Fall 2024
Planting the Rain: A Guide for Passive Rainwater Harvesting
by Lori Palmquist
Autumn has arrived, and we anticipate (actually, hope for) a healthy bounty of liquid refreshment from the sky this season and the next. We all know that fall is the best time to plant. It’s also the best time to start planting the rain.
I’d like to pose a question to you, my fellow Californian. Why do we insist on irrigating our landscapes with water collected in the Sierra, pumped through pipes for hundreds of miles, and then treated with chemicals to meet human drinking water standards? Does this seem even a little nonsensical to you? If that were the only water available to us, it would make perfect sense.
But it’s not the only water source available to us. And the alternative I’m suggesting is free, local, has no chemicals, and is much easier to tap into than you think. It’s known as rainwater. And it’s that water that customarily runs off our landscapes, roofs, driveways, sidewalks, and streets, right into the concrete funnel, (AKA storm drain), that carries it directly to our local creeks and ultimately lands in the bay or the delta, unfiltered and untreated.
So, how can we catch and store that water so we can use it during the dry season to water our landscapes rather than sending it to the creeks, bay, and delta? First and foremost, the biggest water tank you have on your property is your landscape itself. Secondly, you can purchase cisterns, tanks, or barrels to catch and hold onto the water.
Rain tends to fall fairly reliably in our county during the months of November through April. Concord is near the center of the county (on an east-west axis), so I’ll use this city as my reference for the following analysis. I’ll also use the U.S. Climate Data website to gather historical rainfall totals for those months.
https://www.usclimatedata.com/climate/california/united-states/3174
The chart below shows the number of inches of rain that fall historically, alongside the number of inches of water requirement for moderate-water plants and low-water plants during the rainy season.
What we’re looking for is excess rainfall that gets captured passively by the landscape. By passive, I mean you’re using your landscape itself as your storage tank. And we’re talking inches, not gallons of water. Rainfall is measured in inches. And conveniently, plant water requirement is measured in inches as well. For this discussion, we’ll deal exclusively in inches. If we were to talk about cisterns, tanks, and barrels, we’d have to talk in gallons. And that complicates things unnecessarily. So let’s keep it simple.
If you’d like to see the actual number of inches represented by the chart above, they’re provided in the three tables below. Looking at the chart above, we can see that for a moderate-water landscape, we have excess water stored in the landscape November through March. And for a low-water landscape, the excess water stored is from November through April.
Data for the Chart Above
Looking at the numbers above, we can see that for a moderate-water landscape, there is an excess of 7.12 (15.32 minus 8.2) inches of rainwater to use for the dry season. And low-water landscapes have a whopping 12.04 (15.32 minus 3.28) inches of rainwater available.
Now let’s look at the two water-requirement tables below to see how far into the dry season the excess rainfall would take a moderate-water landscape and a low-water landscape.
Plant Water Requirement
Is it Really That Simple?
Of course, storing water in the landscape is not as simple as filling up a container and then using that water in the container to water the plants. There are many factors that come into play: soil type, water-holding capacity of the soil, slopes, whether there are levels of compaction, etc. In the analysis above, I simply stated how many inches of rain occur historically and how many inches of water plants require historically.
Mulch
The fact is that enough rain falls in our county to provide for low-water landscapes all the way through the dry season. Mulch is a huge factor in making that happen. Please see the article I wrote in the previous issue of this newsletter. I explain mulch’s magical properties that not only turn the landscape into a water container but essentially put a lid on the landscape to hold the water in. https://ccmg.ucanr.edu/News_to_Grow_By/?story=2768
Three to four inches of mulch alone should suffice to provide landscape-stored rainwater well into the summer and to suppress weeds to a great extent. The best type of mulch to use for this purpose is freshly chipped tree trimmings. They can be obtained for free from local arborists. And please do put these arbor chips in direct contact with the soil, with no synthetic weed barrier between the mulch and the soil. Weed cloth or cardboard will prevent rainfall from seeping into the landscape to be stored.
Mulch, low-water plants, and rainfall are the trifecta of ultimate water conservation in the landscape. And if you were also to direct the rain gutters onto the landscape, you could sculpt the landscape gently to slow the water down, spread it out, sink it down, and store it. Swales, dry creek beds, and rain gardens are great ways to grab the roof water and put it to good use. And, of course, cisterns, tanks, and barrels could be connected to rain gutters as well. These would be icing on the cake.
Conclusion
Are you ready to plant the rain this fall and winter? Can you see how easy this is? I invite you to start with a healthy three- to four-inch layer of arbor chips before the rainy season starts. This alone will help you wean your landscape off the drinking water supply. It’s a win-win situation for all. Your water bill goes down, your plants thrive, and the benefits just keep rolling in.
Resources
Mulches for Landscapes, a UCANR publication
https://anrcatalog.ucanr.edu/pdf/8672.pdf