Rainwater Collection Technical Brief—King County

Rainwater collection is the process of collecting, storing and reusing water that falls on a building. Typically it is easier to harvest rainwater in this manner, rather than treating water from the site that has accumulated ground-based contaminants.

Goals for Rainwater Collection

Rainwater is collected in order to offset the use of municipally treated potable water. Use of rainwater benefits the local environment by reducing the draw from local water bodies and by decreasing the volume of runoff and contaminants that must be municipally treated. Storage is critical to rainwater collection, as it accommodates the inconsistent nature of rainfall. However, in order to minimize the size of storage tanks, the first goal should be to reduce the building and site water demand. Then, after water use is minimized, calculations can be performed to estimate the ability of the rainwater system to offset potable water use.

How Much Rainwater Can Be Collected?

The amount of rainwater available for collection is inherently limited by the local climate. Typically rainfall is assessed in terms of average daily or monthly volumes. However, it is important to understand that rainfall is a seasonally variable resource that varies from day to day. Figure 1 shows the difference between average daily rainfall and actual daily rainfall in both volume and consistency. This underscores the importance of storage, and how collection capacity will affect the capability of a system to provide a consistent source of water through extended periods of dry weather.

Figure 1: A comparative graph showing average daily rainfall (shaded area) and actual rainfall data (dark line) for the Seattle climate.

Matching Use and Rainwater Volume

Selecting the appropriate use for rainwater is dependent on the volume, consistency and type (potable or non-potable) of the demand. There are three uses for rainwater that are typically considered: irrigation, toilet flushing, and domestic potable water. Treatment requirements vary for each use, as water used for toilet flushing or irrigation requires less purification than water supplied to sinks and lavatories. Most regulated water fixtures within a building, such as toilets and lavatories, are used consistently throughout the year. Process (sometimes called industrial) loads and irrigation, however, may vary seasonally or monthly. A water balance analysis can aid in matching the volume of rainwater that can be collected with an appropriate water demand of a building.

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