Why Use
Is Collecting Rainwater
Legal in CA?
Rainwater Harvesting System Basics
Passive and
Rainwater Harvesting
Why use Rainwater?
By using rainwater to supply water for some, or in certain cases all of our water requirements, we can reduce our dependence on municipal or well water. In many areas fresh water supplies are decreasing and in California water  restrictions have been set in many communities to reduce water usage in order to protect our supplies.

Rainwater falls for FREE – once rain harvesting systems are installed overall fresh water use decreases which can reduce water bills or the long term cost of operating private wells. Water suppliers will increase water service fees as they look to recover the true cost of providing water to communities. The construction of dams, pipes and treatment plants is huge and ultimately tax and rate-payers foot this cost.

By capturing any amount of rainwater and reusing it, we can help reduce the need to use fresh drinking water for landscape irrigation, toilet flushing, laundry, heating and cooling and much more.

Rainwater harvesting is a simple and practical way to engage in watershed stewardship and to use our shared
water resources more wisely
Benefits of Using Rainwater
  • A long term water solution for landscape water use
  • Every gallon of rainwater that is collected and utilized saves a gallon of fresh drinking water
  • Reduces the flow of stormwater to urban streets, reducing flood risk
  • Prevents erosion; capturing the rainfall instead of letting it run-off the property, taking valuable soil with it
  • Reduces the amount of pollutants which are picked up by stormwater flows through streets and storm drains
  • Conserves energy from potable water processes
  • Fosters an appreciation for this essential and precious resource
  • Reduce water bills and demand on community drinking water supplies
The Eight Rainwater Harvesting Principles
by Brad Lancaster
  1. Begin with long and thoughtful observation. Use all of your senses to see where the water flows and how. What is working, what is not? Build on what works.
  2. Start at the top – or highpoint – of your watershed and work your way down. Water travels downhill. Start at the top where there is less volume and velocity so it is easier to follow the next principle…
  3. Start small and simple. Work at the human scale so you can build and repair everything. One thousand small strategies are far more effective than one big one when you are trying to infiltrate water into the soil.
  4. Spread and infiltrate the flow of water. Rather than having water run erosively off the land’s surface, encourage it to stick around, walk around, and infiltrate INTO the soil.
  5. Always plan for an overflow route, and manage that overflow water as a resource. Always have an overflow for the water in times of extra-heavy rains, and use that overflow as a resource.
  6. Maximize living, organic groundcover. Create a living sponge so the harvested water is used to create more resources, while the soil’s ability to infiltrate and hold water steadily improves.
  7. Maximize beneficial relationships and efficiency by “stacking functions.” Get your water-harvesting strategies to do more than hold water. Berms or swales can double as high and dry raised paths. Plantings can be placed to cool buildings. Trees can be selected to provide food.
  8. Continually reassess your system: the “feedback loop.” Learn from your work – we begin again with the first principle.

For a more detailed description of the Eight Rainwater-Harvesting Principles, see pp. 29-38 of Rainwater Harvesting for
Drylands and Beyond, Volume 1
by Brad Lancaster.

Is Collecting Rainwater Legal in Calfornia?
Rainwater collection is legal in California. In 2012, the Rainwater Capture Act was passed in Califonia which allows the use of rainwater collected from rooftops and does not require a water right permit from the State Water Resources Control Board. Indoor, potable use of rainwater does require a permit and can be the most challenging system to get a permit for. Rainwater tanks over 5,000 gallons require a permit, while smaller tanks don’t.

According to California law the definition of “rainwater” means precipitation on any public or private parcel that has not entered an offsite storm drain system or channel, a flood control channel, or any other stream channel, and has not previously been put to beneficial use.
As of 2017, the "Nonpotable Rainwater Catchment Systems" code is found in Chapter 16 of the California Plumbing Code (CPC).

Before the Rainwater Capture Act in 2012 was enacted it was not legal to harvest rainwater in California without a water right permit. Millions of systems were installed anyway as rainwater harvesting is an ancient technology and has been used for millenia.
When a Permit Is Not Required
You can install a rainwater catchment system without a permit if you follow these requirements:
  • You do not store rainwater in a tank, barrel or any other container (i.e. it's ok in the ground)
  • The system will be used for NON-SPRAY irrigation with a maximum storage capacity of 5,000 gallons where the tank is supported directly upon grade and the ratio of height to diameter or width does not exceed 2 to 1 and it does not require electrical power or a makeup water supply connection
  • The system will be used for SPRAY irrigation with a mximum storage capcity of 360 gallons

When a Permit Is Required
Any other situation not list above such as:
  • A system that will be used for spray irrigation from a 1,000 gallon tank using a pressurized system requiring electricity
  • A potable system that will be used indoors for flushing toilets

Rainwater Harvesting System Basics
Normally when it rains rainwater pours off the roof, down through gutters, off the driveway and other impermeable surfaces and runs into the street or other areas of the property. Rainwater harvesting is the practice of collecting and using this rainwater. Rainwater harvesting systems can be as simple as using a barrel to capture water or infiltrate the water into soil storage or it can be as complex as installing a large underground cistern or storage tank.

Always start at the top of the "watershed". Think of the term watershed in this instance to be part of the larger natural watershed but more specifically the property boundaries that the rainwater harvesting system is being used for. In the urban or residential setting the top of the watershed is often the roof.

The basic principal of rainwater harvesting is to channel rainwater from a catchment area (roof or other surface), through a conveyance system (gutters, downspouts and pipes/channels) to storage -whether that be in the soil or in containers such as a barrels or tanks. Often there may be rainwater filtration of some type to filter debris or clean out pollutants. Rainwater distribution take the water to desired areas of the landscape for use as irrigation or passive infiltration for nearby trees and shrubs . Finally there is the overflow system that ensures surplus rainwater is used beneficially in the landscape and prevents flodding and erosion.
  1. catchment area -rainwater
Catchment Area
The total area of a landscape that drains to a particular site or drainage. As used in rainwater harvesting practice, catchment area refers to the area, such as a roof, that drains to a rainwater storage system -be that in a tank or in the ground.

The highest water quality comes from smooth, impervious catchment surfaces. Impervious materials improve the efficiency of rainwater catchment systems by absorbing less water and reducing the chance for microbes and debris to collect in the pores and/or seams of the catchment surface. Smoother surfaces also allow the rainwater to flow faster and thus “clean” the catchment area. Metal, clay, and concrete tile roofing will generate the cleanest water.

Catchment areas are prone to water loss to the environment know as a "run-off coefficient" -which is used when calculating the volume of water one can collect from a specific catchment area.
Conveyance System & Filtration
Rainwater is usually conveyed from a catchment area to some type of storage (containers, infiltration basins or rain gardens) by gutters connected to downspouts. Downspouts with the aid of piping help move rainwater further. The number of downspouts needed depends on the surface area of the roof and intensity of rainfall.

Downspouts can be fitted with filtration devices such as leaf eaters and first flush devices which exclude leaves and debris and filter out the first few gallons of water. Many types of first-flush diverters are readily available; all must be installed “upstream” of the storage tank.
  1. first flush -rainwater
  1. metal rainwater tank
Passive rainwater harvesting systems utilize storage systems such as mulch basins, swales and other earthworks to collect and hold rainwater until it infiltrates into the ground.

Active rainwater harvesting systems utilize manmade containers such as tanks, barrels and cisterns to store rainwater for use at a later time.

See the next section for more information on Passive and Active Systems.
Distribution Systems & Use
Passive system distribution of rainwater happens before storage and can be simple overland flow of water using gravity and water's least path of resistence. Piping from downspouts to earthworks like infiltration trenches can also be constructed to move rainwater where we want it to go -such as mulch basins or raingardens to irrigate specific plants.

In an active system water distribution happens after storage and includes all piping, pumps, and other devices that moves water from storage to the point-of-use in the landscape -such as the vegetable garden.
  1. rainwater distribution
  1. rainwater infiltration trench & basin
  2. rainwater infiltration trench & basin
You cannot turn off the rain! Once your storage is full it's vital to have a plan in place for where the water will go. Design the rainawater overflow to be a resource so it can passively flow or spill over to mulched/vegetated areas, mulch basins or another series of earthworks (as shown in the photo above). Spillways (also called level sills) are low points in a berm-basin or swale system and are level surfaces to allow water to flow evenly across the land to another point of use. Diversion drains are a gradually sloping drains and can also be used for overflow; they are on a slight slope (<1%) to direct water to another area.

Overflow routes should always be stabilized with rock, dense vegetation or other durable materials so fast moving water will not erode the land. Make sure the overflow route and spillway are large enough to handle all surplus water. The last and lowest point in your site's "watershed" should be directed to a natural drainage or in the case of urban settings, the stormwater system in the street. Whenever possible try to make sure all rainwater that falls on your land can stay on site to be fully maximized in the landscape.

Passive and Active Systems
SLOW IT            SPREAD IT           SINK IT!
Passive Systems & Components
Passive rainwater harvesting is the practice of slowing water down and encouraging it to soak into the ground. By building simple structures, rain and stormwater can be used beneficially, encouraging plant growth in landscapes and natural areas, healing erosion cuts, and can even replace the need to irrigate with tap water. Infiltration trenches and basins, swales, dry streambeds, and even pervious concrete or pavers are types of passive collection systems. Passive systems can be relatively inexpensive and are generally simple to design and build.

Passive systems consist of a catchment area, a conveyance system, often a distribution system such as infiltration trenches and a landscape holding area or storage. Runoff is directed from the catchment area to the holding area, where water can be used by landscape plants. Catchment areas include soil surfaces, roofs, roads and sidewalks. Passive water harvesting can be used along with a active rainwater storage system, or can be used alone.
Methods of using earthworks to harvest rainwater for new or existing landscapes:
  • Berms and swales are created perpendicular to the flow of water. The ground is shallowly excavated outside the plant dripline to hold water, while the berm helps detain the water a while longer. If using a berm with a swale, soil excavated from the swale can be used to build the berm. Berms can be constructed of soil, rock, straw bales, or other materials.
  • Create infiltration trenches and basins. This distribution to catchment area includes shallow trenches which lead to basins or depressions (or raingardens) around plantings to supply water to several plants at once.  They can also be shaped as donut rings around individual plants. Locate trees and shrubs in raised areas within or near the basin edge and do not allow water to stand around plant trunks or stems.
  • French drains or dry wells are holes or trenches filled with gravel. Pipe‐less French drains direct rainwater water away from clean impervious areas (so silt does not clog them) allowing water to infiltrate into the soil in a more appropriate location and supply water to plants.
  • Dry streambeds meandering through landscapes with rock “speed bumps” along the channel will slow runoff and  encourage infiltration for use by nearby plants. Streambeds can have aesthetic value, creating a focal point in your landscape and provide the illusion of water.
Passive Systems are inexpensive, simple to build, require little maintenance and turn your land into a sponge!
Active Systems & Components
NOTE: The basic components described here are for a non-potable storage system. A potable system would need to consist of all food grade materials, and water would need to be properly treated to meet drinking water quality standards.  Potable rainwater catchment systems require a permit in California.

Active rainwater harvesting refers to systems that actively collect, filter, store and reuse water. The storage is usually the most visual aspect of an active system (i.e. large tanks), but they also generally incorporate pumps, and sometimes filters that require electricity. These are active components that require regular ongoing maintenance to run efficiently and effectively.
Five Key Components
  1. Catchment Area – roof surfaces provide an opportunity for rainwater capture though driveways can be used for underground tanks
  2. Conveyance System – used to transfer water to storage and is comprised of gutters, downspouts and piping
  3. Filtration – to keep debris out of the system
  4. Water Storage – may be above or below ground and can be comprised of a single container or multiple containers
  5. Distribution System – used to move water from a container to the desired area of the landscape for irrigation.

Catchment Area:
The smoother and more impermeable the collection surface, the less debris will accumulate, keeping the stored water cleaner.

Conveyance Systems:

Catchment area to gutters to downspouts to pipes to tanks.

There two different methods to moving rainwater into containers:
  • Dry Pipe – This method involves a larger storage volume. Essentially, the collection pipe “drys” after each rain event since it empties directly into the top of the tank.
  • Wet Pipe – This method involves locating the collection pipes underground in order to connect multiple downspouts from different gutters. The rainwater will fill the underground piping and the water will rise in the vertical pipes until it spills into the tank. The downspouts and underground collection piping must have water-tight connections. The elevation of the tank inlet must be below the lowest gutter on the house. This type of system allows for more flexibility in storage tank location. Proper pipe sizing is important to prevent flow backup, and pipes are susceptible to freezing.

To keep water clean, prevent clogging and sediment build-up, basic filtration is needed. The type and number of filtering components on a system depend on the amount of roof debris.

Filtration may include:
  • Gutter and downspout screens
  • First flush diverters keep the first flush of poor quality roof runoff from entering the tank. Diverters must have a drainage outlet for emptying standing water, and be emptied as needed. Diverters are usually not needed unless water quality is especially poor (i.e. significant bird droppings on collection surface).
  • A strainer basket or screen at the tank water inlet serves as further protection from debris and animals. Do not allow the basket to sit below the water line and clean frequently.

Water Storage:
Multiple containers can be connected together to increase storage capacity; they can be linked at the top or the bottom.

Storage containers require:
  • Inlet for rainwater to enter
  • Outlet to access water such as a hose bib
  • Overflow pipe which should be as large as the inlet pipe
  • Air vent for air to escape while the tank is filling; if open to the air, the overflow pipe can serve as the vent

Distibution systems:
The distribution component of active systems includes all of the piping, pumps, and other devices that move water from the storage to the point-of-use in the landscape. If irrigation is by gravity flow, the distribution system may be a length of drip tubing or simply a hose.

Optional Components
Pumps: If rainwater is used in a drip irrigation system, moved long distances or uphill, a pump may be needed to pressurize the system. Without a pump the water will be pressured by gravity. There are many different types of pumps, either external or submersible.

  1. rainwater stoage tank
  2. rainwater stoage tank
  3. downspout leaf guard
  4. rain barrel

Rainwater System Considerations
Each rainwater harvesting system is unique and should be thoroughly evaluated when designing the system. How much rainwater could be collected in specfic rainfall events and how much water is expected to be used are only two of the many things to consider. All decisions will affect installation cost, operation, and on-going maintenance.
General Considerations
  • How much rainwater can be collected? (see Calculating Volume below)
  •  What will the rainwater be used for?
  • How will water get from the roof to storage and to the final area of use?
  • Do you want a passive system that moves water to a raingarden?
  • Do you want a simple active system that moves water to a rain barrel?
  • Do you want a complex active system with multiple tanks?
  • Will the system be gravity fed, or will it need a pump?
  • If you use storage containers, where can they be located -above or below ground?
  • Will you want the option to expand the system later?
  • Where will rainwater overflow go?
  • If you want potable water indoors, can you get a permit? Is the roof material suitable?
  • Can the containers serve several purposes where they are located- like serving as shade for the garden, a windbreak or the edge of a structure?
site conditions
  • Catchment area material (e.g. roof), condition, and possible sources of contamination (e.g. bird droppings)
  • Gutter material and condition
  • Location of downspouts
  • Location of area drains, if any
  • Slope, drainage, and water flow direction
  • Soil type (e.g. loam, clay, sandy)
 Water Balance 
SUPPLY – The volume of rainwater captured and stored
  • Annual rainfall amount
  • Seasonal rainfall patterns
  • Size of catchment area
  • Hydrologic properties of catchment area
  • Potential losses
DEMAND –The volume of rainwater used
  • Intended end use
  • Estimated water demand
  • Seasonal and annual use
  1. mulch basin diagram
Calculating Volume
There are two methods for calculating the potential volume of rainwater one can collect in a designed system. There is the easy way and the longer way. As with most things there is more accuaracy with the longer route. The first thing that needs to be done however is to determine the square feet of catchment area. Our example is for a rectangular rooftop (for different shapes and sizes of catchment areas visit Brad Landcaster's website here).
Determining Catchment area (for a rooftop with eaves)
Rooftop catchment area is equal to the total square feet of the house plus the extension of eaves.  To calculate the square feet of the house’s catchment area, measure the area of the outside walls and then include the overhang of any eaves.

For example, let’s say the house is rectangular with outside wall dimensions of 36 feet by 46 feet.  The overhang of the eaves is 2 feet on all sides.  Add 4 feet of the eaves to each wall length (2 eaves of 2 feet equals an additional 4 feet for each wall) to get the total length of the walls plus the eaves (36 + 4 = 40 feet and 46 + 4 = 50 feet). Now multiply 40 times 50 (length times width) to get the total roof catchment area.

40 x 50 = 2,000 square feet (ft2) catchment area
  1. rooftop footprint
Roof Footprint
When working with a rooftop take the view of a falling raindrop, and only look at the “footprint” of the roof’s drip line. It does not matter whether the roof is flat or peaked; the roof dimensions at the drip line are the same. It is the footprint of the roof’s drip line that matters.

Gabling of the roof is another consideration when determining volume as it will split the drainage area and rainwater will flow to one downspout or another.
Method 1 
One inch of rainfall on 1 square foot catchment area provides approximately .6 gallons of water.

The theoretical house has a 2,000 square foot catchment area so you multiply .6 gallons by 2,000.
.6 gal x 2,000 = 1,200 gallons with one inch of rain

If the average rainfall is 20 inches per year, one has the potential to collect 24,000 gallons of water in one year.
1,200 gal x 20 inches of rain = 24,000 gallons in one year

Below shows the average precipitation for 4 cities in the local area. You can find data here for your area.
  1. rainwater averages by month -chico, oroville, red bluff, paradise
Method 2 Rooftop (or impervious surface) Catchment –more involved but most efficient method 1
Catchment area (ft2)  X  Rainfall (ft)  X  Runoff coefficient  X  7.48 gal/ ft=  Maximum Runoff/Rainfall (gal)
  1. To convert inches rainfall to feet rainfall for the calculation below divide inches rainfall by 12. [eg. 20"/12 = 1.67']
  2. To convert cubic feet (ft3) to gallons multiply cubic feet by 7.48 gal/ft3.               [eg. 50 ft3 x 7.48 gal/ft3 = 374 gallons]
  3.  If the roof consists of two or more rectangles, calculate the area for each rectangle and add them together. With conical, octagonal, or other non-standard roof shapes, again calculate the area based on the drip line. For a round roof, use Pi times radius squared to figure the catchment.
  4. You should consider that rainwater harvesting systems aren't necessarily 100% efficient. All rainwater harvesting systems lose some of the rainwater to the environment, known as a ‘runoff coefficient’.

Rooftop catchment using the same house measurements from the Method 1:
40 ft x 50 ft = 2,000 ft2 total catchment area (including eaves)
With 20” annual rainfall (1.67ft) on an asphalt shingle roof works out like this:
2,000 ft2  X  1.67 ft  X  0.95  X  7.48 gal/ ft3  =  23,734.04 gallons
1 From Catchment Area of Rectangular Surface in Rainwater Harvesting for Drylands and Beyond Vol. 1 & 2
  1. run off coefficients

Read City of San Francisco's Rainwater Harvesting Manual
Rainwater Harvesting Services
Active Rainwater Systems
Design: $200-$1,500 +
Full Design & Installation Cost: $500-$10,000 +
Passive Rainwater Systems
Design: $200-$1,500
Full Design & Installation Cost: $500-$5,000 +
Check out our Other Services
  1. rainwater infiltration trench and basin
  2. rainwater infiltration trench and basin
  3. rainwater harvesting in forest garden
  4. infiltration trench with bridge
  5. mulch basin
  6. rainwater overflow pipe
  7. rainwater harvesting system
  8. rainwater harvesting graphic
  9. rooftop rainwater to pond
  10. rainwater mulch basin network
  11. stormwater retention basin
  12. rainwater harvesting network