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Sizing cisterns and other water holding devices


January 28, 2014
By Lorne Haveruk

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“How do I determine the water requirements for a site’s irrigation requirements?”

Good question, and I am afraid there’s no 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 device 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), passive irrigation where onsite stormwater drainage is designed to redirect water to ground level plantings. Are the street trees to be watered which, by the way, are 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.

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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 & 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 & 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.

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 newly-landscaped site is huge compared to sizing and having installed the right size of water-holding vessel.

Lorne Haveruk, CID, CWCM-L, 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. For more information, visit http://www.dhwatermgmt.com or email  lorne@dhwatermgmt.com . This material is for information purposes and is not intended to provide legal advice.