Winter 2025
Detaining the Rain
by Lori Palmquist
This article is the third of a three-part series of articles. The goal is to convince you that enough rain falls in Contra Costa County to sustain your landscape either well into the dry season, or all the way through it, depending on where you live in the county and how much water your landscape requires.
The central theme and goals of these three articles is the following:
- Capture rainwater passively using the landscape itself as a container (“Planting the Rain: A Guide for Passive Rainwater Harvesting”, fall issue of News to Grow By.) *
- Hold onto captured water as long as possible by putting a “lid” of mulch on the landscape “container” (“The Magic of Mulch”, summer issue of News to Grow By and webinar available on our YouTube channel.) *
- Capture roof water in barrels, tanks, or cisterns for use if/when the landscape “container” is empty (“Detaining the Rain”, discussed in this article).
(*) The links to these articles and the webinar can be found at the end of this article.
The focus of this article is on capturing and storing rainwater in barrels, tanks, and cisterns. Unlike passive storage, this method requires a bit of planning, designing, purchasing components, and careful installation. It’s particularly useful for properties on a slope where water naturally runs downhill when not actively detained.
Rainwater harvesting does sound cool. But other than being cool, why should we do it? Let’s begin by addressing why we should even care about putting forth the effort to capture and store rainwater in the first place.
In the past several years we have been experiencing wild swings in our climate between drought and deluge. Furthermore, the droughts are getting longer and drier and the deluges are getting fiercer. Each end of this scale carries consequences that have us scrambling to deal with either the lack of or the overabundance of water.
Drought is a pervasive reality in California. The following graphic shows the state’s drought profile from the Drought Monitor website https://droughtmonitor.unl.edu/Maps/MapArchive.aspx
for most of this century (2001-2021). The graphics show a snapshot of the level of drought in November for each of the indicated years.
Here is a guide to the color-coding of the graphics:
Yellow: Abnormally dry
Beige: Moderate drought
Orange: Severe drought
Red: Extreme drought
Dark brown: Exceptional drought
The important thing to note is that in all but 5 years out of the first 21 years in this century (2005, 2006, 2010, 2011, 2017), California was in some level of drought. That’s 76% of the time!
That’s one end of the scale. At the other end, we have experienced bumper crops of rainfall in the past two winters, with deluge in the form of atmospheric rivers wreaking havoc by causing flooding, landslides, fallen trees, property damage, and other storm-driven consequences.
Capturing rainwater is a solution to both drought and deluge. During drought, stored water can keep our gardens alive and thriving when the drinking water supply is diminished and/or restricted. During a deluge, it helps to reduce the overwhelm of our stormwater management systems and protects against the calamities caused by floods. In both cases, rainwater harvesting reaps and stores a precious resource essential to all life.
When we’re talking about creative ways to provide water to your landscape during the dry season (generally May through October) it’s a great idea to establish just how much water your landscape needs to not only survive but to thrive. How much water does your landscape require during the dry season? It’s best to think of this number in terms of inches. Rainfall is measured in inches. Likewise, plant water requirement is measured in inches. And when we’re talking about inches of water, this pertains to any size of the area of your property. It could be a few plants or the whole property. We only need to convert to gallons once we start trapping water in containers. So let’s keep it simple and talk inches for now.
The tables below show the plant water requirement in inches per month during the dry season for representative cities in the three climatic regions of our county. The top table is for moderate-water plants, and the bottom table is for low-water plants.
Below is the amount of rainfall that these cities get historically. The data was acquired from the US climate data website https://www.usclimatedata.com/climate/california/united-states/3174.
Average annual rainfall
Richmond: 24.9 inches
Concord: 16.5 inches
Brentwood 13.2 inches
Comparing the annual rainfall with the amount of water required by the landscape, we can see that both Concord and Richmond get enough rainfall to provide for low-water and moderate-water plantings all the way through the dry season. The only shortfall above is that Brentwood’s annual rainfall comes up short by almost five inches of rain for moderate-water plantings (13.2 as opposed to 18).
Now we can start talking in gallons. One inch of rain falling on 1,000 square feet of roof will yield 620 gallons. Here is the formula for calculating rainwater yield off a roof:
Gallons of rainwater = Inches of rain x Roof square footage x 0.62
Common sizes of rain barrels range from 50 to 90 gallons. So storage space is gobbled up quickly when using rain barrels. It would take more than 12 fifty-gallon barrels to store one inch of rain from a 1,000 square-foot roof (620 gallons 50 gallons per barrel = 12.4 barrels). Slimline tanks are taller and narrower, and sit up against a building to conserve space. They hold around 200 to 300 gallons. So you would need at least two of those to store one inch of rain from a 1,000-square-foot roof.
Sometimes, water districts offer rebates on rain barrels, but currently, neither EBMUD nor Contra Costa Water District offers them. The cost of rain barrels and tanks ranges from $1 to $3 per gallon of capacity. Permits may also be required for their installation in certain cities and counties. Check with your local agency to get information on codes and regulations. If the barrel or tank requires a concrete base to sit on, or that it be strapped to a building for stability, you’ll need to factor in the cost of hiring a professional to do the work. Additionally, consider the tasks of modifying the rain gutters and downspouts, excluding debris from the storage containers, and distributing the water from the barrels to your landscape.
Given the complexity and cost of watering landscapes with rainwater captured off a roof and stored in barrels, tanks, and cisterns, I recommend prioritizing passive rainwater harvesting using the landscape itself first. This method leverages the free water that falls from the sky most efficiently. Then, if you’re so inclined, look into also providing extra water storage in the form of barrels, tanks, or cisterns. Remember, when it rains, your landscape is getting the water it requires—and much more. So that should be the container you think about first. The water is already being captured and stored in the landscape without any actions on your part. The trick is to hold onto it as long as possible. You can do that with a generous layer of mulch.
In conclusion, detaining the rain by whatever means you choose is a great idea. In times of drought, it provides water when it is being restricted or is unavailable. In times of deluge, it helps relieve the extensive damage done by torrential rainfall. In both cases, it reduces the strain on the public drinking water supply during the dry months and lowers your water bills. Below are links to extensive free information on rainwater-harvesting systems, the articles mentioned above, and more.
SF Public Utilities Commission (SFPUC) Rainwater Harvesting Manual
https://www.sfpuc.gov/sites/default/files/learning/RWH_Manual_Final-APR2018.pdf
Bay Area Stormwater Management Agencies Association (BASMAA) Rain Barrel Fact Sheet
https://www.ebmud.com/application/files/6915/8992/7018/BASMAA_Rain_Harvesting_Fact_Sheet.pdf
Newsletter article: “Planting the Rain: A Guide for Passive Rainwater Harvesting.” https://ccmg.ucanr.edu/News_to_Grow_By/?story=2816
Newsletter article: “The Magic of Mulch” https://ccmg.ucanr.edu/News_to_Grow_By/?story=2768
Webinar: “The Magic of Mulch: Water Conservation at its Best” https://www.youtube.com/watch?v=W28fWlgypwY&t=21s
A captivating 27-minute documentary on the work of Brad Lancaster, Rainwater Harvesting Guru
https://www.youtube.com/watch?v=iIYoU2PTPLk