We have been designing large-scale residential and commercial rainwater harvesting systems in California since 1997, primarily for irrigation use in the landscape. We like to consider our landscapes that incorporate rainwater harvesting as “closed loop systems,” as we begin the design by determining our end-water usage.
It Starts with Plant Selection
It starts with the type of plants we choose. We look at the irrigation requirements for the proposed planting areas. In our design we hydrozone, i.e., put plants together with similar water needs and choose a high percentage (75–90 percent) of our plants from a rich, diverse native and drought-tolerant plant palette. This way we get the most aesthetics and the most efficient water usage out of our site. Many of these plants add more water-saving benefits to the site, in that they help bind the soil and prevent erosion and excess runoff.
Lawns, of course, are the biggest water users, so reducing lawn areas reduces water needs, which in turn requires a smaller rain harvesting system. Our goal is to supply most (or all) of our plant irrigation needs with harvested rainwater.
Other site-specific factors we consider in determining our end-use needs for irrigation water include water percolation rates (dependent on soil type), and evapotranspiration (ET) rates—how fast the water evaporates from or absorbs into the soil. These rates determine how often and how much we need to apply water to the plants. My experience is that most homeowners over water their plants by a factor of two to three times. With the use of “Smart” ET- based automatic irrigation controllers we can now plug in the data for each zone, including the types of plants, soil type and topography. The controller sends this data to a local satellite station to determine the ET rate for the day. Only the amount of water needed on any given day is then administered to the plants. Usually included is a rain sensor that automatically turns the irrigation off if there is any precipitation. When operating on a rainwater system, the irrigation water supply is limited for the year, so we also recommend the irrigation system includes a flow sensor, which when a leak is detected informs the owner or landscape management team of the problem via email or cell phone.
We use drip irrigation as our preferred method of irrigation. Subsurface drip is becoming more popular in the U.S. Subsurface drip is said to deliver 90 percent of the water directly to the roots of the grasses and plants, compared to 60 percent when using a spray system. It is also estimated that subsurface drip irrigation can save up to 70 percent more than spray heads, and up to 25 percent more than regular drip systems. We then add a two to three inch thick top layer of woodchip mulch to the planting areas to add to the efficiency of the system.
Filtered Roof Water
In most cases, the roof surfaces of the home (or other buildings on the lot) can provide the quantity of water needed for a full planting design, so importantly, we determine in advance our irrigation needs and design for the greatest efficiency. Roofs are the cleanest rainwater source, as opposed to surface drained storm water. Because of the particulates that storm water picks up on landscape surfaces, such as asphalt and concrete, it is typically of a lower quality. From the roof downspouts the rainwater may be directed through filters, which are usually quite simple, to appropriately sized storage tanks and then pumped and filtered again before entering directly into the efficiently designed irrigation system. It is important to size the pipes, filters, and storage system appropriately, based on the runoff rates of a heavy storm in a particular area. There also must be a well-planned solution to accommodate overflow during a particularly heavy storm.
Above or Below Ground Tanks
There have been many successful rain-harvesting systems installed in California. An above ground rainwater storage solution is the least expensive, but most space-consuming alternative. We typically install above ground tanks when there is enough room to screen them from view, although some landscape architects are now specifying attractive rainwater tanks as site features if the planting area is small and does not require much water storage, such as a small inner-urban back yard.
Underground tank systems usually cost twice as much to install because of the excavation, but of course have far less visual and spatial impact. One must always consider the level of the water table, surface pressure and an anchoring system when installing underground storage tanks. A great solution, if feasible, is to install underground tanks under a driveway, building or patio with access through a fitted manhole, such as is typical on a city street. Either way, the overall water savings is well worth the investment, if the system is well designed, and there is enough water captured to handle most of the irrigation needs of the site.