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Building Manager Green Tip

August 2012

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The Impact of Inefficient Irrigation

According to the EPA, at least 30% of a typical suburban household's water use is dedicated to landscape irrigation, while 50% of that water is wasted due to "evaporation or runoff caused by over-watering." Such practices account for over 7 billion gallons of water consumption each day in the US and are believed to cause significant strain on watersheds and rivers in Massachusetts

While plant selection has the largest impact on irrigation demand, this post will focus on how to efficiently supply water using controls and delivery systems.

Delivery Methods

Water wasted using spray irrigation techniques

Photo Credit: WholeWheatToast on Flickr

There are two primary means for distributing water. The photo above is of a spray irrigation system, one of the least efficient methods for ensuring landscapes receive the water they need. The efficiencies reported below are those provided for use in landscape irrigiation efficiency calculations used by the LEED rating system.

    • Spray Nozzles - Even if the system is designed in a manner wherein the path of the spray falls completely within the landscaped area (i.e. it is not watering streets and sidewalks), only about 63% of the water leaving the nozzle ever reaches the roots. This is primarily due to the fact that a portion of the mist from the spray evaporates before it hits the ground.
    • Drip Irrigation - Drip irrigation systems supply water through a series of small tubes either on or just below the surface.  As a result the evaporation rates are much lower than spray systems, and typical efficiencies of these systems result in 90% of the water entering the ground.

Control Systems

Irrigation controller including weather station (at left)

Photo Credit: slworking2 via Flickr

While the delivery method is important, the controller that determines how and when your system is functioning can also have a dramatic impact on water consumption.  

    • Manual Watering - According to the EPA, the most efficient means of controlling overwatering is to simply to do it yourself.  "[H]ouseholds that manually water with a hose typically use 33% less water outdoors than those who use an automatic irrigation system" of any type.
    • Timer Controls - If automatic controls are required, schedule based controllers are the least efficient, as there is no way for the timer to know whether it just rained or the landscape actually requires watering.  The EPA states that these systems typically consume 47% more water in households than those without.
    • Weather Based Controls - A variety of companies now manufacture systems that are capable of adjusting watering schedules based on past or predicted rainfall. Some rely on signals from connected websites while others use on-site weather stations (pictured above), but in both cases the systems are designed to turn off irrigation systems when it has recently rained or is predicted to do so in the near future. 
    • Soil Moisture Sensors - Some systems are capable of detecting moisture in the soil and will further limit watering if adequate moisture is detected.  This can have a large impact in swing seasons when the sun isn't drying out the soil in a consistent manner.  Coupled with drip irrigation and weather controls, this is the most efficient way to automatically water a landscape.

The EPA has included irrigation controllers in the WaterSense certification program. A full list of approved efficient products is available by clicking this link.

Green Building Services provides consulting services to ensure that the design, construction and operation of Harvard's built environment has minimal environmental and human health impacts, maximizes occupant comfort and generates an awareness of sustainable design and building operations. To learn more about our work and services, visit http://green.harvard.edu/gbs.