asphalt roof was a mess, so we removed it and installed 26-gauge
galvanized steel metal roofing instead, which harvests rainwater in a
potable form. However, as long as you’re only harvesting rainwater for
use in landscape irrigation, this isn’t a necessary step. (Rainwater
harvested off a conventional asphalt roof can also be made safe for
consumption with the installation of an appropriate water filtration
Take a look at your roof. Where do the gutters drain? Where is
rainfall currently being directed? This is where you should begin with
mulched water-harvesting basins and plantings (at least 10 feet from
the building’s foundation.) On our property, just under half of the
roof runoff is directed to earthworks and fruit trees north of the
house. The rest is directed to an above-ground cistern west of the
garden along our property boundary on top of a 2-foot (60 cm) high
Our cistern is a custom-modified new ferro-cement septic tank, but a
number of good alternatives exist. (See, Choosing a Tank.) We selected
the location of our cistern to provide multiple functions. By placing
it on the western boundary of our yard to shadeing out the hot
afternoon sun, it creates a beneficial microclimate for our garden. By
acting as part of the property line, it provides a privacy screen from
a peering neighbor. And by placing the cistern on an elevated platform,
the system utilizes gravity in circulating water from the roof’s gutter
to the tank, and from the tank to the garden.
Whatever type of cistern you choose, having your garden located
nearby will keep hose length to a minimum (25 ft. ideal) This will
reduce water-pressure loss to surface-friction inside the hose and make
watering with rainwater a convenience. (Your plants will love it too!)
Principle #3: Always plan an overflow route, and manage overflow as a resource.
Eventually, all water-harvesting systems will meet a storm that exceeds
their capacity, so don’t get taken by surprise. All rainwater
harvesting structures should be managed in such a way that the system
can overflow in a beneficial, rather than destructive way.
In that spirit, overflow from our backyard cistern is directed via a
4-inch diameter overflow pipe gutters to a series of adjoining mulched
basins that passively irrigate a citrus tree and our garden. In
addition, all of our sunken earthworks have an overflow “spillway.”
Typically, one earthwork overflows to another and another, until all
are full and then, if needed, the lowest earthwork can overflow to a
natural drainage–-or, in a typical urban context, the street.
Your goal should be to harvest the rain, but never get flooded by it. This is key.
4. Start with small and simple strategies that harvest the rain as close as possible to where it falls.
When people think of rainwater harvesting, usually it’s cisterns and
tanks that spring to mind. But the water collected off your roof is
typically much less than what’s actually falling on your property.
Simple water-harvesting earthworks, such as basins, terraces, contour
berms, and check dams will harvest the rain where it falls, on the land.
The water-harvesting earthworks Rodd and I created collect the vast
majority of our rain. We dug level-bottomed basins and deeply mulched
them (about 4 inches) in order to infiltrate rainfall and runoff
throughout our watershed—once again starting at the highest points of
the yard and working down. Overflow water was directed from the upper
basins to the lower basins, which brings us to principle number five.
5. Spread, slow and infiltrate the flow of water into the soil.
Cisterns along with mulched and vegetated earthworks basins with
overflow routes will effectively transform your erosive runoff during
heavy rainfall into a calm, productive resource while reducing water
loss to evaporation and downstream flooding.
Raised pathways and gathering areas are also a great strategy for
spreading water through the landscape. This pattern of “high and dry”
regions that drain to adjoining basins kept “sunken and moist” will
help to define those areas through vegetation while spreading and
sinking the flow of water. (This also helps keep ice off walkways and
driveways in colder regions.) At our place, we also used earthworks to
redirect the runoff that used to pool against our house to planting
areas 10 feet or more away from the building’s foundation.
6. Maximize living and organic groundcover.
All your basins and other water-harvesting earthworks should be well
mulched and planted. This creates a “living sponge” effect that will
utilize the harvested water to create food and beauty in your
surrounding landscape while steadily improving the soil’s ability to
infiltrate and hold water due to the vast network of growing roots and
Groundcover is equally important in helping to ensure that, in your
enthusiasm for harvesting rainwater, you don’t wind up creating a haven
for mosquitoes. Mosquitoes need three days of standing water to
transform from eggs to adults. Water-harvesting earthworks allowing
water to infiltrate below the surface of the soil (typically within one
hour) where it won’t be lost to evaporation.
Take a hike in the natural unmanaged areas near your home to
determine what native vegetation would be best to plant within or
beside your earthworks. Out in the wild, you’ll notice which plants
grow naturally in depressions – they can be planted within your basins.
Wild plants preferring better drainage can be planted beside, but not
Blue palo verdes, velvet mesquite, chuparosa, oreganillo, and desert
lavender are a few of the native plants found along the ephemeral
washes in our area of Tucson that we plant within our earthworks.
7. Maximize beneficial relationships and efficiency by “stacking functions.”
As mentioned previously, water-harvesting strategies offer maximum
benefits when they’re integrated into a comprehensive overall siteplan.
We focused on locating the earthworks where we wanted to stack
functions with multi-use vegetation.
Through rainwater harvesting earthworks, we’ve nurtured a solar arc
of deciduous trees on the east, north, and west sides of our home that
cool us in the summer, but let in the free light and warmth of the sun
in winter. A living fence of native plants along the property line
(along with an existing citrus tree) form part of a sun trap. This
suntrap shades our garden from the afternoon sun, creates on-site
stormwater control, and enhances habitat for native songbirds and
The Big Picture
Within our generative landscape, rainwater has become our primary water
source, greywater our secondary water source, and municipal groundwater
a strictly and infrequently used supplemental source (meeting no more
than 5% of our exterior water needs). Most of our established landscape
has even become regenerative by thriving on rainwater alone.
Our household consumes less than 20,000 gallons of municipal water
annually, with over 90% of that being recycled in the landscape as
greywater. Additionally, we harvest and infiltrate over 100,000 gallons
of rain and runoff into the soil of our site (and, by extension, the
community’s watershed) over the course of our annual average rainfall.
As a household, we’re shifting more and more to living within our
rainwater “budget”: the natural limits of our local environment. As a
result, we’re enriching the land, growing up to 25% of our food on
site, creating a beautiful home and neighborhood environment – and
giving back more than we take!
The further we go, the easier and more fun it gets, which brings us to the eighth and last principle:
8. Continually reassess your system and improve it.
Three years ago, Rodd and I set up an outdoor shower so the bather
could either use pressurized municipal water at the showerhead or
cistern water distributed from a shower bucket on a hook. Other
strategies have included a solar-powered greywater “laundromat” in our
backyard (utilized by seven neighboring households) along with a
reduction in impermeable hardscape by replacing our asphalt driveway
with lush plantings and earthworks.
One of our most rewarding recent improvements has been the process
of working with our neighbors and the city to replace 26% of the
pavement from the corner intersection with a water-harvesting traffic
circle planted with native vegetation. We also succeeded in
implementing a system that harvests street runoff within curbside
mulched basins to grow a greenbelt of trees along the street and
sidewalk, so the street now passively irrigates the trees.
As a result, our neighborhood—once the victim of urban blight—is now one of the greenest and most livable areas of the city.
My advice to anyone who wants to get started living more sustainably
is to start with rainwater-harvesting. Start at the top. Start small.
But above all—start!
Sidebar: Choosing a Rainwater Cistern
Our cistern has a 1,200-gallon (4,560 liter) capacity. We selected this
size after calculating the average annual roof runoff, assessing our
water needs, and determining the resources we wanted to commit to the
system. We opted for a precast concrete septic tank for a number of
reasons, but primarily because it was affordable as well as a workable
size and shape for our space (5 foot wide, 6 feet tall, 10 feet long).
Our septic tank was custom-made for use as a cistern, and further
reinforced for above-ground installation. The cost back in 1996 was
$600, which included delivery and placement. It’s been working great
Other options for pre-manufactured cisterns include light-free dark
green or black polyurethane plastic, corrugated metal, and fiberglass.
See www.watertanks.com for options and look in the yellow pages under tanks for local suppliers.
Calculating Your Rainwater Resources
To calculate the volume of rain falling in an average year on a
specific surface such as your roof, yard, or neighborhood, use the
following calculation: CATCHMENT AREA (in square feet) multiplied by
the AVERAGE ANNUAL RAINFALL (in feet) multiplied by 7.48 (to convert
cubic feet to gallons) equals the TOTAL RAINWATER FALLING ON THAT
CATCHMENT IN AN AVERAGE YEAR: CATCHMENT AREA (ft2) x RAINFALL (ft) x
7.48 gal/ft3 = TOTAL AVAILABLE RAINWATER (gal/year).