Rainwater Catchment for Coastal Rural Areas

In many coastal communities, reliable access to fresh water is limited and watershed health is a concern. During summer months, when stream flows and groundwater supplies are lowest, human demand is highest and endangered fish populations are under extreme stress. Additionally, climate change forecasts indicate that greater seasonal variations in rainfall could affect water security. Roofwater harvesting systems are a “low-tech” way to capture winter rains for use during dry periods. The following is a brief overview of design and construction considerations for roofwater harvesting systems.

The target communities are residents and businesses within all coastal California communities, especially those with water supplies directly linked to waterways supporting threatened or endangered salmon and steelhead fisheries.

A well-designed roofwater harvesting system can reduce or eliminate demand for surface and groundwater supplies, increase water security, improve fire protection, and result in more reliable instream flows for fish and other aquatic life during the dry season. In addition, capturing and infiltrating storage tank overflows onsite can recharge groundwater supplies while reducing erosion, flooding, and pollution during rains.

The Salmon Creek Water Conservation Program Manual addresses design elements common to both potable and non-potable systems. Special considerations for residential, non-residential, and agricultural uses are addressed next, with a final section of additional web and print resources that offer in-depth analysis of the information in this Conservation Strategy. For a recent rainwater harvesting case study, see this report.

First Steps in System Design Conservation—Efficiency First

A roofwater harvesting system is not intended as supply augmentation for inefficient use, waste, or increase in demand. Roofwater harvesting in coastal California is one approach to seasonally offset demand for instream flows and groundwater with stored rainwater. In any water system design, conservation, and efficiency are always the first steps.

Before starting to design a system, research existing policies or ordinances in your area that regulate the use of rainwater, and be sure your intended system will be in compliance. In many counties, tanks over 5,000 gallons will need a building permit for their grading and installation. For more information visit your county’s building department website.

Intended Use: Potable or Non-potable

Anyone who has their water supply impacted during the dry season or who uses water from a stream will benefit from installing a roofwater system. The type of system selected will depend on the intended use of the stored water. Simple non-potable systems provide fire protection, irrigation, and livestock water supply independent of instream flows and groundwater. Potable systems need filtration, treatment, and possibly a backflow preventer. Consider these factors to determine which roofwater system is most appropriate:

• If irrigation or livestock water supplies are insufficient or unusable, or there are water needs in remote/inaccessible areas (even those currently served by stream diversion or pumping), consider a non-potable system.
• If the current potable water supply requires trucking in water, seasonal changes diminish well capacity or reliability, or there are concerns about water quality, then a potable water system may be worth developing.

Site Survey and Water Audit

To determine how much water will be needed during the summer, perform a water audit on the structures and surrounding landscape. A Residential Self Survey Conservation Strategy is available at www.salmoncreekwater.org, and will help in estimating storage capacity needed for the rainless months of the year. For help performing a water audit and designing systems for larger scale agricultural needs, contact your local Resource Conservation District (RCD). The Gold Ridge RCD also has information on roofwater systems for dairy operations. Their website is: www.goldridgercd.org.

Roofwater harvesting systems range in complexity from rain barrels under downspouts to municipal-scale systems.

Collection Capacity

To calculate the collection area of a structure’s roof, measure the horizontal length and width of your roof line (not the sloped roof) and multiply the two measurements. Next, gather data on average annual rainfall for the area. On-site rain gauge data is optimal, but contacting the local weather service, agricultural extension agent, or public water agency will suffice.

Then, estimate the water quantity the structure’s roof could harvest per year using the following formula:

(Collection area square footage) x (Average annual inches of rainfall) x (600 gallons) / 1000 = Total gallons of rainfall harvested per year. While average annual rainfall numbers are a good starting point, it is a valuable exercise to do this calculation for 25- and 50-year drought figures in order to plan for the worst- case scenario.

A capacity calculator is available at www.oaec.water.org/calculators.