Water Harvesting To Irrigate Landscapes


Rain water, gray water, and blackwater systems can aid arid areas.

Reducing the amount of water used for landscape irrigation is challenging, yet extremely important to the drought affected areas of the U.S. Landscape watering needs within the U.S. vary greatly based on many different factors: local climate, soil conditions, native plants, plant selection, and micro-climates on the site. In many locales, rainfall can be nature’s irrigation system, while in drought affected areas precious potable water provides most of the source for landscape irrigation. Yet even with these differences, saving water is very possible. In most cases, simple changes can result in water savings between 10% to 20%. By reusing water, savings of 50% to 100% are achievable.

Water Harvesting Landscape Irrigation
(Photo: HarvestH20.com)

First Steps

The first step in saving water is maximizing the efficiency of the irrigation system. Once that is accomplished, incorporating a water reuse system (i.e., rain water harvesting, gray water, or blackwater) makes sense. (Efficient irrigation systems will obviously reduce the cost of any new water-reuse systems.)

Irrigation System Efficiency. Obvious simple system fixes such as addressing leaks, adjusting heads to not over- or under-spray, getting the controller set right, adding a rain sensor, and upgrading the controller to an EPA WaterSense weather-based model will result in a reduced water bill and a less costly water reuse system. The EPA provides a water budgeting tool which is handy when determining how much water should be applied to a landscape. It can be found at epa.gov/watersense/water-budget-tool.

Soil & Mulch. Another step to take be-fore diving into water reuse is optimizing soil health. Improving soil health is a very important variable in water savings. Just by adding mulch and organics, water use can be reduced, plants will be healthier, and infiltration by irrigation water can be raised substantially. The Environmental Protection Agency (EPA) estimates that only 25% of rainfall that falls on landscapes actually stays on landscape. The rest is runoff or evaporates. Adding mulch can increase this number, thereby improving the use of irrigation water. Every site should be designed for zero runoff. Another impact of zero runoff is reducing the cost of a water reuse system.

Good quality mulch is one of the best investments for using water efficiently.  Soils without mulch lose rainwater because the moisture never percolates to the plant’s roots. Soil without organics will not allow as much water to infiltrate and thus cause increased runoff. Two to four inches of mulch are recommended, with four inches being ideal. Another strategy for reducing unnecessary runoff is building berms and swales. Local nurseries and Master Gardeners are good sources for help on determining the health of soil.

Water Reuse

When it comes to types of water reuse, some states (e.g., California, New Mexico, Arizona, etc.), allow a choice between rainwater, gray water, or blackwater, while in others, regulation constraints limit the type of systems that can be installed. The first step is to determine what can be done locally by contacting the local building department.

For example, in the state of California, gray water is legal, but some local jurisdictions are very hesitant about these systems, making it difficult to permit and install a gray water system. In most cases in Colorado, the limit is two 50-gallon rain barrels to harvest rain water, and installing even a small gray water system is currently not allowed in most of the state. Installing a blackwater system for subsurface irrigation is illegal in a local community, Los Campanas, NM, even though the state permits these systems. Knowing these limitations upfront will save time and aggravation.

Water Harvesting Landscape Irrigation
Always check your state and local regulations on water reuse before planning a system.


The next step is to determine how much water is generally needed to meet irrigation needs. Assuming the current irrigation system is efficient and effective, start by reviewing the water bill or water meter since it provides detail on monthly water usage. Find out how much water is used in the peak summer months and how much is used in January. In many locations, subtracting January usage from July usage provides a reasonable estimate of gallons required for irrigation. Some water utilities charge in cubic feet, therefore requiring a conversion from cubic feet to gallons. There are many articles and videos on the Internet for analyzing water bills.

Once it’s determined the water capture required on a specific site by gallons (i.e., rain water, gray water, or blackwater), the next step is choosing the best alternative water source for the site. All methods have pros and cons.

Rain Water

Rain water is free. Figuring out how much can be captured is easy. This equation provides gallons that can be captured:

Rain Water Gallons Harvestable = Roof square footage x annual (or monthly) rainfall x .623

For example, a 2,000 square foot roof in an area with 12″ of annual rainfall can harvest 14,952 gallons a year. The equation above provides an optimal collection. In most cases, the reality will not be this total number due to inefficiencies in the system (e.g., not being able to capture certain sides of the roof, overflows, etc.), so it should be reduced by 10% to 25%.

Rain water systems can range from simple rain barrels to complex large tanks with pumps, filters, piping, and controls. Each design is customized for site requirements. Organizations like the American Rainwater Catchment Systems Association and the Watershed Management Group train individuals and professionals to design, install, and maintain these systems properly.

In areas blessed with adequate rainfall in the summer, passive rain water catchment may be adequate. These systems direct the rain water to specific areas of the property where it can be optimized for plant irrigation, rather than runoff. Examples include rain gardens, swales, berms, underground perforated PVC pipes, and/or catch basins that allow for infiltration and longer landscape plant use, instead of runoff. These systems work well, are inexpensive to build, and require very little maintenance.

In other areas, rainfall alone will not suffice as the sole source of irrigation water unless it can be stored onsite in tanks for use as needed. The addition of tanks adds filters and a pump to the system, creating an active rain water system. There are thousands of these systems installed for irrigation throughout the U.S

Rain water harvesting has significant benefits: the water is free, usually very clean, and requires minimal filtration before it can be used for irrigation. Harvesting and reusing rain water is legal in most states and countries. Colorado, Nevada, and Utah have restrictions, but most other states promote or even offer incentives to install rain water harvesting systems. (Check with the local water conservation office to determine what is possible and if rebates are available.)

Gray Water

Another source for water reuse is gray water. (Although we cannot agree on how to spell it, it is all the same—grey-water, graywater, gray water, or grey water.) Gray water is everything used in the household water except for toilet water, wastewater with food particulates in it, or sink water with harmful chemicals in it. It is about 40% of the water used in a household. Small graywater recapture systems can be easy to install and are definitely a viable reuse option in many states, especially in the West

Gray water is an excellent water source for landscaping if allowed. Gray water is available almost every day of the year and in quantities that can rival rainwater in many areas. It can pose health issues, so the use of gray water for irrigation needs to be done correctly.

Water Harvesting Landscape Irrigation
In a typical L2L, or Laundry-to-Landscape system, the laundry pump acts to pressurize an irrigation line running out to the landscape, often to mulch basins around trees or plants.


The WaterShed Management Group in Tucson, AZ (watershedmg.org); Grey water Action (greywateraction.org); Qualified Water Efficient Landscaper (qwel.net); and the Santa Fe Community College (energysmartacademy.com/water) are a few of the organizations that offer gray water training courses. These courses provide the education necessary to understand how to harvest and reuse gray water safely. Unless adequately filtered and treated, gray water should always be used for subsystem irrigation or mulch basin. Gray water, like rainwater, can be implemented in both existing and new homes. It is nutrient-rich, which reduces the need to add fertilizers, and can be used on most plants. In some cases, a property owner’s choice of cleaning products may need to change. Fortunately, many eco-friendly washing detergents are now avail-able and can be used in gray water systems.  In the plumbing codes, the general calculated amount of gray water generated in a house is based on the number of rooms in the home. It is:

Annual Gray Water Production = (Bedrooms in the House + 1 ) x (25 gallons +  15 gallons) x 365 days a year

For instance, with this equation, a three-bedroom house would produce 58,400 gallons of water. This formula generally overestimates the amount of gray water produced by a household. This is due to the efficiency improvements of faucets, clothes washers, and shower-heads that have occurred over the last few decades. The number above—or a formula like it—will be required on any permit application, but for more realistic usage it should be reduced by 25%. Gray water systems can be simple or complex, inexpensive, or very expensive. Simple Laundry-to-Landscape (L2L) systems can be installed in many existing homes and, in many cases, with only a few hundred dollars of parts. In an L2L system, the laundry pump acts to pressurize an irrigation line running out to the landscape, often to mulch basins around trees or flowering plants.

Simple gray water systems, such as L2Ls, or rain barrels with pumps and drip hoses, can reduce onsite potable water usage, can be easily installed (where allowed) and increase the interest in more sophisticated solutions.

If the site requires more landscape water than these systems provide, com-pare how much onsite gray water can be generated versus how much rainwater can be harvested. This comparison will assist in determining which solution might be a better option.

Using gray water onsite is an excellent option for homes and offices occupied full-time. L2L systems can be installed inexpensively and are a great way to start. Many new fully packaged systems are entering the market that make capturing and reusing all the gray water onsite possible and practical. These systems capture all gray water generated from a household, filter it, and pump it to the irrigation system. These packaged systems are an excellent option for new homes, but may be cost-prohibitive in many existing homes. These new systems handle one of the most significant drawbacks of gray water systems; how to filter out hair, lint, and other debris without frequent system cleaning.


Blackwater is all household water, including gray water as well as toilet and kitchen sink water. While it has a “yuck” factor associated with it, blackwater should be considered when evaluating onsite water reuse in drought-prone areas. A treatment system processes the blackwater to be suitable for sub-surface irrigation. However, the water cannot be used on all plants (i.e., no root crops or food crops in many jurisdictions). Plumbing codes allow these systems, but as mentioned previously, many states have strict regulations on how and where these systems can be installed. Like gray water systems, these systems do pose a health risk. It is highly recommended that these systems only be installed by a trained professional. They also require annual maintenance usually performed by a trained septic system professional.

Blackwater reuse systems are widely installed in Arizona, Texas, and New Mexico. The extra holding tank, hardware, and components needed to upgrade an existing septic system for reuse capabilities are fairly straightforward to install. Retrofitting/upgrading a current septic system is also possible if the area exists near the old system. Installing a new system where the blackwater is hooked to a municipal sewer system will be more expensive and generally harder to permit. Blackwater treatment systems produce nutrient-rich water that can be used on trees, grasses, and non-edible shrubs and plants.

Annual Blackwater Harvestable = (Bedrooms in the House + 1)  x 75 gallons x 365 days per year

Like gray water, the above formula produces a high number and should be scaled back to be more in line with the actual indoor household water use. For example, with this equation, a three-bed-room house would produce 109,500 gallons of water a year.

Both gray water and blackwater systems have the advantage that water is produced and available year-round (i.e., when the house is occupied) yet rainwater harvesting is naturally limited to when it rains. Due to the cost of each system, either a gray water or blackwater reuse system is generally installed—not both. A blackwater system will harvest more overall water (since it includes gray water), but since the water is all mixed in the one system, none of the harvested water can be used on edible plants, such as fruit trees or non-root crops. A gray water system, while harvesting about 50% less water, can be used on many plants in the landscape as local laws permit.

When deciding which of the water harvesting systems might be best, choose the one that will: create the most water; is legal for the location; is possible to implement onsite; and meets the needs of the owners and the landscape. In some cases, two different types of systems may be needed to fill all landscape water requirements.

Always start with improving the efficiency and effectiveness of what is already in place and then move to reusing water onsite. Using clean, filtered drinking water to water landscapes will become a thing of the past. Reusing water is not only a good thing to do; it is better for plants, better for community, and can be better for the pocketbook. Learn more about water reuse in your area today to build a more sustainable tomorrow.

Pushard founded Santa Fe, NM-based HarvestH2O.com over 20 years ago to stimulate education and awareness on the subject of rainwater catchment and water conservation. Today, he designs and works with homeowners and local contractors to install small-to-large residential water management systems to maximize water capture; while optimizing water efficiency and minimizing overall costs and ongoing maintenance. Pushard has presented on rain water and water conservation at conferences around the U.S. 

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