Passive rainwater harvesting allows you to collect rainwater runoff from roofs, patios and driveways as well as the overflow from rain barrels and cisterns (active rainwater harvesting sources) and direct it into the landscape. The water then gets stored in the soil where plants can take advantage of the moisture.
Several rainwater harvesting techniques can turn potential problems (i.e. erosion, flooding or excess moisture near foundations) into water for your plants. Rainwater contains extra nutrients not always found in potable water, so the plants receive additional benefits. Passive rainwater harvesting also helps filter out pollutants, increase infiltration to the aquifer and provide healthier ecosystem habitat.
Start by calculating the amount of water that flows from your potential runoff sources. For every 1,000 square feet of hard surface, 1 inch of rain will generate approximately 600 gallons of water. It’s easy to see why rain barrels or cisterns can quickly fill during medium to larger rain events, so a passive collection system for overflow is always a good idea. You want to make sure whatever system you create will be able to handle a
good-sized rain event (refer to the link above). It’s very important to create a directed overflow so that when there is a large rainstorm, the excess water will flow into a basin or other feature onsite or into the street (if your capacity is limited) without creating erosion.
There are many different techniques, and some can be used together to enhance the benefits. These techniques include rain gardens, swales, basins, soil sponges, curb cuts, French drains and pumice wicks, check dams and gabions. Study your site and these techniques to determine the best option for your garden. Watch your site during a rain to see where the runoff comes from and where it goes. You may want to take advantage of existing patterns.
Rain gardens are depressions to catch surface runoff from slopes and are best planted with native plants that can take occasional flooding.
Swales are sloped depressions that convey water into basins or rain gardens from canales, downspouts or other drains. Make sure the slopes consistently move the water downhill and are large enough to not get overwhelmed. Swales are usually lined with large gravel or cobble to prevent erosion. If you have extra material onsite, this is a great recycling solution. Make sure that swales are designed to catch the slow drip of a light rain as well as the powerful flow of a heavy downpour.
Basins are catchment ponds designed to handle large rain events and provide a great place to plant trees and larger shrubs or heavy water use plants that can benefit from the extra water. Calculating the size to match your runoff is vital here. The area where the runoff from your swale(s) enters the basin will need gravel to dissipate the erosive forces. Shredded wood mulch works best in the bottom of the basin. The capacity of the basin and the health of the plants can be further enhanced by installing soil sponges (see below) in the low points. In larger yards, you may want to create a series of basins to increase capacity and opportunities for plantings.
Soil sponges are post holes dug and filled with a soil mix containing compost to increase infiltration and introduce organic matter and beneficial microbes to the soil sponge and surrounding soil. These also provide an avenue for plant roots to quickly grow deeper, improving health and drought tolerance.
Curb cuts are made with a concrete saw to channel runoff into a basin. This can be done on your own property, but permission is needed to cut into a public curb to direct excess water into the street.
French drains and pumice wicks are progressively deeper trenches lined with geotextile fabric (to prevent silting up) and filled with gravel or pumice. These are best used when you have existing trees or large shrubs and the base of those plants is on the same level or higher than where your runoff originates. Aim them directly at the tree or shrub to minimize root damage while still providing additional water to the plants’ root zone. The open space in pumice provides additional water capacity.
Check dams and gabions are small dams used in areas where existing runoff has started to erode and cut deeply into the soil. These small dams slow down the water so it infiltrates and lessens erosion. Gabions are a type of check dam constructed of riprap (oversize gravel/cobble) held together with a wire basket. Generally, both have an area of cobble below to prevent erosion.
Passive rainwater harvesting is a great way to lessen your use of potable water, reduce erosion and provide a supplemental water source for your garden. Remember, you still will need to water your plants (through irrigation or hand watering) to establish them and, in most cases, to maintain their health.
Passive rainwater harvesting is a great way to optimize your landscape while minimizing water use. So you may ask, “What are active and passive rainwater harvesting?”
Active rainwater harvesting involves collecting rainwater runoff from roofs and other impermeable surfaces into containers such as rain barrels or cisterns for later use. These containers can vary from small (50 gallons or less) to very large (thousands of gallons). The saved water can be distributed with a hose or drip system for use in the garden.
Passive rainwater harvesting channels water from roofs, patios or driveways directly into the landscape via swales (channels) into basins (depressions in the landscape) or into French drains where the water will be stored in the soil for use by the plants. For every 1,000 square feet of hard surface, 1 inch of rain will produce about 600 gallons of water, so it’s easy to see how active systems will produce overflow and why it’s always recommended to send that overflow into a passive rainwater harvesting system. This will lessen the amount of potable water you will need for your landscape while improving plant health.
So, what plants should you choose for your passive rainwater harvesting garden? First you need to realize there will be three different zones with different amounts of water available to the plants.
1. The High Ground Zone is the area around the outside perimeter of your basin or swale where there will be limited amounts of extra water available to the plants. This will be your most xeric (low water use) zone. Plants for this area could include:
Desert willow (Chilopsis linearis)
Grasses such as sideoats grama (Bouteloua curtipendula) or sand dropseed (Sporobolus crytandrus)
Desert four o’clock (Mirabilis multiflora) or blackfoot daisy (Melampodium leucanthum)
Sideoats Grama
2. The Transition Zone is partway up the sides of the swale or basin. Plants growing here will get some extra moisture but will not be at the low points of the basin or swale.
Escarpment live oak (Quercus fusiformus)
Grasses such as blue grama (Bouteloua gracilis) or little bluestem (Schizachryium scoparium)
Salvias such as autumn sage (Salvia greggii), Mexican blue sage (Salvia chamaedryoides) or Mexican red sage (Salvia darcyii), or dwarf goldenrod (Solidago sp.)
Escarpment Live OakLittle BluestemSalvia darcyii
3. The Inundation Zone will be the wettest area during large rain events. Only plants that can tolerate periods of standing water will work here. The basins can be enhanced with soil sponges to increase storage capacity and soil quality.
Netleaf hackberry (Celtis reticulata)
New Mexico olive (Forestiera neomexicana) or fernbush (Chamebatiaria milefolium)
Grasses such as giant sacaton (Sporobolus wrightii) or Indiangrass (Sorghastrum nutans)
Horsetail milkweed (Asclepius) or creeping germander (Teucrium chaemadrys)
Always make sure the sun or shade exposure needs of the plant match your site and remember that you will need to water these plants at least until established.
The Albuquerque metro area has a wide variety of landscape types, which include desert-friendly landscapes, lush pollinator-friendly xeriscapes and wildscapes or prairie plantings where native grasses and wildflowers bring the feel of a prairie to a yard. Rain gardens are specifically designed to make the most of natural precipitation, while edible landscapes have garden beds or edible plants interspersed throughout them. Efficient turfscapes have smaller areas of turfgrass and diverse plantings. Conventional turfscapes, with large areas of cool-season grasses and high-water foundation shrub plantings, are no longer considered desert-friendly landscapes. The term “desert-friendly” now describes a variety of landscapes that include beautiful, diverse plantings with increased resiliency achieved through efficient watering.
Rain gardens, a subset of xeriscapes, maximize the benefits of active and passive rainwater harvesting. Plant choices become more critical, particularly if the intent is to use little supplemental irrigation.
Pro Tips for Rain Gardens:
* Direct any overflow from a rain barrel or cistern into a passive water harvesting system.
* Prioritize trees for passive rainwater harvesting.
* Choose low-water and rainwater-only plants for the best results.
* Contour the soil to create gentle berms that drain better. Put water-loving plants in the low spots (swales) where rainwater and irrigation water collect.
Do you want to learn how to harvest rainwater to support a beautiful, regionally adapted, low -water-use landscape in your yard? A new video series launched in June provides instruction on how to design, install, and maintain residential-scale, passive rainwater harvesting features. In passive water harvesting, rainwater is conveyed directly to a landscaped area and infiltrated in the soil.
Landscapes that incorporate rainwater harvesting and native/drought-tolerant plants require little supplemental irrigation. Rainwater harvesting provides the deep watering that large plants like trees and shrubs need in our hot and dry climate. Capturing and infiltrating rainwater also reduces runoff that carries pollutants to the Rio Grande River.
In this series, you will learn how to construct some of the most common passive rainwater harvesting features in residential applications – basins, swales and soil sponges. The training addresses site selection, sizing basins, grading swales, installing soil sponges for enhanced infiltration and selecting plants. It is presented by four local experts on rainwater harvesting and arid-adapted gardens: Jim Brooks, Tess Houle, Judith Phillips, and Hunter Ten Broeck.
The video series was developed through a collaboration between Bernalillo County, Ciudad Soil and Water Conservation District, and the Arid LID Coalition, and included the installation of a Rainwater Harvesting Learning Landscape at the Gutiérrez-Hubbell House History and Cultural Center. Please visit the Rainwater Harvesting Learning Landscape in person to see an example of a rainwater harvesting feature designed for our arid climate.
505Outside sat down with local landscape contractor Hunter Ten Broeck, owner of WaterWise Landscapes Incorporated, to chat about Water Harvesting in Albuquerque.
There are two categories of water harvesting techniques: passive water harvesting and active water harvesting. Passive water harvesting is the practice of slowing water down and encouraging it to soak into the ground. Water is collected from the source (roof, canale, downspout, or sloped earth) and directed to the landscape where the water can be immediately used by the plants. Active water harvesting uses rain barrels, cisterns, and other types of containers to store rainwater for later distribution. These typically hold as few as 200 gallons of water up to 10,000 gallons. A good calculation to remember is 1” of rain on a 1000 sq. ft. roof equals about 600 gallons of water.
Hunter
says the key to any type of water harvesting system, whether active or passive,
is to incorporate it into the landscape design. Thinking through how the rain
water will interact with your landscape and designing for that should be first
and foremost.
When asked what a homeowner could easily do to start water harvesting in their yard, Hunter recommended starting small and trying passive water harvesting. Passive systems can be as simple as a swale, a French drain (or rock burrito, as nicknamed by his staff), or more complicated systems that include building an underground soil sponge area. He says to start by finding the roof’s downspouts and canals, then provide a catch area underneath them. A catch area could be a concrete splash block or cobbles. Then direct that water away from the home via a cobble swale that runs into a planting area. This can be done easily by digging down a few inches and creating a slope away from the house into the planting area. Once the swale is dug, add cobble on top to stabilize the soil and prevent erosion.
Cobble swale located at downspout next to house.
To make Hunter’s “rock burrito” simply dig an 8” wide trench at least 12” deep and as long as necessary for the space. Slope the bottom of the trench toward the drainage area. Layer the trench with filter fabric followed by cobble or rocks. Roll the fabric over the rocks then backfill with soil. This will provide an underground collection area for water. Water will slowly soak into the ground providing moisture to adjacent planting areas.
“Rock Burrito” French drain under construction in an Albuquerque landscape.
For homeowners who are ready to tackle an active water harvesting system like a rain barrel or cistern, Hunter recommends adding a passive water harvesting system next to the active one for when the container overflows.
Passive water harvesting swale next to active water harvesting cistern.
Using every drop of water through water harvesting is an important feature Hunter incorporates into all his landscape designs.
Local Expert: Hunter Ten Broeck, Landscape Contractor and owner of WaterWise Landscapes Incorporated in Albuquerque, NM. Author: Jill Brown, ASLA Landscape Architect and owner of My Landscape Coach in Albuquerque, NM.
