Cisterns and Stormwater

Rainwater collection system at Northside Family Learning Center, Richmond, VA

Cisterns make sense, why not use stormwater as resource and reduce the demand on potable water supply systems. I have successfully designed cistern systems, I encourage the use of cisterns, and I look forward to the standardized methodology to calculate benefits in the proposed regulations . However, I feel that there are some misconceptions about cisterns that need to be addressed:

  • If the cistern is used only for irrigation, then the cistern does nothing for reducing stormwater impacts in the 6 or 7 months out of the year when there is no irrigation. Cisterns used solely for irrigation need to overflow to another measure, such as bioretention.  Is there a stormwater benefit for a measure that works for only half of the year
  •  Rainfall is unpredictable. In the last ten years in Richmond, the annual rainfall varied by a factor of two, meaning that even though the annual rainfall is 43”, the wettest year was 60.2 inches and the driest was 28.3 inches.  Also, 90% of the rainfall in a year can come from only half of the rain events. This makes cistern sizing very difficult. Given that cisterns can be very expensive, actual rainfall data should be used to determine the most cost effective size. For example, if doubling the cistern size results in reducing the make-up water cost by only 10%, there’s no point to a larger cistern.

I have more to say about cisterns, most of it good. I would love to hear from other people on their experience.