Even though more than 70 percent of the earth’s surface is covered in water, we are headed toward a significant water crisis on a global scale.
The earth’s water is 97.5 percent salt water, leaving a mere 2.5 percent available as a fresh water resource. Almost 70 percent of that fresh water is frozen solid in the polar ice caps based on data provided by UNESCO (United Nations Educational Scientific and Cultural Organization) in the United Nations World Water Development Report #2 in 2003. It’s probable that some of that frozen fresh water has blended with ocean salt water as a result of climate change by now.
A majority of our planet’s remaining fresh water resources are captured in underground aquifers. It’s estimated that less than one percent of the world’s fresh water resources are accessible for human use. Still, the world’s population continues to grow.
It’s easy to see why the use of fresh water for landscape irrigation will continue growing less available, as the demand for those resources for human consumption and agricultural endeavors continues to increase. The irrigation industry is responding to this by increasingly moving toward using water that’s available on-site instead of using potable supplies. There are alternatives to using potable water for landscape irrigation purposes.
On-site water reuse strategies
Historically, landscape irrigation systems have drawn from the same potable water sources that service the buildings and aesthetic features of a site.
In times of drought, the typical regional strategy is to mandate significant reductions in potable water use for irrigation.
This can cause serious damage to and even loss of plant material. The development of passive and active on-site water reuse strategies for irrigation may save or significantly reduce the loss of plant material during a temporary water shortage.
Passive on-site irrigation systems make use of natural rainfall for irrigation and site grading for delivery of it. Some common examples of passive irrigation systems are bioswales or rain gardens. These are created by effectively grading a site to direct rainwater into a channel or basin planted with drought-tolerant landscape species.
Depending on the region of the country and the likelihood of a consistent source of rain, these swales and basins may not require supplementary irrigation to maintain an attractive planting aesthetic.
In urban areas of the U.S. and Canada, this planning and design strategy is referred to as low-impact development, or LID. In many urban settings this practice has become an alternative to the historic “best management practices” of capturing stormwater and directing it to an underground pipe infrastructure that carries the water to a canal, stream or lake.
In this scenario, LID enables the immediate beneficial use of the stormwater as an irrigation source before it finds its way into the atmosphere through evaporation; or into an underground aquifer; or is disposed of off-site through the storm sewer infrastructure.
Using rain barrels to store captured rainwater runoff from roof surfaces and grading the site or using downspout extensions to direct that water to specific planting areas is another example of a passive on-site irrigation system. Water captured in the barrels can be used to fill watering cans and buckets for spot watering of individual plants or planter pots.
The use of downspout extensions typically requires a reasonable site gradient that enables the water to flow by gravity to the planting destination. These techniques are generally used at individual residential sites on a smaller scale, usually to augment a larger scale automated irrigation system.
Active on-site water reuse systems are designed to capture, store, treat and deliver water from rainfall or other unrelated onsite uses so that it’s available for irrigation use on an “on-demand” basis. This concept forms the basis for “net zero” water use, which effectively means that any available water is used and reused so efficiently onsite that there’s almost no loss of water and only a minimal amount of “new” water from an off-site source is required.
While that concept sounds admirable, actually achieving a true net zero reuse system is extremely difficult, often costly, energy intensive, and frequently not sustainable without a strong ongoing commitment from the property owner and those responsible for operating and maintaining the system.
An educational program focused on onsite water reuse systems has been recently developed through a collaborative effort between Jeffrey L. Bruce and Co., Green Roofs for Healthy Cities and the American Society of Irrigation Consultants: “Integrated Water Management for Buildings and Sites.”
The GRHC sponsors a series of seminar and webinar courses for this program, which describe in detail the major components and concepts for this type of on-site water reuse system. The major features of these systems, identified by the IWMBS program, include the following.
1 Water harvesting. This involves capturing water from multiple onsite sources such as rainwater from building roofs, stormwater from impervious surfaces and foundation drain systems, graywater from sinks and showers, condensate from building cooling systems and others.
The capture, storage and blending of any of these sources may be strictly regulated or prohibited by local codes and ordinances, so it’s critical that the designer and installer of the system do an appropriate level of due diligence prior to starting the design.
2 Entry treatment. Often the water captured from these on-site sources contains debris or constituents that degrade its quality to the point that it can’t be stored or used effectively for irrigation purposes.
Most active reuse systems require initial filtration or a “first flush” to remove as many of these undesirable constituents as possible so that the water leaving the storage facility is the highest quality possible.
3 Conveyance system. This provides a means for transporting the water from the point of capture to storage. Generally, this is accomplished through a piping system using gravity. This usually requires that the onsite storage facility be installed at a lower elevation than the features that are used to capture and initially filter the water.
When gravity conveyance is not possible, other means of moving the water may be necessary, such as low head pumping systems. But this introduces an additional need for energy that may offset the sustainable benefit derived from the reuse system.
4 Storage. The need for on-site water storage is usually driven by the fact that the amount of water supplied by the water source(s) does not match the volume or timing required by the irrigation system.
There are almost always delays between the frequency and timing of the water supply, and when the property owner or water manager needs or wants to apply the water to the landscape through irrigation.
An appropriately sized storage facility provides a necessary buffer between supply and demand, enabling efficient water application and system management.
5 Storage treatment. Water that sits in an on-site storage vessel or pond tends to degrade in quality over time, potentially to the point of becoming toxic or unusable for irrigation. Stagnant water that is exposed to sunlight has a tendency to develop algal blooms and may also become a breeding ground for mosquitoes.
Appropriate treatment for maintaining acceptable irrigation water quality may include chemical injection, filtration, aeration and other processes depending on the quality of water within the storage facility and the quality of water required to maintain healthy plant material.
Consistent quality testing of the stored water will assist the owner or water manager in determining the level and type of treatment strategy required.
6 Irrigation distribution and application. Reuse water is pressurized, usually by a booster pumping system, for distribution to the various control zones throughout the system, where it’s applied to landscapes via control valves, lateral piping, and sprinklers, bubblers or drip emitters. Supplementary filtration at the control valve may be necessary for low-flow and drip applications due to the small orifices involved.
It’s usually beneficial to supplement a reuse irrigation system, whether passive or active, with an emergency backup potable water source that can be tapped if the reuse system should require repair, or if one or more alternative water sources becomes unavailable. This can be as simple as providing a hose-end sprinkler for manual watering, or extending hose from a building to supply water to an on-site storage facility. This backup system acts as an insurance policy, reducing the potential for loss of plant material if unusual weather conditions occur.
Local and municipal strategies
As the availability of fresh water sources continues to decline, and the cost of treating and delivering water at a quality that’s safe for human consumption continues to rise, many developers, communities and municipalities are searching for alternatives to using potable water for irrigation.
Strategies are being put into place at the local and regional levels to provide the necessary infrastructure for delivering recycled (reclaimed) water — sewage water that’s partially treated but not enough to be potable — for use in irrigation.
Population growth is causing the expansion of the urban footprint into areas that have historically been used for agriculture. At the same time, farmers and landowners in many rural areas near major cities are finding it more difficult financially to continue their ranching and agricultural operations. They are finding it more lucrative to sell or lease their rights to surface raw water and groundwater resources to developers and municipalities for urban and landscape irrigation use.
Irrigation districts and water purveyors that have raw water surface infrastructure intended for agriculture are converting them for urban use.
In the desert Southwest, some municipalities are developing ways to pump recycled water from treatment facilities into the aquifer via injection wells to obtain storage credits. They then withdraw that water directly from the aquifer via recovery wells using those credits.
Often recovery wells are put in new developments for direct use or to supply a lake amenity, which can also act as a water storage facility. In some cases, these injection and recovery wells are combined into single facilities known as “Aquifer Storage and Recovery Wells.” This reduces the need for piped infrastructure under existing streets because the aquifer itself acts as the storage and delivery infrastructure.
In many cases, multiple water sources (i.e., raw surface water, groundwater, recycled water and potable water) may be provided to a single storage facility and blended into a “flavor” of water that is suitable for landscape irrigation use.
Property owners, irrigation designers, installation contractors and water managers need to work together to seek out alternatives to potable water for irrigation use wherever possible. Our future depends on it.
The author is a principal and vice president of Fort Collins, Colorado-based Aqua Engineering Inc., where he manages the firm’s Phoenix office.