rain water collection converting inches of rainfall into gallons of potential irrigation water

Rain Water Collection

Converting inches of rainfall into gallons of potential irrigation water


In most landscapes rainfall is diverted into drain lines and directed through a hole in the curb then flushed to the sea as quickly as possible


Most any flat, impermeable surface within a landscape can be a source of potential, future, irrigation water

Definition: impermeable—not allowing fluid to pass through


This includes:Rooftops using rain gutters and

downspouts Patios & patio coversDriveways & sidewalks


Water from these flat surfaces can be channeled and captured in large, closed collection tanks for future use


The most typical flat surfaces include:

RoofsPatio coversPatios And walkways


First – determine the potential for water savings per year

Ex. water costs in Westminster are $2.29/unit (as of summer 2012)

1 unit = 100 cu.ft. of water or 748 gallons of water

Rain Water CollectionMy bill for 3 residences averages

about 18 units/month or about 13,464 gallons per month

For 6+ people3 toilets3 kitchen sinks3 tub-showersAnd whatever yard watering


We average about 14 inches per year of rainfall in the Orange/LA County areas (more or less)

How much usable, potential irrigation water does that amount to?


Let’s say we have a storm that deposits 1” of rainfall on your roof and your roof covers 1,000 square feet of area

How many gallons of water does that amount to?


The first thing we need to do is to convert the amount of rainfall into a comparable measurement—to something we get billed for


We start by asking ourselves: “How many gallons of water are

there in 1” of rainfall over an area of 1,000 sq.ft.?”


Since we are ultimately dealing with cubic “FEET” of water – we should probably begin by turning inches of rainfall into “FEET” of rainfall

Rain Water CollectionSo 1” of rainfall equals how many

“FEET” of rainfall? 1 foot = 12 inches so we could divide: 1” rainfall = 0.083 ft.

12”/foot(don’t forget the inches cross-cancel and feet come to the top)

So 1” of rainfall = 0.083 ft. of rainfall

Rain Water CollectionNext figure out just how many cubic

feet of rainfall in our 1000 sq.ft. area that amounts to:

Remember that cubic feet represents a volume

Volume = Area x Depth orV = L x W x D provides an answer in

“cubic ‘something’”Remember: Area = L x W


So if we take the area (in sq.ft.) covered by rainfall and multiply it by the depth of rainfall (in ft.) that will give us the amount of cubic feet of rainfall

Volume = area x depth (V = A x D)


We started with an area of 1,000 sq.ft. covered by rainfall and 0.083 ft. of rainfall

1,000 sq.ft. X 0.083 ft. = 83.3 cu.ft. of rainfall

Rain Water CollectionHow many gallons does that convert to? Since there are 7.48 gallons per cubic

foot of waterWe could divide:

7.48 gallons/cu.ft. X 83.3 cu.ft.= 623.3 gal. 1 cu.ft.

(don’t forget the cu.ft. cross-cancel)


So . . . For every 1” of rainfall on an

impermeable area of 1,000 sq.ft. we could conceivably collect 623 gallons of possible

Rain Water CollectionAnd in an average year, with an

average of 14” of rainfall, that means about 8,727 gallons per year of potential irrigation water!

Almost 12 units per yearOr about $28 yearIs it worth it?

Rain Water CollectionNow how much water does that lawn

need to stay green? On average of about 1” of irrigation

per weekOr 624 gallons per 1,000 sq.ft. per

week At 38 weeks per year (arbitrary

number assuming 14” of rainfall over 14 weeks)


That’s about 25,000 gallons of irrigation water per 1,000 sq.ft. of lawn per year

About 33 units per yearAbout $75 per year


It probably wouldn’t be practical to irrigate lawns using captured rainfall

It might make sense to use that captured rainfall in a “gravity fed” drip system in the planters

Wouldn’t that offset water costs for residential landscape irrigation uses?


The costs may not sound like much now, but water rates continue to rise!

The future price of tap water:Tap water = bottled waterThat might make it worth it.

rain water collection converting inches of rainfall into gallons of potential irrigation water

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