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Features Agronomy
Cisterns for irrigation purposes


February 9, 2015
By Lorne Haveruk

Topics

“What storage systems are available? How do I size a storage system?”

Good question, and I am afraid not a short answer. I have spent many hours trying to come up with a generic formula that can be applied to a cad design area calculation. This does work, but it only takes into consideration one of the many parameters that come together to create the picture of a realistically-sized water cistern or other water-holding device for irrigation.

Right from the start I would need to know what the landscape architect’s desired outcome for the project is in regards to watering the plant material. Is the system to provide full coverage of all plantings? This will certainly increase the size of devices used to capture and hold water for future use. Or is the overall goal to provide high-efficiency subsurface irrigation for the intensive green roof(s) or passive irrigation where onsite stormwater drainage is designed to redirect water to ground level plantings?

Is the street tree to be watered—which, by the way, is now law in some cities? And, if they are to be watered, how? City water, even though allowed and available, will not score well for a LEED project if your system is for a LEED project. Plus, what space is available for the tank installation? In ground or above ground?  

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LEED (Leadership in Energy & Environmental Design) strives to not use irrigation at all. See the following credit explanations for scoring points striving to attain a level of LEED certification for a project.

WE Credit 1.1: Water Efficient Landscaping:  Reduce by 50 per cent = 1 Point

Intent: Limit or eliminate the use of potable water, or other natural surface or subsurface water resources available on or near the project site, for landscape irrigation.

Requirements: Reduce potable water consumption for irrigation by 50 per cent from a calculated mid-summer baseline case. Reductions shall be attributed to any combination of the following items:
• Plant species factor
• Irrigation efficiency
• Use of captured rainwater
• Use of recycled wastewater
• Use of water treated and conveyed by a public agency specifically for non-potable uses

Potential technologies and strategies: Perform a soil-climate analysis to determine appropriate plant material and design the landscape with native or adapted plants to reduce or eliminate irrigation requirements. Where irrigation is required, use high-efficiency equipment and/or climate-based controllers.

WE Credit 1.2: Water Efficient Landscaping: No Potable Water Use or No Irrigation: 1 Point in addition to WE Credit 1.1
Intent: Eliminate the use of potable water, or other natural surface or subsurface water resources available on or near the project site, for landscape irrigation.

Requirements: Achieve WE Credit 1.1. and: Use only captured rainwater, recycled wastewater, recycled greywater, or water treated and conveyed by a public agency specifically for non-potable uses for irrigation. OR, install landscaping that does not require permanent irrigation systems. Temporary irrigation systems used for plant establishment are allowed only if removed within one year of installation.

Potential technologies and strategies: Perform a soil/climate analysis to determine appropriate landscape types and design the landscape with indigenous plants to reduce or eliminate irrigation requirements. Consider using stormwater, greywater, and/or condensate water for irrigation.
Two cistern systems

There are a variety of systems available to choose from when considering the water storage container. You can use a new septic tank to capture the water or you can form your own out of concrete. An easier way is to utilize a system that is straightforward to work with which is similar to a Lego set known as the Atlantis System.

Bushman premade tank systems are precast tanks, ready for use. Be sure to check around to see what might be available in your area. Locally sourced saves money while scoring additional LEED points if yours is a LEED project.

Atlantis rainwater harvesting tank system

With the Atlantis System, all that is required is a pre-dug hole, liner, and assembly of the snap-together panels, plus items listed in the checklist.

Rain tank assembly

Rain tanks arrive on your project in flat panels which need to be assembled on-site. Building the units should take roughly three minutes per segment. Connect four small panels into one large panel using the short pegs. Place the panels on each end of the large panel, and two more into the holes closest to the centre. Do NOT use the row of pinholes directly in the centre or the two middle rows nearest the edges. Next, working from one end to the other, attach a second large plate on the opposite side of the first. Once the top and bottom large plates are attached, two more side plates are attached to complete the sides of the rain tank unit.

Rain tank pre-construction checklist

If you’re building rain tanks from panels:
√ Dead-blow mallet
√ Work table
√ Duct tape
√ Band clamps to fit inlet and outlet pipes
√ Razor knife
√ String line and marking paint
√ Reciprocating saw (if inspection or maintenance ports are required)

Materials You’ll Need

√ Rain tank units√ Geotextile
√ Geogrid (only for load-bearing applications)
√ 95 per cent compactable backfill material (NOT clay)

Equipment You’ll Need

√ Powered mechanical compactor

√ Small skid steer with tracks

√ Small vibratory roller

Note: This list does not include equipment or tools needed to excavate or level the floor of the excavation.

Bushman tank system

Bushman tank systems come in a variety of shapes and sizes and are installed above or below ground, depending upon the model and its intended use.

Rainwater harvesting is now viewed as realistic and a green thing to promote and do where possible. Sizing of the holding vessel is key. Sized to small and you run out of water in which case you now need to provide landscape water from a city source or trucked in which is cost-prohibitive if multiple applications throughout a drought are required. The alternative that I am being told will occur is to not water the new plants if the tank runs dry. Not good! The cost of losing the plantings on a new landscaped site is huge compared to sizing and having installed the right size of water-holding vessel.

Tank sizing rule of thumb

Store enough water to allow high-efficient, low-volume irrigation to occur three times a week for three weeks. In hotter climates this may be too infrequent. Adjust frequency for your area, compute the landscape water requirements, multiply by the watering days, work out for one week, and multiply by the length of the dry spells. Sometimes this number is a large volume and the tank price would be uneconomical. 
Time for hard decisions as to what gets water and what goes dormant or dies. Reality is tough.

Lorne Haveruk, CID, CLWM, CIC, CGIA, CLIA, WCP, is principal, DH Water Management, a water resource management consultancy firm focused on innovative rainwater, groundwater, stormwater, greywater, and  city water supplied indoor and outdoor watering solutions. Visit  www.dhwatermgmt.com or email  lorne@dhwatermgmt.com for information. This material is for information purposes and is not intended to provide legal advice.

References
• LEED for New Construction Version 2.2, October 2005
• Atlantis Rain Water Systems. www.rainharvest.com
• Bushman USA. www.bushmanusa.com