By: John W. Bartok Jr.
Rainwater collection systems trace their roots to both Greek and Roman civilizations. In 2014, why consider this system?
As supply dwindles, water harvesting is becoming an increasingly necessary option. Here’s what you need to know.
- Water quantity is becoming limited in some sections of the country. Rainfall collection can help to supplement existing sources.
- Rainwater is generally soft with very few chemicals. It is clean except for any debris that gets into the system. In some parts of the country the water is slightly acidic and may have to be treated.
- Significant amounts of water can be collected. A 1” rainfall on 1 acre amounts to 27,100 gallons. At this rate, a 30’ x 100’ greenhouse will collect about 1200 gallons. A common yield is about 65 percent with losses due to evaporation, wind, leakage of the piping system and diversion of the first few minutes of the rainfall to remove debris. To calculate the amount of rainfall, multiply the footprint of the greenhouse by 0.4 to get a quantity in gallons.
In gutter-connected greenhouses, it is fairly easy to divert the water collected in the gutters to a storage tank. Large greenhouse operations usually store water in ponds. Even with hoophouses, water can be captured in gutters attached to the side rail at the top of the roll-up sidewall.
Besides the collection gutter, a rainwater system consists of PVC piping to direct water to a roof washer where trash, leaves and other debris are removed by a screen mesh. Dust, bugs, bird droppings and other small debris that gets through the screen settles in a small tank and is drained away. Roof washers are commercially available or can be homemade.
Fiberglass, polyethylene and corrugated steel tanks used for storage are available as above- ground or buried tanks. Cost is usually between $0.75 to $1.50/gallon depending on the style and size of the tank and how far it has to be shipped. A typical fiberglass 10,000 gallon tank that is 12 feet in diameter and 12 feet high will cost about $18,000. Tanks usually come with threaded fittings for pipe connections and large manholes for cleanout.
Water silos are corrugated steel tanks that can be assembled on site to provide a large water holding capacity. A chemical resistant liner is installed to contain the water. As they are formed in sections from sheet steel, they are easy to ship and bolt together. Tanks that are located inside a greenhouse or headhouse allow the water to be warmed before it is used for irrigation.
Growers with several acres of greenhouses usually build a pond to retain rainwater. These can be made to any size but usually require a design by a licensed engineer and a permit from the local wetland agency. If the soil is porous, a vinyl liner is placed on the bottom of the pond to prevent seepage. The disadvantage to a pond is that the water may become contaminated from algae or other plant growth.
All water storages need an overflow to handle excess water once the tank is full. This is usually an overflow pipe that is one size larger than the inlet pipe. The water needs to be directed to a wetland or drainage area where it doesn’t flood neighboring property.
Once the water is collected, it can be distributed to the greenhouses with the normal irrigation system.
As good water supplies become more difficult to find, water harvesting may become a necessary option for many growers. Greenhouses provide the collection area for large quantities at a low cost.