Using a spreadsheet to model rain barrel efficiency and cost-benefit for homeowners
Logan, J.  2014.  HortTechnology, 24:156-158.

Abstract:
By collecting rain from a roof during rain events and storing it in a barrel or cistern, homeowners can create an alternative supply for their home gardens and landscaping that won’t over pump the groundwater or increase the water bill. The typical homeowner rain barrel is a 55-gal heavy-duty plastic barrel with a spigot located near the bottom of the tank. Water is either pumped or allowed to flow by gravity to a nearby garden. The objective of this study was to assess the amount of supplemental irrigation water that a 55-gal rain barrel could provide for a typical 400 ft2 vegetable garden for 21 locations across the eastern U.S.A. Daily rainfall (1981-2010) data for the growing season (May 1- Oct 15) was used in an Excel® spreadsheet that allows for multiple, user-selected criteria such as the size of the barrel and the catchment area as well as the watering habits of the homeowner. By limiting the criteria to only the capacity of the rain barrel, an average of 70% or more of the irrigation requirements could be met by a single barrel. However, most homeowners do not use their rain barrel to maximum efficiency, so a more realistic scenario of watering habits shows that only 23-35% of the seasonal irrigation requirements are fulfilled by the rain barrel. However, by installing two or three rain barrels under different catchment areas, as much as 100% of the supplemental irrigation needs can be met.