by Charlene Burgi
All reports are that you’re been enjoying some good rain in Marin! Are you considering ways to start catching some of this precious stuff to use during the dry times? Do you have a plan?
You might ask what’s to plan except to run down to the local hardware store, pick up a barrel or two, and direct your downspouts into the open barrels. Or the plan may entail setting out buckets, pots, and pans around the garden with the thought of capturing any raindrops that happen to fall into them.
Several questions come to mind when thinking about these types of plans. Did you calculate the amount of water that will be collected from the roof going into the barrels? What happens to the overflow? Will the excess water erode the area around the barrel, or is it directed away to protect your foundation and prevent flooding? What becomes of the harvested water until it is used? Will the uncovered barrels, buckets, pots, and pans become a breeding ground for mosquitoes or see critters falling in with no way out? So many questions!
Harvesting rainwater comes with a responsibility that is often overlooked. The concern isn’t about taking advantage of falling rain for conservation purposes, but doing it in a way that considers the health and welfare of your property and our environment. If you don’t have a plan yet but would like to collect rainwater, let’s outline a plan that is a win-win for all concerned.
Let’s do the easy step first. If the collection containers are not covered, move the water into an enclosed container immediately. Many commercial barrels are sold with a water faucet already attached for easy hose or drip assembly. Next, consider what you are going to do with the collected water. Rain barrels are a good fit for watering a few plants under the eaves of your house during the winter. However, watering the entire garden will require a much larger vessel in the form of a tank or multiple connected rainwater catchment containers or bladders—which leads me to the next step.
Calculate the amount of rain runoff from your roof so you can anticipate what size tank/container to purchase. Measure the square footage of the portion of the roof that directs water to the downspout(s) you are using for collection (length x width = square feet). Now, multiply the square footage by the number of inches of rainfall, then multiply that times a conversion factor of 0.623. For example, let’s assume the roof collection area is 1,000 square feet and that during the big storm earlier this month your neighborhood received 10 inches of rain. The calculations would look like this:
1,000 x 10 x 0.623 = 6,230 gallons of water
If you only have two rain barrels collecting a total of 90 gallons of water, where, might I ask, does that remaining 6,140 gallons go? The plan must include directing excess water into rain gardens or bioswales to soak it up, spread it out, and sink it into the richly prepared soil.
At the Marin Art & Garden Center, runoff from a shed roof collects in a 2,500-galllon tank.
If you like working out these problems, calculate the gallons of rainfall you can collect for the year based on the above formula using your average yearly rainfall. Now calculate the water needs of the plants in your garden for the year based on the average evapotranspiration rate for your area. (Hint: WUCOLS can help with water needs of your plants and CIMIS can help with yearly ET averages.) Is your water storage big enough to support your garden for the year? How about a month? And where on your property would you install a container able to store all that water?
I hope this exercise has been fun as well as informative. Meanwhile, enjoy the pitter patter of raindrops! I know I will!
Go Green with Graywater
The County of Marin is hosting a workshop Saturday, March 15, to teach the community about graywater reuse and installing laundry-to-landscape systems. The workshop is open to all and will be 9:30 a.m. to 12 p.m. at San Rafael Corporate Center, 740 Lindaro Street in San Rafael. Get the complete details here.
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