Harvested rainwater can supplement or even replace municipal water for outdoor use. The amount of rainwater that can be captured depends on both the amount of rainfall and the area of the surface from which it is collected. If you know the amount of water you use and the average rainfall you receive, you can estimate the catchment area required to obtain that amount of water. By comparing this to the area you have available, you can determine whether you can meet all of your outdoor water needs with harvested rainwater.
Estimate the amount of water you use outdoors. Multiply the number of minutes you irrigate your lawn and garden per month by 3 gallons per minute. For all other outdoor water uses, such as filling a swimming pool or washing a vehicle, multiply the number of minutes per month the water runs by 10 gallons per minute. Add these numbers together to find your average monthly water usage.
Obtain an estimate of the average monthly rainfall for your area. Find a nearby city in the National Climactic Data Center's records of normal monthly precipitation, or contact a local weather station and ask for historical monthly precipitation records for your area.
Multiply the gallons of water needed for the month by 2.41 and divide by the average inches of rainfall for that month to obtain the required catchment area in square feet. Perform the calculation using data for a summer month, since outdoor water usage is highest during the summer.
Measure the length and width of the roof of the building from which you collect rainwater from eave to eave and from front to rear. If rainwater only runs into your catchment system from part of your roof, measure only that part of the roof. Catchment area depends on the footprint of the roof and not its surface area, so there is no need to determine the slope.
Multiply the length and width of the roof together to determine the available catchment area in square feet. Compare this number to the required catchment area you calculated previously. If the available catchment area is less than the required catchment area, you may only be able to fulfil a percentage of your outdoor watering needs using captured rainwater.
Maintain consistent units to ensure a correct calculation; measure water usage in gallons, precipitation in inches, and catchment area in square feet.
Don't include uses that require potable water, such as watering pets, in your calculation of outdoor water use; collected rainwater is non-potable and cannot be used in such cases.