and can be of any size. Discovery of microcatchments where the smallest of native plants flourish are a delightful surprise. So, too, is the wildlife these plants attract. Whole yards can be converted in this manner and may then be certified as “Wildlife Habitat” by the National Wildlife Foundation. The Texas Parks and Wildlife Department has a similar program. Austin is proud to be one of America’s first “Wildlife Habitat Communities.”
Rain gardens address the concerns of the TWDB for meeting Texas’ future water needs. They make use of rainwater that naturally runs off a site, an alternative to using expensive potable water for landscape irrigation. In addition, if runoff is not slowed or stopped, detained on the landscape and retained in the soil, it might cause problems.
In a natural landscape, plant surfaces capture or intercept runoff and aid its movement into the soil where it may flow underground. As natural landscapes shift from vegetated to urban, there is a loss of these plant surfaces. There also is reduced evaporation and transpiration by plants, or evapotranspiration, as a result of the decrease in plant material or biomass. Impervious surfaces in suburban and urban areas, such as roofs, streets, parking lots, sidewalks and patios do not let rainwater soak into the ground. Instead, these hardscapes increase the amount of runoff that later collects to cause flooding and increased transport of pollutants, matters of concern for the EPA and the Texas Commission on Environmental Quality. This is in contrast to the normal flooding and pollutant transport in a vegetated landscape that is serviced by nature’s processing and filtering.
Fertilizers, pesticides, pet wastes, oil and gasoline, paint products, eroding soils, effluent from failing septic systems, overflowing combined storm and sanitary sewer systems and atmospheric mercury from coal-fired power plants are all washed down with rainwater into the nearest drain, then down to rivers, lakes and oceans, thus impacting water quality. Texas state law defines the four general categories for water use as aquatic life use, contact recreation use, public water supply use and fish consumption use. Bacteria from pet and human waste may impair water bodies for contact recreation — or swimming and other sports having direct contact with the water. Many of these bodies of water used in contact recreation also have low dissolved oxygen rates because of algae growth. Fertilizers and other nutrients promote algal bloom, which uses up oxygen and results in unhealthy environments for aquatic life. Fish contamination originates from the use of pesticides in the landscape and atmospheric deposition of mercury.
The illustration at right depicts some of the services of plants. Plants trap and filter pollutants, thereby cleaning the air, while photosynthesis sequesters carbon and oxygenates the air. Evapotranspiration cools the air. Leaves and stems soften the impact of raindrops in throughfall, lessening soil erosion, while stems and roots carry water along their surfaces into the ground, increasing moisture available in the soil. Vegetated ground helps rain infiltrate into the soil and percolate farther down. Rainwater not used by plants remains stored for later use during dry spells or may travel into aquifers, then bubble up in springs and rivers.
The depression storage of rain gardens traps pollution and sediment, mitigates flooding, cleans water for recharge, promotes infiltration and nurtures plantings. Living plant material or biomass aids the environment in many ways; however, these ecosystem services are often discounted, if acknowledged at all.
“Green infrastructure,” including rain gardens, employs the beneficial use of plants or porous materials to mitigate flooding and pollution and to improve air quality. San Antonio hosted a green infrastructure workshop in February 2009. Some incentives discussed for using green infrastructure were development incentives, rebates and installation financing and awards and recognition programs. Green infrastructure is also known as Low-Impact Development because it lessens the impact of urban development.
Houston has hosted a LID design competition. Additionally, the “Cool Houston Plan” intends to take advantage of the ecosystem services plants provide by filtering and cooling the air through evapotranspiration. The plan is for homeowners and other landowners to plant 10 million new trees in 10 years. Together with the conservation of existing trees, Houston aims to lower air temperatures, provide shade to cool buildings and save energy and improve air quality, storm-water management and quality of life.
Rain gardens benefit the missions of both the TWDB and the TCEQ. The TWDB values the resulting water savings while the TCEQ recognizes the positive impact on water and air quality. It is believed, though, that a rain garden’s greatest benefit may lie in its ability to increase the available soil moisture for plants. The wetter niches created from catchments provide opportunities for native plants during times of water shortages. Even with conservation efforts and water development projects, water resources will still be scarce. Rain gardens may provide a refuge for native plants as the increased demands of population growth collide with water supply limits.
While landscape needs in many areas of the state will continue to suffer from greater water use because of increasing population, rain gardens create critical refuge, especially in more arid environments. In the less humid parts of Texas, this ancient ingenuity born of the necessity of prehistoric droughts is particularly important. By increasing available soil moisture and sliding microclimates up the scale to slightly wetter ones, these refugia could ensure our environmental legacy.
The most important role of rain gardens may be to save native plants in living seed banks for generations to come. While TWDB is focused on water supply and TCEQ’s mission relates to water quality, there is also a concern for our Texan heritage of native biodiversity and genetic resources. Through implementation by Native Plant Society members, rain gardens may help to keep Texas looking like Texas.
Selected references
http://www.twdb.state.tx.us/innovativewater/rainwater/docs.asp — Texas Manual on Rainwater Harvesting, Third Edition. Texas Water Development Board, 2005
www.growgreen.org — City of Austin Landscaping for Clean Water
www.nwf.org/backyard/ — Garden for Wildlife: Create a Certified Wildlife Habitat. National Wildlife Federation, 2009
www.tpwd.state.tx.us/huntwild/wild/wildlife_diversity/wildscapes/ — Texas Wildscapes
www.sustainablesites.org/ — The Sustainable Sites Initiative, ASLA, 2008
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**ARCHIVED POST AUTHOR: dryden
