Harvested rainwater can provide a key source of landscape irrigation for federal facilities. Rainwater harvesting systems capture, divert, and store precipitation from rooftops for later use. The Federal Energy Management Program (FEMP) developed a map showing rainwater availability specifically for landscape irrigation to help federal agencies strategically identify locations in the United States that may be particularly well suited to supplement irrigation with harvested rainwater.
The rainwater availability for landscape irrigation map was developed using zip-code-level monthly precipitation and evapotranspiration (ET) data across the United States1 ET is the amount of water a plant requires to stay healthy over a given time period.
A metric was developed from the data that compares the total precipitation to ET of a traditional landscape, such as turfgrass, which typically requires supplemental irrigation in most regions of the United States. This metric provides the total amount of supplemental irrigation for a traditional turf landscape needed in a given area. This comparison reveals areas in the United States that likely have more rainwater available to supply supplemental irrigation. 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 features rankings of the amount of rainfall that is typically available for irrigating a traditional turf landscape during the irrigation season. The intent of the rankings is to help prioritize locations that may be optimal for rainwater harvesting projects. The map provides the following designations:
- Rainfall Exceeds Irrigation: Areas where abundant rainfall exceeds the ET of a traditional turf landscape most of the year and rainwater harvesting for irrigation may not be necessary
- Highest: Areas where the ET of a traditional turf landscape closely matches rainfall and that have more than 4 and up to 9 months where rainfall exceeds ET during the irrigation season
- High: Areas where the ET of a traditional turf landscape closely matches rainfall and that have 1 to 4 months where rainfall exceeds ET
- Medium-High: Areas where the ET of a traditional turf landscape typically exceeds rainfall and that have more than 3 and up to 6 months where rainfall exceeds ET
- Medium: Areas where the ET of a traditional turf landscape typically exceeds rainfall and that have zero to 2 months where rainfall exceeds ET
- Low: Areas where there is high ET for a traditional turf landscape and low precipitation, and that have 5 or fewer months where rainfall exceeds ET
- Lowest: Areas where there is very high ET for a traditional turf landscape and minimal precipitation, and that have 4 or fewer months where rainfall exceeds ET
- Not Recommended: Areas where there is very high ET for a traditional turf landscape and insignificant precipitation, and that have 1 or fewer months where rainfall exceeds ET.
The data used to develop the map have limitations. Daily precipitation is best for determining how much rain is available for harvesting. The maps were based on historical, monthly average rainfall. This does not account for monthly variations such as large rain events or rainy versus dry periods, and does not consider daily variations in irrigation requirements.
Rainwater harvesting for landscape irrigation may be appropriate for many areas across the United States, even in areas of low availability. Important considerations when planning for harvesting projects 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
- Permits: Rainwater harvesting permits may be required; check with local or state government
- Turf replacement: Consider replacing traditional turf with native landscaping that requires significantly less water and can make rainwater harvesting a viable option in many areas of the United States.
- FEMP's Best Management Practice #14: Alternative Water Sources
- U.S. Environmental Protection Agency’s WaterSense at Work: Best Practices
- Alliance for Water Efficiency's Introduction to Rainwater Harvesting
1 Precipitation and evapotranspiration data were provided to PNNL by the U.S. 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.