Last night's downpour covered New York City in an inch of rain and caused flooding in some neighborhoods. Storms place a huge burden on the city's sewer system, the effects of which are felt by the local watershed. GrowNYC has helped build over 60 rainwater harvesting systems in community gardens throughout the city. These systems help mitigate stormwater runoff, reduce the demand on the public water supply system, and make it easy for gardeners to water their gardens. But just how much water do they save? The general formula we use for calculating rainwater harvesting potential is that one thousand square feet of catchment area (like a roof) will yield 600 gallons of water per inch of rain. We also take into account the efficiency of the systems and say that 75% of potentially harvestable rain is actually collected. If a typical building's footprint is 20' x 50' (1000 square feet), that means 450 gallons of water could have been harvested by last night's one inch rainfall. A typical 10' x 10' shed could have harvested 45 gallons. For reference, a typical watering can is about 2.5 gallons. More information about GrowNYC's rainwater harvesting program is here. For a PDF guide to building your own system, click here.