The Pacific Northwest National Laboratory (PNNL), in collaboration with the Federal Energy Management Program (FEMP), developed an online, Geographic Information System (GIS)-based Alternative Water Supply Tool to help federal agencies strategically prioritize commercial alternative water projects. The mapping tool provides four layers of data for alternative water supplies:
- State-by-state Rainwater Harvesting Regulations: Information on requirements and technical resources for using rainwater harvesting by state.
- Rainwater Harvesting Potential for Year-round Collection: Relative potential to collect rainwater throughout the year across the United States.
- Rainwater Harvesting Potential for Landscape Irrigation: Relative potential to collect rainwater specifically conducive for irrigating landscape.
- Condensate Capture Potential: Relative potential of collecting condensate capture from air-conditioning systems.
Learn how to use the tool and gather data.
For information on the methods used to develop these maps, see the tool’s Help Guide or read PNNL’s technical report. The help guide also provides information on applying the results of the tool, tips for alternative water projects, and other considerations.
Alternative Water Sources
The tool provides data layers for two types of alternative water supplies—rainwater harvesting and condensate capture.
Rainwater Harvesting
Rainwater harvesting systems capture, divert, and store precipitation from rooftops and other hard surfaces for later use. The tool provides a range of available rainwater for harvesting across the United States, both for general applications and specifically for landscape irrigation.
Most commonly, harvested rainwater is used for non-potable applications such as:
- Landscape irrigation
- Toilet and urinal flushing
- Vehicle wash
- Dust suppression
- Cooling tower makeup
- Ornamental pond and fountain filling.
In addition to providing alternative water, harvesting rainwater can also potentially prevent stormwater from entering waterways, helping agencies meet federal requirements for stormwater management. Reducing stormwater eases flooding and erosion by slowing runoff and allowing it to soak into the ground, turning stormwater problems into water supply assets. Less runoff also means less contamination of surface water from sediment, fertilizers, pesticides, and other pollutants that runoff might transport.
The data used to develop the tool has limitations. While daily precipitation is best for determining how much rainfall is available for harvesting, the tool utilizes historical, monthly average rainfall. This does not account for variations across the month such as large rain events or rainy versus dry periods.
Rainwater harvesting may be appropriate for many areas across the United States, even in areas of low rainfall availability. Important considerations when planning for harvesting projects should include the following:
- Size of catchment area (roof size): A 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 (See FEMP's Best Management Practice #14: Alternative Water Sources).
- Regulations: Some states limit rainwater harvesting, so check the tool’s mapping layer on state regulations to make sure it is allowed.
- Permits: Rainwater harvesting permits may be required; check with local or state government.
- Turf replacement (landscape irrigation only): 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.
- Size of catchment area (roof size): A larger roof area can capture significant precipitation even in areas of low rainfall availability.
Condensate Capture
Water condenses on air-handling units (AHUs) and cooling coils when warmer humid air contacts these cool surfaces. A large amount of condensate can form on cooling equipment in areas with hot, humid summers such as the southeastern United States.
Condensate water that collects on the AHUs and cooling coils must be drained to prevent damage to the equipment or the building from water buildup. Typically, the condensate is collected in a central location and discharged to a sewer drain. In a condensate capturing system, the condensate is directed to a central storage tank or basin, treated appropriately, and then distributed for reuse.
The condensate capture potential presented on this map is based on local climate conditions of the selected cities. Within the regions, the condensate capture potential may vary due to local annual weather patterns. Other factors affecting the potential for condensate capture should also be considered when assessing the potential at a specific site; these include the following:
- Size of cooling system: Larger systems can potentially produce more condensate (depending on cooling system operation).
- Cooling system operation: Hours of operation and temperature set points will influence condensate production.
- Size of cooling system: Larger systems can potentially produce more condensate (depending on cooling system operation).