1. Rainwater Harvesting: Cistern Size Optimization Study
Washington DC Precipitation ( 50,000 SF Roof )
Assumed Design Criteria: 4,000 Gallons Per Day Use for Toilet Flushing
Overflow
Captured & Used
Use of Potable
Proposed Tank Size
16,000 Gallons
Total Water (Millions of Gallons)
This study is based on the historical rainfall data over a twenty year period in Washington DC.
These graphs were 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 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 for toilet flushing by approximately 50%.
The key variables in this type of study are: location of roof, size of roof (sf), and rate of water use (gallons per day).
A simple computer algorithm was written to generate the lines on the graph based upon these variables.
Historical precipitation data was found at 16
Cistern Size (Thousands of Gallons)

2. Rainwater Harvesting: Cistern Size Optimization Study
Washington DC Precipitation ( 50,000 SF Roof )
Assumed Design Criteria: 5,000 Gallons Per Day Use for Toilet Flushing
Overflow
Captured & Used
Use of Potable
Proposed Tank Size
22,000 Gallons
Total Water (Millions of Gallons) 22
Cistern Size (Thousands of Gallons)

3. Rainwater Harvesting: Cistern Size Optimization Study
Washington DC Precipitation ( 50,000 SF Roof )
Assumed Design Criteria: 6,000 Gallons Per Day Use for Toilet Flushing
Overflow
Captured & Used
Use of Potable
Proposed Tank Size
34,000 Gallons
Total Water (Millions of Gallons) 34
Cistern Size (Thousands of Gallons)