Sink it or shed it? When to keep rain on your land and when to drain it

By The Downspout · Posted Oct 12, 2026 · 9 min read

Most properties need both. Sink clean water where the soil can take it and nothing will be harmed. Shed water, slowly and safely, where soaking it in would threaten a foundation, a slope, a septic system or a neighbor. The craft is deciding which water goes where.

Start with your own roof: the rain catch calculator shows how many gallons it sheds in a year. One inch of rain on a 2,000 square foot roof is about 1,250 gallons.

Two philosophies of rain

For most of the last century, the goal was simple: get water off the property fast. Gutters to pipes, pipes to the street, the street to the creek.^[1]^[3]

Sonoma County's Resource Conservation District puts it bluntly. That approach "only shifted problems downstream."^[1]

The newer idea runs the other way: slow it, spread it, sink it, store it. Treat rain as something to hold, not get rid of.^[1]^[3]

Both ideas are right somewhere. Neither is right everywhere.

	Sink it	Shed it
Goal	Hold water in the soil	Move water safely off site
Good for	Groundwater, plants, creeks downstream	Foundations, slopes, septic systems
Tools	Rain gardens, basins, level swales, mulch	Sloped swales, drains, pipes, safe outlets
Fails when	Soil will not drain, or water lands in the wrong place	Water leaves fast and dirty, or lands on a neighbor

What paving does to rain

Source: California NEMO fact sheet.^[4] Soaks in = shallow plus deep infiltration.

On natural ground, about half of the rain soaks in and only a tenth runs off.^[4]^[1]
When three quarters or more of the ground is hard, runoff jumps to 55%.^[4]

What that means on a quarter acre

Ground	Runs off	Soaks in
Natural ground	about 23,000 gal	about 115,000 gal
35 to 50% paved	about 69,000 gal	about 80,000 gal
75 to 100% paved	about 126,000 gal	about 34,000 gal

Our calculation from NOAA's Santa Rosa normal (33.78 inches a year, 1991 to 2020)^[5] and the fact sheet's percentages.^[4] Method: 10,890 sq ft x 33.78 in x 0.623 gallons per sq ft per inch = about 229,000 gallons a year, times each share. A rough guide: the split is a general one, not measured on any one lot.

Paving over most of a lot sends about five and a half times as much water off it in an average Santa Rosa year. That water has to go somewhere.

What runoff carries

Rain starts out fairly clean. Running over roofs, streets and yards, it picks up sediment, chemicals, fertilizers and pesticides.^[1]^[6]
In Sonoma County, outdoor drains carry it straight to the nearest creek, untreated.^[1]^[6] See only rain down the storm drain.
Fast water also moves soil. Bare, steep ground is the most at risk.^[1]

Shedding water is not neutral. It moves the load downstream: to the creek, the salmon gravels and the next property.

When to sink it

Clean water. Roof water is the easiest to soak in. Driveway water can carry oil, grease and soap.^[1] Our advice: run it through gravel or plants first.
Soil that drains. Heavy clay drains poorly and suits few soak-in features.^[1]^[2] A simple test: in the rainy season, dig a hole 6 inches deep and fill it with water. If it is gone within 24 hours, the soil can take a rain garden.^[2] The free USDA Web Soil Survey shows your mapped soil type.
Room to work. Keep a rain garden at least 10 feet from the house.^[1]^[2] Keep it 25 to 40 feet from a septic system, depending on the source, and 40 feet from a steep slope.^[1]^[2]
Time. Standing water should be gone within two to three days.^[1]^[2] Mosquitoes take about five to seven days to go from egg to adult.^[7]

The rewards: water stored in the soil for plants, a recharged water table, and less sediment carried to the creek.^[1] See can your yard recharge groundwater?

When to shed it

Heavy clay or a high winter water table. The ground is already full. Soak-in features there need an overflow, or should not be built at all.^[1]^[2]
Near buildings. California's residential code requires the ground to fall at least 6 inches in the first 10 feet away from a foundation.^[8]
Hillsides. Water from downspouts, irrigation and poor drainage can help set off landslides. Send it well away from unstable slopes.^[10]^[1]
Septic systems. Keep rain and surface water off the drainfield.^[9]^[1]
Dirty water. Our advice: do not soak oil, fuel or chemicals into the ground. Clean up spills instead of washing them away.

Shedding well is still a craft. Slow the flow, spread it out, armor the spot where it lands with rock, and send it to a stable outlet, not your neighbor's yard.

Most yards do both

Our chart, drawn from published siting rules.^[1]^[2]^[8] A guide for thinking, not a site design.

Think of your property as a small watershed. The roof might feed a rain garden. The driveway might drain through gravel to a planted strip. The steep bank behind the house might need a lined channel and a rock outlet.

Brad Lancaster, author of Rainwater Harvesting for Drylands and Beyond, names the bridge between the two ideas in his fifth principle: "Always plan for an overflow route, and manage that overflow water as a resource."^[11]

Every sink needs a shed for the storm it cannot hold.

One word, two jobs: the swale

Swales built across the flow of water^[12] and conveyance swales sloped at least 2%.^[1] "Level swale" is our name for the first kind.

Rainwater harvesting books often mean the first kind. Stormwater manuals often mean the second. If a designer says "swale," ask which. More in our full guide to swales: what they do, how to build one, and when not to.

The rules

California neighbor law. In Keys v. Romley (1966), the California Supreme Court said every landowner must "take reasonable care" with surface water. The lower owner must also take "reasonable precautions." If both act reasonably, the loss falls on the one who changed the natural drainage.^[13] This is not legal advice.
Unincorporated Sonoma County. A drainage permit is needed before any drainage improvement, and grading over 50 cubic yards needs a grading permit.^[14]^[15] Details: when you need a county permit to change drainage.
Creek setbacks. On smaller, non-designated streams, the county keeps grading at least 25 feet back from the top of the bank. Designated streams carry setbacks of 50, 100 or 200 feet. Septic systems sit 50 to 100 feet back.^[16]
Cities. Each city has its own rules. In the City of Sonoma, a home project that creates or replaces 2,500 square feet or more of hard surface needs a stormwater control plan.^[17]
Big projects. Disturbing one acre or more of soil needs coverage under the state's Construction General Permit.^[18]
Catching rain. Rooftop rain capture is legal in California. See are rain barrels legal in California? Not every state is as open. Colorado passed a law in 2016 that lets homes use up to two rain barrels, 110 gallons in all.^[19]

Words worth knowing

Infiltration. Water soaking into the soil.
Impervious surface. Anything water cannot soak through: roofs, concrete, asphalt, packed dirt.
Sheet flow and concentrated flow. A thin film of water across the ground versus water gathered into a stream. Sheet flow soaks in. Concentrated flow cuts gullies.
Green and gray infrastructure. Plants and soil that soak up runoff, versus pipes that carry it away.^[3]
Low impact development. Managing runoff close to where it falls, often what city stormwater rules require.
Berm and basin. A low mound with a shallow dip uphill of it, built across the flow to catch water.^[12]
Overflow, or spillway. The planned exit for water a feature cannot hold.^[11]
Energy dissipater. Rock or another hard surface where fast water lands, so it does not dig a hole.
Induced meandering. Bill Zeedyk's method of nudging an eroded channel to rebuild its own floodplain. As one review puts it: "intentionally eroding selected banks while encouraging the deposit of sediment."^[20]

Going deeper

Slow It. Spread It. Sink It. Store It! Sonoma Resource Conservation District. Free, local and practical.
The Eight Rainwater-Harvesting Principles, Brad Lancaster. His books go much further.
Rain Garden Design for Homeowners, University of Nebraska-Lincoln Extension. Sizing and soil tests.
Passive Water Harvesting, University of Arizona Extension. Berms, basins and French drains, with drawings.
Let the Water Do the Work, Bill Zeedyk and Van Clothier. Healing eroded gullies and streams.

What it means for you

Our view: soak in what your land can safely hold, and shed the rest slowly and cleanly. Every gallon that sinks in is a gallon that is not carrying soil to the creek.

Walk your property in the next big storm. Watch where water gathers, where it runs fast, and where it disappears.
Start at the top, with the roof, and start small.^[11] A downspout turned toward a mulched bed is a real first step.
Get help for anything near a slope, a foundation or a septic system.

Next: the step by step guide to slowing, spreading and sinking rain.

Sources

Checked 2026-10-12. Rules and programs change. If something here is out of date, the source above wins. Spot an error? See our corrections policy.