An Atmospheric River May Hit the West Coast Next Week: The Runoff and Drainage Math for Your Garden

The West Coast has a storm to plan around. In a November 4 report for the Sacramento Bee, Sean Macaday pointed to roughly November 11 to 16 as the likely window for Central California's first atmospheric river of the season. Early projections show Sacramento getting close to its full November average, nearly 2 inches, in a day or two. Farther north, Meteored US reports early modeling of 4 to 6 inches of rain for the Cascades. NOAA's Climate Prediction Center has flagged heavy rain risk for parts of the region, too.

These are early numbers, and reports already disagree. UNILAD notes that the latest guidance may lower rain volumes in Washington. So don't plan around one total. Plan around the formula below, and put your local forecast into it when the National Weather Service publishes one. For most gardens, the biggest number won't be the rain falling on the bed. It will be the water your roof sends toward it.

Step One: Compare the Bed to the Roof

One inch of rain on one square foot is about 0.623 gallons. That conversion ignores small losses, so treat the results as upper estimates. Here is a 2-inch storm on two surfaces:

• A 4-by-8-foot bed: 32 × 2 × 0.623 = about 40 gallons.
• An 800-square-foot roof section feeding one downspout: 800 × 2 × 0.623 = about 1,000 gallons.

That one downspout carries 25 times what the bed receives from the sky, because the roof is 25 times larger. For a 5-inch storm, like the middle of the early Cascades range, the same roof sheds about 2,500 gallons. Before you buy anything, walk the house and note where each downspout lets out. If one empties beside a bed, moving it will likely do more than any soil work you could finish this week.

Step Two: Spread the Water, Then Do the Division

The fix depends on how much ground takes the water. To find the depth, divide the roof area by the receiving area, multiply by the storm total, and add the rain that falls directly on that ground. Then divide by your soil's drainage rate to get a rough number of hours:

Hours to drain = ((roof sq ft ÷ receiving sq ft × storm inches) + storm inches) ÷ inches per hour

Using the 800-square-foot roof and the 2-inch storm:

• Into a 100-square-foot patch: 16 + 2 = 18 inches. That takes 36 hours at ½ inch per hour and 72 hours at ¼ inch per hour.
• Into a 400-square-foot lawn area: 4 + 2 = 6 inches. That takes 12 hours at ½ inch per hour and 24 hours at ¼ inch per hour.

A downspout extension or a shallow swale that moves water to a larger area sloping away from your house, beds, and neighbors cuts the time by two-thirds in this example. The result is only a planning number, not a hydrology model, since saturated soil and sloped ground won't behave this neatly. It still shows which fix matters most.

Without the roof, the math is much less alarming. Two inches spread evenly over 24 hours is about 0.08 inches per hour, well under the rate of even slow soil. Actual storms come in bursts, and hard, dry ground can shed water. NWS director Ken Graham warned in NOAA's winter outlook that too much rain on dry soil can fuel flash floods. Standing water in a bed usually needs a second cause, though, and that is often concentrated runoff, a slow soil layer, or both.

Step Three: Measure Your Drainage Rate

Those hours depend on a number only your yard can give you. The Minnesota Horticultural Society's percolation test needs a shovel and a ruler:

1. Dig a straight-sided hole 12 inches deep.
2. Fill it with water and let it drain completely.
3. Fill it again and record the water level every hour.

The society rates less than ½ inch per hour as poorly drained, 1 to 2 inches as good, and more than 3 inches as too fast. Test several spots, since results can vary across a yard. Water also leaves a hole through its sides, so the reading may overstate how fast a large area drains. Use it to compare spots rather than as an exact forecast. Macaday expected California's next seven or so days to stay dry, which leaves time to do this there. Parts of Washington may not get the same break.

Step Four: Size the Fix for a Bed That Stays Wet

A test in dry soil can't show everything, so check again during the first wet stretch. A day after heavy rain, dig a test hole beside the bed and measure down to any standing water. That depth is your drained root zone. To find how much height to add, subtract it from your target:

Inches to add = target root zone − drained depth

Say you want 12 inches of drained soil, and water stands 6 inches down. You need 6 more inches of bed. On a 4-by-8-foot bed, that is 32 × 0.5 = 16 cubic feet, or about 0.59 cubic yards. In bags, it comes to 11 at 1.5 cubic feet each, or 8 at 2 cubic feet each. The 12-inch target is only an example, so adjust it for what you grow.

This is a project for after the storm if time runs short. Moving a downspout takes minutes, while adding soil over planted crops takes planning. The Royal Horticultural Society recommends raised beds on poorly drained ground, and it notes that waterlogging generally does less harm in cold winter soil than in warm summer soil. A short soaking now is usually less serious than the same soaking in June. Look for prolonged saturation, plants that wilt in wet soil, and roots that turn blue-black or smell sour.

Step Five: Barrels and Timers

Rain barrels won't solve a flooding problem. A 55-gallon barrel under that 800-square-foot roof is full after 55 ÷ (800 × 0.623) = about 0.11 inches of rain, and a 1,000-gallon storm would fill 18 of them. What a barrel can do is store water for later. Its 55 gallons equal about 2.75 inches of water on a 4-by-8-foot bed. That's worth having if the dry spell returns, so check that the overflow points away from the house and beds. (Our rain barrel math covers sizing.)

Next, look at your irrigation timer. Peyton Ellas, a UCCE Master Gardener, wrote in the Mid Valley Times that a controller with a water budget setting should run at 50 percent or less of its July level in November. She adds that it takes more than a half-inch of rain to soak the root zone of established trees and shrubs. A 2-inch storm clears that bar, so pause the system and use our rainfall conversion to decide when it should run again.

Step Six: Stay Off the Beds Afterward

The last rule is the cheapest. Clallam County Master Gardener Susan Kalmar warns that working soil that is too wet can cause long-term damage. Her test: squeeze a handful from 4 to 6 inches down. If it oozes water or forms a sticky ball, wait. If it crumbles when poked or dropped, it's workable. Until then, stay on the paths and use boards where you have to step near a bed. She also notes that raised beds usually dry faster than in-ground soil, so test each area separately.

If you don't live on the West Coast, keep these formulas anyway. NOAA's winter outlook favors wetter-than-average conditions from the Southwest to the Southeast and along the East Coast, though those are odds and not promises. Gardeners in those regions can do this work before the first big storm arrives. (If drought has been your main problem, our subsoil recharge math shows how much of that rain a dry bed can bank.)

Quick Recap

• Forecast totals are early and still changing. Use your local National Weather Service forecast in the formulas.
• Gallons = square feet × inches of rain × 0.623. A 2-inch storm puts about 40 gallons on a 4-by-8 bed and sends about 1,000 gallons off an 800-square-foot roof section.
• Spreading that runoff over 400 square feet instead of 100 cuts the depth from 18 inches to 6, and the rough drain time by two-thirds.
• Test drainage with a 12-inch hole: under ½ inch per hour is poor, and 1 to 2 inches is good.
• Adding 6 inches to a 4-by-8 bed takes 16 cubic feet of soil, or 11 bags at 1.5 cubic feet each.
• Pause irrigation for the storm, and stay off beds until a handful of soil crumbles.

You can't control how much rain falls next week, but you can control where the roof's share of it goes. Check your downspouts this weekend, and that one job may solve most of the problem.