Marin Art and Garden Center

Acknowledgements: This project was designed and installed in 2010 as part of the 10,000 Rain Gardens Project. Special thanks to the Marin Art and Garden Center (MAGC), Marin Master Gardeners, Drake High School Mobius students, Americorp Watershed Steward Program interns and Clean Water Components for participation in this project.


Interpretive sign at the project site
Project Overview

The Marin Art and Garden Center receives hundreds of visitors annually and is a premier site for horticulture and landscape education in Marin County, including hosting the Marin Master Gardeners and the Bay-Friendly Qualified Landscaper training programs. The location for this demonstration project was selected for easy access for visitors and integration with existing educational activities. The project is adhacent to an outdoor classroom used for many activities at MAGC as well as the compost demonstration area that regularly brings visitors to this area. Interpretive signage at the project site will provide visitors the opportunity to see rainwater harvesting in action and gain a better understanding of the technology.

The system is designed for rainwater catchment from the roof of half of a shed, measuring approximately 115 square feet, to feed into a 2,500-gallon cistern. The overflow from the cistern is directed to a basin-and-berm rain garden situation in an adjacent planting area. Water stored in the cistern is distributed by low-pressure gravity irrigation to existing and new native and water-wise plantings in and around the rain garden, and will also be available for use in the composting process and for cleaning hand tools.

2,500-gallon cistern & shed

Basin-and-berm rain garden
General System Statistics

Area of collection:
Gallons per inch of rain (90% efficiency):
Gallons per year (44-inches average rain):
Rain catchment capacity:
Rain garden basin area:
Rain garden design capacity:
Irrigated landscape area:

115 sq. ft. (1/2 of shed roof)
64.5 gallons
2,837 gallons
2,500 gallons
60 square feet (30 cubic feet)
1 inch of rain
750 square feet

Inlet and outlet
Rainwater Catchment System Unique Features

A gutter was installed on the half of the shed roof to convey rainwater run-off to the cistern. Gutter screens were added to filter leaf debris from entering the tank. Due to the limited height differential between the gutter and the inlet for the cistern (which had already been on-site and waiting to be put to better use in a more visible location), no first flush or additional debris exclusion was used in this system. In the future, a Brazilian Ball Pre-Filtration system could be installed to provide a first flush (for a case study on this technology created by the WATER Institute at Occidental Arts and Ecology Center, see "Roof Water Harvesting For A Low Impact Water Supply," available at

The 2,500-gallon polypropylene cistern was placed on a gravel foundation ring filled with pea gravel. An additional 2-inch bulkhead was installed just below the inlet to allow overflow water from the cistern to travel by gravity through 2-inch PVC pipe to the rain garden located approximately 50 ft. away.

To build the rain garden, undesired vegetation was removed and the existing mulch was raked away from the area. A basin was dug, and the excavated soil used to build a berm on the downhill side. Rocks were place to create a spillway so that if a rain event exceeds the rain garden's capacity, water would have a safe way to exit the system without causing erosion. Mulch was pulled back in to cover the exposed soil, and additional mulch was added to aid in the rain garden's water-holding capacity.

A low-pressure gravity drip irrigation system was installed to convey water from the bottom of the tank to plantings around the rain garden without the need for a pump or additional gravity pressure. The system uses a Holman battery controller to regulate the flow of water, which then passes through a 400 micron IrriGRAY filter to remove any sediments from the water.  
Students prepare the rain garden
3/4-inch poly tubing carries the water to the planting area, and then connects to 3/8-inch in-line dripper line with emitters at 12-inch spacing. This dripper line is run in three laterals through the planting area, and has a screw-on end caps to allow the system to be easily flush for maintenance.

Watershed Friendly plants were installed on the berms of the rain garden, including: Elderberry, Snowberry, Silk Tassell, Oregon Grape, White Sage, Yarrow, Sedge, Lambs Ear and Yerba Buena.