Rainwater Harvesting for California’s Central Coast
Although we’ve seen a bit of rain this year, as Californians we know that every drop we get is precious. It seems as though drought years are becoming more common, and wet winters are few and far between. Capturing and using rainwater has many benefits. Rainwater is very healthy for plants because it is 100% soft, free of salts, minerals, and chemicals, slightly acidic and a natural source of nitrogen. When it comes to the landscape, there are several techniques that can be used to harvest and utilize rainwater.
Bioswales and Detention Basins
You can beautifully maximize the effect of rainfall with passive techniques, using it to water deep-rooted plants such as trees. It is common to direct roof water and stormwater to bioswales or detention basins to allow for deeper infiltration in specific zones of the landscape. Keeping water on site reduces runoff and erosion downstream from your property.
Bioswales can be beautiful additions to the landscape if made to look like a natural creek or pond with rocks and plants. Concealed detention basins can be created by with underground gravel leach fields around tree groves.
For the 3,000 square foot home, you can get almost 500 gallons from downspouts with a light ¼” rainstorm. Even in a drought winter, your trees can get some good deep watering.
The most efficient way to harvest rainwater is to collect it from roof surfaces by piping downspouts into a cistern system. With a properly designed rainwater harvesting system, you can essentially transfer 100% of the rainwater that hits your roof into storage.
For every 1” of rainfall, you can capture 0.62 gallons of water per square foot. This means that a home with a 3,000 square foot roof will collect 1,870 gallons from 1” of rain. With an average annual rainfall of 19” in San Luis Obispo and 13” in Paso Robles, that same home has the potential to collect between 24,000 and 37,000 gallons over one winter. Once stored, the rainwater can be filtered and pumped into an irrigation system to supplement the water supply during the dry season.
Options and Costs
One major constraint for rainwater harvesting systems is the cost. In California, most of the rain comes during our short winter season, with little need for irrigation between storms. To maximize the harvest, you need to have a lot of storage for the water. Most commonly, above-ground tanks are used to store collected rainwater. There are a lot of options for above or underground storage tanks, with plastic being the least expensive material. Collected water can also be stored in a holding pond, but this method allows for water loss to evaporation.
To have a system installed with a storage capacity between 5,000 and 40,000 gallons, you can expect to pay between $2 and $5 per gallon for overall cost installed by a qualified contractor. If a full system isn’t in the budget, you can certainly keep costs low and use simple rain barrels to harvest water from downspouts. You can use collected water for indoor plants or landscape areas that don’t get direct rainfall.
When to Plan
With all of the different ways to think about harvesting rainwater, planning is key. It is advantageous to think about your system now, during the wet season. That way, you can have your system designed and installed during the dry season to get ready for the following year’s rain. We recommend 3-8 months to allow for design and installation without rushing decisions. When June comes around, rain may be the last thing on your mind – just remember watching that precious rainwater running down the drain and plan, plan, plan!
At any time of the year, consult with your landscape designer. If you are already working on a landscape plan, be sure to consider your rainwater harvesting system. It can be designed alongside planting and irrigation designs to allow your contractor to take care of everything at once.
Want to learn more about including bioswales or dry creeks in your landscape design? Contact our team at [email protected] or (805) 466-6263.