4.9.1 Applications & Limitations

Rain water harvesting systems are typically used where rainfall or other environmental conditions limit the availability of domestic water supply, but can provide multiple environmental and economic benefits. Some of the applications and limitations on the use of rain water harvesting systems include:
  • Arid and semi-arid climates where water availability is scarce.
  • Residential and commercial sites with high irrigation and/or non-potable water demands (see Figures 4.9.2 and 4.9.3).
  • Indoor re-use of harvested water for toilet flushing and cold water for laundry.
  • Exterior re-use of harvested water for cleaning, irrigation, and other non-potable uses.
  • Combined sewer overflow (CSO) reduction in urban areas.
  • Only appropriate for collection of stormwater runoff from roof surfaces and not from vehicular or pedestrian areas, surface water runoff, or bodies of standing water.

A challenge in eastern Washington is that the majority of the rain fall occurs during the winter and spring. Summer water demand for irrigation typically exceeds the amount of rain water harvested on the site. Consequently, rain water harvest systems that are used as the sole supply for irrigation will need to be connected with a water supply so that the tanks do not go dry in the summer.

Indoor re-use typically requires pumping and treatment, which may increase the long-term costs for this BMP. However, in highly urbanized areas where land rents are high, these systems may allow scarce land resources to be placed into use and provide valuable educational opportunities. Check with the local jurisdiction and applicable plumbing code requirements for allowable indoor and outdoor re-use applications and associated design requirements.

Urban areas with CSO problems can use rain water harvest systems to capture, store, reuse, and/or slowly release detained stormwater, thereby minimizing peak stormwater flow rates to the combined sewer systems and helping to reduce CSO frequency and volume.

Figure 4.9.2 - Cistern used to meet irrigation needs
Rain water harvest system used to meet the irrigation demands at the Eagle Veterinary Hospital in San Antonio, Texas. Source: Innovative Water Systems, LLC

Figure 4.9.3 - Cistern used to meet non-potable water needs
Rain water harvest system used to meet the non-potable water demands of the Denton Fire Station.
Source: Innovative Water Systems, LLC