Every monsoon, rain that could water your trees runs off into the street. Here's how curb cuts, basins, swales and a few other simple pieces catch it, slow it and sink it where your plants can use it.
Permascaping guide Β· About a 15 minute read Β· Based on the Greater Phoenix Metro Green Infrastructure Handbook (2019), adapted for home yards
Green stormwater infrastructure, or GSI, is a fancy name for a simple idea: treat rain like the resource it is. Instead of rushing water off roofs, driveways and streets into storm drains, you guide it into the ground where it lands.
In the desert that matters more than almost anywhere. We get most of our rain in a few short, heavy storms. Hard ground sheds it fast, it floods low spots, and then it's gone. A yard built to catch it stays cooler, greener and needs a lot less from the hose.
This guide walks through each piece of the toolkit: what it is, where it goes and the rules of thumb for sizing it. Every piece can be placed on your own yard in our free watershed planner.
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Why it matters in the desert
Our rain comes fast and leaves fast. GSI gives it somewhere to go.
Monsoons dump it all at once
An inch of rain on a 2,000 square foot roof is about 1,200 gallons. In a monsoon that can fall in under an hour, and most yards let nearly all of it run into the street.
Shade and cooler streets
Trees watered by rain grow bigger and faster. Their shade cools sidewalks, walls and roofs, and that's one of the best tools we have against the urban heat island.
Cleaner water, less flooding
Water that soaks in near where it lands doesn't pile up downstream. Soil and roots also filter the oil, dust and litter that rain picks up on the way.
Rain running down a Phoenix street. Every gallon here could have watered a tree.
The five steps
Every good GSI plan follows the same order, from the roof down.
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Watch the water
Go out in the next storm (or run a hose) and see where water actually goes. Note the downspouts, the low spots, where it leaves the yard and where it puddles. That map is your plan.
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Slow it down
Fast water erodes and carries soil away. Rock aprons, check dams and mulch take the energy out of it so it can soak in instead of scour.
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Spread it out
Send water across the yard with gentle swales and shallow grading, so more ground gets a drink and no one spot floods.
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Sink it in
Shallow basins (rain gardens) hold water long enough to soak in. That's where the trees and shrubs go, so the water lands right at their roots.
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Plant and mulch it
Plants are what make it work. Roots open up the soil, mulch keeps it from crusting, and shade keeps the water from evaporating. Start small, add more each season.
The toolkit
Most yards use a few of these, not all of them. Start with where your water already goes.
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Curb cuts
An opening cut in the curb so water running down the gutter flows into a planted basin behind it instead of the storm drain.
Where:Along the street, upslope of the basin, where gutter flow actually runs in a storm.
Rules of thumb:At least 2 feet wide so it doesn't clog. Saw-cut the curb, and slope the floor down into the yard with at least a 2-inch drop. Put a sediment trap or rock pad right inside so the edge doesn't wash out. In the right-of-way it needs the city's okay.
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Sediment traps
A small pad right where water enters a basin. Street grit, leaves and trash drop out there instead of clogging the basin.
Where:Just inside every curb cut and at the end of any pipe or channel that carries water from paving.
Rules of thumb:A flat concrete pad with a low lip is easiest to clean; a rock bottom is hard to shovel out. Clean it at least twice a year and after any storm over half an inch.
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Rain gardens (basins)
A shallow, mulched dip that holds water for a few hours while it soaks in. The heart of any GSI plan.
Where:Wherever water already flows or can be sent: below downspouts, along driveways, at the low side of the yard. Several small basins work better than one big one. Keep them at least 10 feet from your foundation.
Rules of thumb:About 9 to 12 inches of water, plus 3 more inches of rim. Sides no steeper than 3 to 1. Standing water should be gone within 36 hours. Keep plants out of the spot where water pours in, and skip the filter fabric.
A mulched basin with a tree on its rim: the water soaks in right at the roots.
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Swales
A long, gentle channel, usually planted and mulched, that moves water across the yard and lets it soak in along the way.
Where:Along a slope or a wall, to carry water from where it lands to where you want it.
Rules of thumb:Keep the grade gentle so water creeps rather than rushes, with sides no steeper than 3 to 1. Add check dams on longer runs.
A planted swale carrying runoff along a parking lot. Photo: Wikimedia Commons, public domain.
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Check dams
Low lines of rock set across a swale or a slope. Each one makes a small pool that slows water and drops its silt.
Where:In swales and washes, every so often down the slope.
Rules of thumb:Angular rock at least 3 inches across, no taller than 2 feet. Make the middle about 4 inches lower so water spills over the center, not around the ends. Space them so the bottom of each dam is level with the top of the next one down.
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Rock aprons and splash pads
A pad of rock where water lands hard: under a downspout, at a curb cut or where a pipe empties.
Where:At every point water drops or speeds up.
Rules of thumb:Use rock big enough that a storm won't move it, and set it into the soil, not on top.
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Overflow spillways
A low, rock-lined notch in a basin's rim where water leaves once the basin is full.
Where:On the downhill side of every basin, pointed at the next basin or a safe place, never at a wall or a neighbor.
Rules of thumb:Line it with rock so it doesn't erode, and set it at least 3 inches below any nearby sidewalk or patio. Plan where the overflow goes before you dig.
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Sidewalk scuppers
A short covered channel under a sidewalk (sometimes called a curb core) that carries water from the street side to a basin on the other side.
Where:Where a sidewalk sits between the street and the spot that can hold water.
Rules of thumb:Work in the public right-of-way, so it needs a permit and a contractor who has done them.
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Permeable paving
Pavers, pervious concrete or gravel laid over a stone base so water soaks through instead of running off.
Where:Patios, paths, parking pads and driveways.
Rules of thumb:Use at least 6 inches of clean crushed stone underneath and keep the slope gentle. Never put decomposed granite on or next to it, since the fines clog the gaps. Sweep it twice a year.
Permeable pavers in an alley soak up rain instead of shedding it. Photo: U.S. EPA, public domain.
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Dry wells
A gravel-filled pit that lets water soak deeper into the ground where the surface soil is too tight or there's no room for a basin.
Where:Where water collects and can't drain, after you've tried a basin.
Rules of thumb:Put a sediment trap ahead of it so it doesn't clog. In Arizona, dry wells have to be registered with ADEQ, so check the rules before you dig one.
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Rain tanks
A tank that stores roof water to use between storms, then overflows into a basin when it's full.
Where:Under a downspout, on a level, solid base, close to what it will water.
Rules of thumb:Screen the inlet, cover it against mosquitoes and light, and send the overflow to a basin. Size it in our planner from your roof and what you water.
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Mulch and plants
The living part that makes everything else work: roots open the soil and mulch keeps it soft, shaded and soaking.
Where:In and around every basin and swale.
Rules of thumb:About 3 inches of wood mulch where plants need water, rock where water runs fast. Keep mulch away from tree trunks, and let fallen leaves stay as free mulch.
Every piece here can be placed on your own yard in the watershed planner: draw your roofs and gutters, then drop in curb cuts, sediment traps, basins and the rest to see how much water each one catches.
Good enough to plan with. A design review checks them against your soil and slopes.
What
Rule of thumb
Why
How much rain to catch
The first half inch of a storm, from everything that drains to the basin. More if you have room, up to about 1ΒΌ inches.
About 82% of Valley storms are half an inch or less, and that first flush carries most of the grit and oil.
Water depth
9 to 12 inches of water, plus 3 inches of spare rim
Deep enough to hold a real storm, shallow enough to soak in fast and stay safe.
Drain time
Standing water gone within 36 hours
Mosquitoes need several days of standing water. Most Valley soils soak in about ΒΌ to ΒΎ inch an hour, so test yours first.
Side slopes
No steeper than 3 to 1 where planted: 3 feet across for every foot down
Gentle sides stay put, plant easily and are safe to walk near.
Distance from buildings
At least 10 feet from foundations and property lines
Water soaking in next to a footing can settle or crack it, and overflow shouldn't land on a neighbor.
Roof runoff
About 0.62 gallons per square foot of roof for each inch of rain
A 2,000 square foot roof sheds about 620 gallons in a half-inch storm.
These come from the Greater Phoenix Metro Green Infrastructure Handbook, written for public projects. In a home yard they're a safe starting point, not a code. To test your soil, dig a hole a foot deep, fill it with water and let it drain, then fill it again and time how fast the level drops.
Plants for wet and dry
A basin isn't wet all the time. It's soaked for a day and bone dry for weeks, so plants above the water line and below it need different strengths.
The rim: trees and big shrubs
The rim stays driest, but roots reach right into the wet zone. That's the spot for velvet mesquite, blue palo verde, ironwood, desert willow and fruit trees.
The upper sides: tough shrubs
Above the water line, the sides get wet often and dry fast. Brittlebush, jojoba, Texas ranger, salvias and penstemon handle both. Let them grow to their natural shape instead of shearing them.
The bottom: plants that can take a soak
Below the water line, the bottom floods in storms and bakes in between. Deer grass, pink muhly, chuparosa, desert hackberry and quail bush can take it, or just a thick layer of mulch.
Rain gardens a few years in: the trees sit on the rims, with tougher plants down in the basins.
Street-side work and permits
Curb cuts and basins in the strip by the street are some of the most effective pieces, and the ones that need the city's okay.
Curb cuts, sidewalk scuppers and basins in the strip between the sidewalk and the street are in the public right-of-way. Check with your city's street department before any concrete is cut. In Phoenix that's Street Transportation, and other Valley cities have their own versions.
Before you dig anywhere, contact Arizona 811 at least two working days ahead so buried gas, water, power and communication lines get marked. It's free and it's the law. Leave the flags in place and dig by hand near them.
Keep basins clear of sidewalks so people can still walk by, leave room around fire hydrants, meters and utility boxes, and make sure the overflow can't send water back toward a house.
A street before and after green infrastructure: planted basins along the curb catch the runoff. Photo: U.S. EPA, public domain.
Common mistakes
Most problems come from a few easy-to-avoid habits.
MythDeeper basins hold more, so deeper is better.
ActuallyShallow, wide basins soak water in faster and don't hold it long enough for mosquitoes. A foot of water at most, spread over more ground, beats a deep hole.
MythPut the basin right next to the house so the downspout drains straight in.
ActuallyWater against a foundation can cause cracking and settling. Run the downspout out to a basin at least 10 feet from the house.
MythRock looks clean, so line the whole basin with it.
ActuallyRock heats up and holds no water. Use it where water lands hard, and wood mulch where you want plants to thrive.
MythMore basins means more problems with mosquitoes.
ActuallyA basin that drains within a day and a half can't breed mosquitoes, which need several days. Standing water for days means the basin's too deep or the soil's sealed.
MythIt's a one-time project.
ActuallyClear sediment traps after big storms, top up mulch once a year and keep inlets open. A few minutes after each storm keeps it all working.
Questions
Do I need a permit for a basin in my own yard?
Usually not for a basin or swale on your own property. You do need the city's okay for anything in the right-of-way, like curb cuts and street-side basins, and HOAs may have their own rules.
How big should my basin be?
Start from the area that drains to it: each 100 square feet of roof sheds about 62 gallons per inch of rain. Our watershed planner works it out from your own roof.
Will water near my house cause damage?
It can if it pools against the foundation. Keep basins at least 10 feet from the house and make sure the ground slopes away from walls.
What about mosquitoes?
Basins that drain within 36 hours don't breed them. Cover rain tanks and screen their inlets.
Can I do this myself?
Most of it, yes. Basins, swales, check dams and mulch are hand-tool work. Curb cuts, sidewalk scuppers and anything structural are best left to a contractor.
Where should I start?
Watch one storm, then build one basin below one downspout and plant it. Learn from that one and grow from there.
