Water is a precious resource. Water is essential for life. As Texas continues to grow, demand for water increases as well. Water demand in Texas is projected to increase by 22 percent between 2010 and 2060.
More than 70 percent of the Earth is made up of water. According to the Texas Water Resources Institute, the oceans make up 97 percent of the Earth’s water in the form of saltwater. The remaining 3 percent is in the form of fresh water. Of this 3 percent, less than 1 percent of the earth’s water is water either in aquifers, wells, rivers, and lakes used by humans.
Rainwater harvesting is one approach to capture and store water for future water needs. Rainwater harvesting captures, diverts, and stores rainwater for in-home use, for the landscape, and for wildlife or livestock use. Rainwater, with proper treatment, can even be used for drinking water.
Rainfall varies throughout Texas. A simple water harvesting system usually consists of a catchment, a distribution system, and a landscape holding area. In this system, gravity moves the water from the catchment to different locations in the landscape. Water in this system may be caught in small containers and stored for later use.
Complex water harvesting systems exist as well. These may cost more to build but they yield greater water savings. Complex water harvesting systems include catchments, conveyance systems, storage, and distribution systems. The amount of water that a catchment provides depends on the size and surface texture. In a complex system, filtration may be used to remove particles and debris from the water before it is stored.
Containers used to catch rainwater may be made of polyethylene, fiberglass, wood, concrete, or even metal. The costs of rainwater harvest containers vary according to the material from which it is built and the size of the container. Many today can be built to fit and blend into the landscape.
How much water can I expect to catch? Supply can be figured by using the following equation. Supply in gallons equals inches of rainfall multiplied by 0.623 multiplied by catchment area in square feet multiplied by the runoff coefficient.
Multiply rainfall in inches by 0.623 to convert inches to gallons per square feet. Multiply the result by the area of catchment in square feet. For example, a 10-foot-by-20-foot roof is 200 square feet. For a sloped roof, measure the area covered by the entire roof which is usually length times the width of the building. Multiply the results by the runoff coefficient to obtain the available supply.
The runoff coefficient is the percentage of the total rainfall that can be harvested from a particular surface. The runoff coefficient varies from the type of surface material the roof is built out of. A chart showing various runoff coefficients is listed in Appendix II in the Extension publication titled Rainwater Harvesting, B-6153 from the Texas A&M AgriLife Bookstore, at www.agrilifebookstore.org/default.asp .
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