Rainwater Harvesting: Cistern Size Optimization Study 1990-2009 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 www.wunderground.com. 16 Cistern Size (Thousands of Gallons)

Rainwater Harvesting: Cistern Size Optimization Study 1990-2009 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)

Rainwater Harvesting: Cistern Size Optimization Study 1990-2009 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)