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The continuous population growth and the growing number of extreme droughts across the world have led to a great increase in consumption of potable and non-potable water. Conservation of rainwater is becoming critical in parts of the United States to meet the growing water demands. Living in a country where water has always been readily available, most people do not realize that rainwater can be used for nearly all non-potable applications including irrigation, toilet flushing, bathroom sinks, mechanical systems, washing machines, car washing, custodial uses, and many more.

Rainwater harvesting is the collection, conveyance, and storage of rainwater. Systems can be as simple as a rain barrel for garden irrigation at the end of a downspout, or as complex as a domestic potable system or a multiple end-use system at a large corporate campus. ParkUSA’s RainTrooper is a solution for both commercial and residential applications to conserve as much rain as possible to store for future use, and reduce consumption of limited treated municipal water.

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A best management practice (BMP) for sizing any rainwater harvesting system is to determine the volume of water that can be captured and stored (the supply) and this should equal or exceed the volume of water used (the demand).

Another factor to consider is the loss if rainwater to first flush, evaporation, splash-out, or overshoot from the gutters in hard rains and leaks. Rough collection surfaces are less efficient at conveying water because water captured in pore spaces tends to be lost to evaporation. Also impacting achievable efficiency is the inability of the system to capture all water during intense rainfall events. For instance, if the flow-through capacity of a filter-type roof washer is exceeded, spillage may occur. Additionally, once storage tanks are full, rainwater is lost as overflow.

The Rational Method is used to calculate the potential supply of rainwater runoff:



  • Q = Average Monthly Rainwater Runoff Rate from drainage area or Average Monthly Supply (GPM)
  • C = Runoff Coefficient, the fraction of rainfall on the drainage area that becomes stormwater runoff. Runoff coefficients can range from as high as 0.80-0.85 (for a well-constructed corrugated-iron roof) to 0.10-0.20 (for a compacted soil surface). 
  • I = Average Intensity of Rainfall (in/hr)
    To solve for the average rainfall intensity, find the annual precipitation for the area then divide by 12 months to determine monthly average and ultimately the monthly supply. Consider seasonal adjustments depending on the application
  • A = Drainage Area (sq.ft.), the area that drains to the design point of interest
    A conversion factor of 7.48 gallons of water per one cubic foot of area will be necessary to change the final result from cubic feet to gallons.

If the catchment area is comprised of a variety of different surfaces, with different runoff coefficients, then a weighted average value should be calculated.

Monthly precipitation is the key calculation to the rainwater harvest supply as it must equal or exceed the monthly demand for water usage. Deficiencies in monthly precipitation are typically “made up” by piping costly city water to the system. No one can outguess the weather month to month so “make up” water piping and associated valves are necessary in any system. The trick is to minimize the use of make-up water through good planning during this phase of the sizing process. Under sizing of a system defeats the purpose of this Green Building BMP and ultimately the owner realizes limited savings from the investment.

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