Rainwater Harvesting by Wayne Bowman, Master Gardener.
The collection of rainwater, or rainwater harvesting as it’s termed nowadays, has been around for centuries. Archeological digs have documented the building of cisterns for more than 10,000 years. All water is rainwater. Rain falls from the clouds and runs into creeks and rivers, where municipalities use the water for drinking or sanitary purposes and then return it to streams. Streams flow into lakes and oceans where it evaporates and again forms clouds and rain to complete the cycle. I’ve read claims that our planet only has a certain amount of water, it just gets continually recycled.
The practice of rainwater harvesting is encouraged here in Texas. Laws enacted since the early 2000’s cover subjects such as limiting an HOA’s restrictions on harvesting and tax incentives for harvesting installations. More details can be found at Texas Water Development Board’s website: http://www.twdb.texas.gov/innovativewater/rainwater/faq.asp#title-06.
As rainwater falls through the atmosphere, it picks up dissolved oxygen and nitrogen. A good soaking by rainfall is much better than watering from a hose. Energy, delivered through atmospheric lightening, splits stable nitrogen atoms into nitrites. Plants take up these nitrites as nitrogen, essential for their growth. Rainwater is also free of the kinds of chemicals typically added to tap water, and lacks the salinity and minerals found in groundwater.
There are four parts to every rainwater harvesting system: catchment, conveyance, storage and distribution.
Catchment is the term used for the collection area. Roofs are the most commonly utilized catchment. Metal or clay tile roofs are superior, but asphalt shingled roofs work fine as well. Patios, driveways or any surface from which you can direct rainfall may work as catchment.
The conveyance moves water from the catchment to the storage area. When planning the conveyance, take into consideration the dirt, leaves, etc. that are flushed off the catchment with each rainfall. A diverter can take the first few gallons of contaminated water to a holding area to be discarded later. Commercially designed filters installed on downspouts can perform the same duty. This link provides an illustrated design of a first flush diverter: https://rainwaterharvesting.tamu.edu/pre-storage-treatment/
Rainwater storage options range from small, simple barrels to tanks holding thousands of gallons. Food grade barrels can be used for a smaller installation, as long as an overflow is provided to direct water away from the house when a barrel is full. Keep in mind that even a small amount of rain can overflow a single barrel, as one inch of rain on 100 square feet of roof surface can equal over 60 gallons of harvested water.
Harvested rainwater can be distributed by pump, hose, or watering can. Many gardeners avoid pumps and pressure tanks, as they add greatly to the cost and complication of a system. Gravity is free, and a spigot at the bottom of the barrel or storage tank will cause the water weight to provide plenty enough pressure. Raising the storage even a foot or two will increase the pressure at the outlet.
Don’t forget that storage barrels contain still water, the perfect environment for mosquitos. Control solutions of bacillus thuringiensis israelensis (BTI) are sold commercially. Look for the brands “Dunks” or “Mosquito Bits.” These provide effective mosquito control in rain barrels, puddles, ponds, etc., and are safe to use where you have aquatic animals.
With a little bit of creativity and probably a bit of sweat, a rainwater harvesting system can provide ideally suited water for your garden and plantings. During our dry spells here in Texas, even when your city restricts use, you can have water.