It’s all around us. It surrounds you as you move around. Without it we could not survive for more than a few days. In some countries it is illegal to capture while in others they have so much of it that they just want it to stop and go away. 
It is water in the form of rain and its many states of water: invisible moisture in the form of humidity, mist of fine water, casual rainfall that is beneficial to the plants and recharging water reservoirs to provide us with the essential water humans require, and then on mass—floods where it is just too much at one time and needs to stop now.
Rainwater arrives at irregular intervals throughout the year. To facilitate the capture and utilization of this free source of water (not in all parts of the country and probably not forever, either), we must devise a capture, storage and utilization system. What do you need to do to capture the rainwater? How much do you need to capture? What are you going to use the rainwater for?
The benefits of utilizing rainwater are that up to 50 per cent of the drinking water that is currently used can be safely replaced with rainwater without any loss in convenience. Rainwater, often referred to as soft water, can be efficiently harvested and used for laundry, flushing toilets, general cleaning and garden irrigation. Based on future water price projections, a complete rainwater recycling system, excluding installation costs, will pay for itself. 1
Rainwater harvesting begins from the ground up. For a successful outcome, information provided by an experienced person who is knowledgeable, up to date and has worked with similar sized projects, would be an asset. Rainwater can be used for many uses as noted above. First thing to do is decide what are you going to use the water for? The simplest form of rainwater collection and a very common use is the spot watering of plants, both indoor and outdoor, by collecting in a rain barrel, filling a watering can and walking around manually watering the plants.
A little small for your project? Don’t worry. The system can be almost any size. Only room for tanks becomes an issue but, if they are to be the larger types, they can be buried underground.
As water becomes an ever increasing issue around the world, the variety of tank systems grows as new players become involved, offering what they hope will be the winning combination of water capture tanks and system components that are utilized to create these systems. This smaller inground tank system by Bushman shows a complete automatic pumped water system that stores 2,500 US gallons (9,463 litres). Wow, tons of water right? Begs me to ask the question once again as to what are you using the water for? If this is to augment a domestic water supply that will be utilized to water a landscape, ok. It should last for watering the plants onsite through a high-efficiency subsurface, on surface and/or point source low-volume irrigation system, but not the turf areas that utilize spray sprinklers for the smaller areas and rotor sprinklers for the large areas. No way will you have enough water.
An average city residential property of 50 feet by 100 feet (15.24 metres by 30.48 metres) (5,000 square feet or 464.5 metres) with a moderate landscape with plants on either side of the front and back of the home and turf areas on the side of the driveway, front yard and the backyard, and not much else, usually requires one inch of water—less rainfall—(2.54 centimetres) per week. This is the guideline established and provided by our local government agencies. Ok, still what does all this mean? An acre, 43,560 square feet (4,046 square metres) requires 27,154 US gallons (102,789 litres) of water to cover the acre with one inch (2.5 centimetres) of water per week if it does not rain one inch. Our site is only 5,000 square feet which is only 8.7 of an acre so it should require 27,154 / 8.7 = 3,121 U.S. gallons of water per week. The tank pictured above holds 2,500 U.S. gallons. For this small city landscape, two tanks should be installed providing 5,000 gallons so that, hopefully, city water does not need to be tapped. Hold on! The one tank is not enough for one week and the two tanks will last about one and a half weeks. What happens when we go into a summer drought and do not receive any rainfall for as long as 60 days or longer? Ok, now you see why tank sizes for rainwater harvesting have been and usually are undersized to be able to perform as per the client’s wishes where city water is not to be used. You need a lot more storage. I usually won’t size a tank at less than 10,000 U.S. gallons (37,854 litres).
Does this mean that I need to size my tank to meet the weather conditions so that I have enough water onsite? Yes, you need to consider the local weather and how this might affect the amount of water to be stored—preferably underground so that the water stays in a dark, cool place so it can remain in a useable state for a longer period of time.
For larger sites, a system that has been used quite successfully is one where a large hole is dug to hold 100,000 gallons (378,541 litres) or whatever you calculated the amount of water you need to store based on the landscape’s daily consumption and the length of time between timely rainfalls that can recharge the tank. Not limited to residential, these system have already been utilized for large commercial sites, parking lots where water is stored beneath the lot, sports field water storage beneath the play surface or alongside the play area, and golf where water can be stored wherever power and pumping is located throughout the course.
If you need help with this, let me know as I have a few computer programs to accurately calculate tank size based on weather and usage. Remember, if you store enough water, this water can be used for many other non-potable uses if the plumbing code laws allow. If they don’t, then maybe we need to have the laws updated as long as all health-related water issues are appropriately dealt with.
The main problem that I am witnessing, regardless of who is designing the rainwater harvesting system, is the undersizing of the storage tank. Error on the overcapacity side if you need to error. If we are going to become independent from city-provided potable water, where we are still using this liquid gold for toilet flushing, car washing, driveway washing, laundry water, and irrigation, then we must do our math correctly. This can only be achieved if you are knowledgeable in the fine art of water use calculations to cover all of yours or your client’s non-potable water uses.
Lorne Haveruk is principal of DH Water Management, a leading irrigation consulting firmt focused on water efficient irrigation. His firm consults on visionary design utilizing rainwater, groundwater, stormwater, greywater, reuse and conventional city water supplies to create sustainable irrigation.
Visit www.dhwatermgmt.com or email lorne1@dhwatermgmt.com for more information.
References
Granita, Above and below ground rainwater harvesting systems. www.h2opic.com
Bushman, www.bushmanusa.com
Atlantis Rainwater Harvesting, www.atlantis-america.com.
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