Rain Power: Harvesting Rainwater

We all know that water is a precious resource. In Northern California, that's especially true. Our Mediterranean climate, characterized by hot, dry summers and mild winters, leaves no room for water waste. Fortunately, Marin homeowners are becoming increasingly educated on ways they can help the cause and do their part. Most people turn off irrigation in the winter before the rains. They embrace the use of native and drought tolerant plants. Some even design their gardens around hydrozones. But only a few pioneers are harvesting rainwater. 

First things first. In Marin, rainwater harvesting will probably never be able to replace your current irrigation system. Our annual rainfall is too light and the drought period is too long, however, used in conjunction with other water-saving strategies it can have a significant impact. There are additional benefits to using a catchment system beyond water conservation, you will also: decrease your water bills, reduce  storm runoff and help control erosion.

Before you get started it's important to sit down and get a clear on a few things:

Know HOW you want to use the captured water.
Intended use drives catchment system design.  Will you be using it just to supplement your landscape irrigation? Or are you going to use it for drinking or feeding pets and/or livestock?  Some homeowners start out using rainwater catchment to help with irrigation and graduate to more sophisticated uses; anticipating how needs may evolve over time is important.

Familiarize yourself with rainfall patterns.
Supply of rain and system capacity need to line up.  Understanding annual and monthly rainfall as well as the intensity of rainfall are critical to buildling a successful system. According to Marin Municipal Water Disctrict the average annual rainfall in Marin is 52".  Once you understand that you can start extrapolating the numbers.  The American Rainwater Catchment System Association (ARCSA), provides a good base calculation to get started:  1" rain X square footage of roof collection area  X .623 gallon. So, a 2000 square foot roof x 1" of rain will result in roughly 1250 gallons of water (this does not include runnoff coefficient based on roofing material).  For more details go to www.arcsa.org.

Complete a site evaluation.Not all sites are created equal. Ideally rain catchment systems are designed at the inception of a building project, but more often than not they are retro-fitted solutions. This make site evaluations critical. You need to understand the strengths and the weaknesses of your site. Things like roof material and condition, air quality, property slope, drainage and space availability all contribute to the design solution, and as such need to be considered.

When it comes to water conservation, every little bit helps. It sounds trite and we've all heard the call, but small changes can have a big collective impact. So next time the skies cloud over, and the rain begins to trickle in, play catch.

Pete Pedersen
Principal Pedersen Associates Landscape Architecture
Green Certified, ARSCA Member, Bay Friendly Coalition Member
http://www.pedersenassociates.com/