Rainwater Catchment

Rainwater Harvesting

Photo by inclinedesign
Rainwater collection, including rain barrels, is certainly nothing new; humans have been doing it for thousands of years.  However, with the advent of cheap, potable water delivered right to your doorstep, those who harvest rain have become somewhat of an anomaly.  This is changing in Washington State, largely for three reasons:

On October 12, 2009, Ecology issued an Interpretive Policy Statement clarifying that a water right is not required for rooftop rainwater harvesting.

If and when the department determines that rooftop or guzzler rainwater harvesting systems are likely to negatively affect instream values or existing water rights, local restrictions may be set in place to govern subsequent new systems (there are currently no restrictions).  However, Ecology generally does not expect the collection of harvested rainwater to cause problems or reduce the amount of runoff that would have occurred from the site in its natural, pre-development state.

The rainwater storage system may also be used to store groundwater withdrawn under the groundwater permit exemption for future beneficial use (i.e. an owner may pump groundwater in the winter under the permit exemption, store it, and then use it in the spring and summer).  The interpretive policy statement limits water use from rainwater harvesting systems (including storage) to the same parcel where water is captured.

Rainwater Harvesting Calculator

Ecology created a Rainwater Harvesting Calculator (RWH) to help Washingtonians size their RWH system based on the typical climate where they live.  The model allows the user to test various RWH scenarios (different roof size, cistern size, indoor use, outdoor use) and see how well the desired use is achieved during a normal precipitation year.

To use this calculator start on the Data Entry page.  Toggle to the city nearest you and begin entering data in the green highlighted cells. Every green highlighted cell must be filled out.  When done, you can review the results in the blue highlighted cells on the bottom half of the page. Furthermore, you can scroll to the different charts to see a more visual representation of how well your RWH system is meeting your needs.  You can change the data entries as many times as you wish.


Barbara Anderson


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Rainwater Harvesting Calculator