Project Scope
The Walker County Office of the Texas Cooperative Extension is developing a home-based rain collection system to provide landscape irrigation. This project is being assisted by the volunteers of the Walker County Master Gardener Program. Harvested rainwater has been overlooked in recent history as an efficient method of providing water for various uses around the home and landscape. Once a common practice, the availability of economical well water and "city" water supplies has decreased the widespread utilization of "harvested" water around the home. Until the mid 1900's, it was unusual for a home not to have a cistern for collection of rainwater. Rainfall was collected and utilized for a variety of applications around the home. Obvious uses included drinking, washing, bathing, watering ornamental plants and small vegetable gardens.

Natural rainfall is a quality product that may still be utilized for a variety of uses. With proper filtering devices, it can be utilized for almost any household purpose. With the addition of suggested chemical treatment to remove any possible pathogens or pollutants, the water could even be safe to drink. Harvested rainwater is such a potential benefit for alleviating overburdened community water systems in some areas of the state and the U.S. that tax benefits have been established to promote the use of this historical water source.

OBJECTIVE:
The Central Texas area (as an example) receives an average of 32 inches of rain per year. A 2000 square foot area can capture 36,000 gallons of water, which would match up 100 gallons per day in water demand. Walker County average annual rainfall is around 45 inches per year.

The capacity of a rainwater harvesting system depends on the amount of rainfall, size of collection area, storage capacity, and the household's level of demand for water.

Since the largest need for irrigation water in our area occurs during the time of lowest rainfall and highest temperature, a rainwater system designed to meet this need will have to capture water prior to the summer.

To determine the square footage of catchment area of a house, use only the houses' footprint. The actual area of roof material will be greater due to the roof slope. However, the amount of rainfall on the roof is not affected by the slope.

For outdoor uses of rainwater, the types of plants, amount of exposure to direct summer sun, soil conditions, presence or lack of mulch, and size of the area will determine how much irrigation water is needed. Large landscapes with large water demands are not readily accommodated by rainwater catchment systems.

Storage capacity of indoor uses of rainwater can typically be more readily gauged; although this is not a precise science due to the vagaries of rainfall and personal habits.

Landscape Irrigation utilization:

The size of the storage system may be prohibitive for using rainfall for the sole source of irrigation water in large or water-intensive landscapes.

A low water demanding landscape is required. The landscape to be irrigated for this example consists of 2,500 square feet. It is determined through consultation with landscape specialists that the plants should receive a minimum of one inch of rain per week to be healthy from June through September. The roof area for collection in this example will be 1,500 square feet. Example: 2,500 square feet of landscape area requires a little over 1400 gallons of water to equal one inch of rain. In 16 weeks (June-September), the water requirement is 22,400 gallons.

Natural Rain:

 Choose if you wish to assume average rainfall or a lesser amount will fall during this period. For this example, we will estimate that only half of the average summer rainfall will occur.(June through September rainfall totals 10.79 inches. We will assume therefore only 5.25 inches will fall.) In referenced material, 5 inches of rainfall over 2,500 square feet equals 7,023 gallons. Additional material shows .25 inches of rainfall over 2,500 square feet equals 351 gallons of water. This totals up to 7,374 gallons of natural rainfall on the landscape.

Figuring the Catchment Need:

Subtract the natural rainfall (7,374) from the required amount (22,400) for the net need of the landscape. This amount equals 15,026 gallons. This is the amount of water that will need to be collected for irrigating the landscape when rainfall is half the average amount.

Collection:

The roof area will similarly receive 5.25 inches of rain which can be collected for irrigation purposes. The 1,500 square feet of roof space will collect a total of 4,425 gallons of water.

Figures 1 & 2 (below) show the expected rainfall which may be potentially caught and utilized at our demonstration location.

Catchment Area Size: 60 feet wide 120 feet length 7,200 Total (square feet) = building foot print Project catchment is 1/6 of the building 1,200.00 Total (square feet) 1/6 of building foot print. Total amount expected to be available for capture per year: Area (ft2) X Average Rainfall (inches) X 600             1200 X 45 X 600                       1000                                      =              1000       =       32,400 gallons (Figure 1)

Total amount expected to be available for capture per inch of rain Area (ft2) X 1 Inch Rainfall (inches) X 600                 1200 X 1 X 600                       1000                                      =              1000       =       720 gallons (Figure 2)

.



MATERIALS AND METHODS: A rainwater harvesting system consists of the following subsystems: catchment area (roof), conveyance system (guttering, downspouts, and piping), filtration, storage (cistern), and distribution.


Catchment Area: Rainwater harvesting can be done with any roofing material if it is for non-drinking use only. For potable use or rainwater, the best roof materials are metal, clay and cement although all roof material types have been used (except asbestos & lead). Asbestos roof materials used in older homes should not be part of a system to provide drinking water. Asphalt shingles can contribute grit to the system and need a pre-filter for the water before it enters the cistern. Lead materials in any form should not be used in the system (i.e. lead flashing).

Conveyance Subsystem: Gutters are used to convey water from the roof to pipes to the cistern. Downspouts should provide 1 square inch of downspout opening for every 100 square feet of roof area. The maximum run of gutter for one downspout is 50 feet. The conveyance piping form the gutter system to the cistern or filter should be Schedule 40 PVC or comparable in a 4 inch diameter.


Storage Subsystem: The storage tank (cistern) must be sized properly to ensure that the rainwater potential is optimized.


Filtering Subsystem: The rainwater may become contaminated by dirt, debris, and other materials from the roof surface. The best strategy is to filter and screen out the contaminants before they enter the cistern. A leaf screen over the gutter and at the top of the downspout is helpful. A primary strategy is to reject the first wash of water over the roof. The first rainfall will clean away any contaminants and is achieved by using a "roof washer". The main function of the roof washer is to isolate and reject the first water that has fallen on the roof after rain has begun and then direct the rest of the water to the cistern. Ten gallons of rainfall per thousand square feet of roof is considered an acceptable amount for washing. Roof washers are available commercially available and afford reliability, durability, and minimal maintenance to this function. Roof washing in not needed for water used for irrigation purposes. However, prefiltering to keep out debris will reduce sediment buildup. A sand filter can also be used.

Distribution Subsystem: Removing the water from the cistern can be achieved through gravity, if the cistern is sufficiently high enough, or by pumping.

The component list and expense costs of the Walker County Extension Water Harvesting project are shown in Figure 3.

RESULTS & DISCUSSION: The day after completion, we received a 1.24 inch rain. The 550 gallon tank on the water harvesting system filled up and overflowed during this first rain event. Since the initial construction phase, we have raised the height of the storage tank to allow additional gravity flow to our end water use area. Due to the low amount paid for the storage tank on this system, this project has been very economical and feasible to install.

ACKNOWLEDGMENTS: Special Thanks go to the Walker County Master Gardeners involved with the installation and maintenance of this project. Additional Thanks is noted to Jacob Bullion, Extension IPM Intern who assisted with a variety of activities toward development of this projects completion.