Rainwater Availability Map

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Rainwater harvesting can provide a key source of alternative water to federal facilities. Rainwater harvesting systems capture, divert, and store precipitation from rooftops for later use. 

Harvesting rainwater can also potentially prevent stormwater from entering waterways, helping agencies meet federal requirements for stormwater management. The captured rainwater can be used for: 

  • Landscape irrigation
  • Ornamental pond and fountain filling
  • Cooling tower makeup
  • Toilet and urinal flushing.

The Federal Energy Management Program (FEMP) developed a rainwater map to help federal agencies strategically prioritize commercial rainwater harvesting projects by providing a range of available rainwater across the U.S.

Map Development

The rainwater availability map was developed using zip-code-level monthly precipitation data across the U.S.1 The map is based on the total available rainfall during frost-free months and the total number of months–with one inch or more of total rainfall–to indicate areas that may be more conducive for storing rainwater for later use. This map is ranked from lowest to highest to show the relative availability of rainwater. 

For more information on the method used to develop this map, read the report by Pacific Northwest National Laboratory (PNNL).

How to Use the Map

The map is categorized into the following six groupings to help identify the relative rainwater available during frost-free months that can help prioritize locations that may be optimal for harvesting projects:  

  • Highest: Areas with abundant rainfall and year-round storage potential that receive a total of 28 inches of precipitation or more during frost-free months and have 9 months or more with at least 1 inch of rain

  • High: Areas that receive between 23-27 inches of total precipitation during frost-free months

  • Medium-High: Areas that receive between 19-22 inches of total precipitation during frost-free months

  • Medium: Areas that receive between 13-18 inches of total precipitation during frost-free months

  • Low: Areas that receive between 8-13 inches of total precipitation during frost-free months

  • Lowest: Areas that receive less than 8 inches of total precipitation during frost-free months.

Considerations

The data used to develop the map have limitations. Daily precipitation is best for determining how much rainfall is available for harvesting. The map was based on historical, monthly average rainfall. This does not account for monthly variation such as large rain events, or rainy periods versus dry periods. For example, if there are large rain events at the beginning of the month, there may be minimal rainwater at the end of the month.

Rainwater harvesting may be appropriate for many areas across the U.S. even in areas of low rainfall availability.  Important considerations when planning for harvesting projects should include the following:

  • Size of catchment area (roof size): Larger roof area can capture significant precipitation even in areas of low rainfall availability

  • Rainwater storage capacity: Areas with lower available precipitation may require larger tanks to provide more storage capacity, increased tank size will increase equipment cost

  • Water rates: Areas with more expensive water rates should also be considered when prioritizing locations for rainwater harvesting projects

  • Operation and maintenance: Rainwater harvesting systems require regular operation and maintenance; when prioritizing sites for projects, make sure to consider available staff that can operate and maintain the system  (FEMP's Best Management Practice #14: Alternative Water Sources)

  • Permits: Rainwater harvesting permits may be required; check with local or state government.

Additional Resources

1 Precipitation data were provided to PNNL by the Environmental Protection Agency in 2010. Data originated from the International Water Management Institute Climate Atlas, which uses 30 years of historical climate data at the zip code level.