Cistern Model

 

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Outdoor uses account for approximately two-thirds of household water consumption. Rainwater stored in cisterns can be used for landscape irrigation, reducing the use of potable water for that purpose. Because cisterns store rainfall that would otherwise run off, they can also reduce the potential for flooding. This model allows you to build a virtual cistern and see how it would have performed during nine different rainy seasons in Los Angeles.

Use the slider controls to specify the physical structure of the cistern:

  1. Capacity is the volume of water (in gallons) that you want the cistern to hold
  2. Collection area is the square footage of your catchment (the surface from which you’ll capture rainfall)
  3. Outflow rate determines the speed at which the cistern can be emptied
  4. Reserve is the amount of water you want to keep on hand for irrigation
  5. Irrigation rate is the daily amount of water required to maintain your landscaping.

Use the procedural controls to specify the operation of the cistern:
During periods of high rainfall, runoff is already high, so releasing cistern water could exacerbate flooding. The text fields allow you to specify periods when water is not released. (Note: if the cistern overflows, the amount of overflow is effectively released.)

Putting it all together
The physical structure and the operating procedures work together. When the cistern contains more than the reserve amount, an amount up to the daily maximum can be released until the water drops to the reserve level.

The line graph shows the amount of water in the cistern for each day of the rainy season.
As you change the specifications or operation of the cistern, the chart reflects the resulting changes in the volume of water stored. You can make the dates and stored amounts appear by moving your mouse across the chart. Press the Daily Results button to view rainfall data and cistern performance through the season. The Reset button makes the slider controls specify TreePeople’s demonstration cistern (the procedural controls are not reset.)

The rainfall data source is online at www.nwsla.noaa.gov/climate.