Rainwater Harvesting

Rainwater Harvesting

We are starting to see some rain this year, but as Californians we know that every drop of rain we get is precious.  It seems as though drought years are common, and normal winters are few and far between.  When it comes to the landscape, there are several techniques that can be used to maximize the harvest and use of rain water.

Rainwater Harvesting Techniques

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.  For example, a 3,000 square foot home will collect 1,870 gallons from 1” of rain.  With an average annual rainfall of 21”,  a home in Atascadero, CA has the potential to collect nearly 40,000 gallons in one winter.  Once stored, the rain water can be filtered and pumped into an irrigation system to supplement the water supply during the dry season.

You can also maximize the effect of rainfall with passive techniques to water deep rooted plants like 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.  In addition, keeping water on site reduces runoff and erosion down stream from your property.  Bioswales can be beautiful additions to the landscape if made to look like a natural creek or pond with rock and plants.  You can also achieve this unseen by creating 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, you can make sure your trees get some good deep watering.

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.  In order 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 rain water. 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 does allow 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 $1 and $3 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 for use.  You can use collected water for indoor plants or landscape areas that don’t get direct rainfall.  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 to Plan

With all of the different ways to think about harvesting rain water, planning is key.  Rainwater harvesting systems can be designed alongside the planting and irrigation design to allow an installation to take care of everything at once.  While it is raining now, it may be a little late to collect this winter.  It is important to think about your system during the dry season to get ready for the next year’s rain.  We recommend 3-6 months to allow for design and installation without needing to rush 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!