1. Rainwater Harvesting: Cistern Performance Simulation
New York City Precipitation ( 40,000 sf roof )
Rainwater Overflow
Rainwater Captured & Used
Potable Water Used
3,000 Gallons Per Day Used For
Toilet Flushing (Projected Estimate)
Total Water (Millions of Gallons)
12,000 Gallon Tank
This cistern performance simulation study is based upon the historical rainfall data over a twenty year period in New York City.
The study includes six different scenarios (two different roof sizes, each with three different water use demand projections).
It is intended that these graphs may help to convey the long term effectiveness of rainwater harvesting strategies with respect to both reduced water use and reduced stormwater overflow.
These graphs may also be used to help to identify the optimal cistern size for various rates of use as toilet flushing water (refer to LEED Credit WE 2 – Innovative Wastewater Technologies).
The goal of this credit is to reduce the use of potable water for sewage conveyance. Water conserving toilets and urinals can also contribute to this credit.
The graphs indicate a cistern size that would effectively reduce the use of potable water by approximately 50% (independent of fixture type).
The key variables in this type of study are: location of roof (precipitation data), size of roof (sf), and rate of water use (gallons per day).
A computer algorithm was written to generate the lines on the graph based upon these variables. Historical precipitation data was found at
Note that the values of red line plus the blue line are always equal to the dotted black line that indicates the projected daily use.
Also note that as the cistern capacity grows, the rainwater harvesting potential continues to increase but at a diminishing rate.
This effect will be countered somewhat by the greater cost efficiency of a large tank/system versus a smaller tank/system.
If a primary project goal is to reduce total stormwater overflow, the cistern size should be made as large as is feasible (e.g. - twice as large as the tank sizes indicated on the graphs).
Cistern Size (Thousands of Gallons)

2. Rainwater Harvesting: Cistern Performance Simulation
New York City Precipitation ( 40,000 sf roof )
Rainwater Overflow
Rainwater Captured & Used
Potable Water Used
4,000 Gallons Per Day Used For
Toilet Flushing (Projected Estimate)
20,000 Gallon Tank
Total Water (Millions of Gallons)
Cistern Size (Thousands of Gallons)

3. Rainwater Harvesting: Cistern Performance Simulation
New York City Precipitation ( 40,000 sf roof )
Rainwater Overflow
Rainwater Captured & Used
Potable Water Used
5,000 Gallons Per Day Used For
Toilet Flushing (Projected Estimate)
30,000 Gallon Tank
Total Water (Millions of Gallons)
Cistern Size (Thousands of Gallons)

4. Rainwater Harvesting: Cistern Performance Simulation
New York City Precipitation ( 20,000 sf roof )
Rainwater Overflow
Rainwater Captured & Used
Potable Water Used
Total Water (Millions of Gallons)
1,500 Gallons Per Day Used For
Toilet Flushing (Projected Estimate)
6,000 Gallon Tank
Cistern Size (Thousands of Gallons)

5. Rainwater Harvesting: Cistern Performance Simulation
New York City Precipitation ( 20,000 sf roof )
Rainwater Overflow
Rainwater Captured & Used
Potable Water Used
2,000 Gallons Per Day Used For
Toilet Flushing (Projected Estimate)
Total Water (Millions of Gallons)
10,000 Gallon Tank
Cistern Size (Thousands of Gallons)

6. Rainwater Harvesting: Cistern Performance Simulation
New York City Precipitation ( 20,000 sf roof )
Rainwater Overflow
Rainwater Captured & Used
Potable Water Used
2,500 Gallons Per Day Used For
Toilet Flushing (Projected Estimate)
Total Water (Millions of Gallons)
15,000 Gallon Tank
Cistern Size (Thousands of Gallons)