How Much Water Can you Collect?

The following illustrates the amount of rainwater that can be harvested from a catchment surface.  A catchment surface is typically a roof, such as on your home or work place, a barn, or shed etc., but could also include any other impervious surface where rain runs off.  Even a small catchment surface of 100 square feet can harvest just under 1000 gallons of water over the average rainfall season on the Monterey Peninsula.  A 5000 square foot catchment surface can harvest over 47,000 gallons per year during the average rainfall season. 
Catchment       Gallons / sq ft / inch of rain       Efficiency Factor      MP avg inches / Year       Gallons / Year
       100                          .623                                               .9                                       17                                  953
    1,000                          .623                                               .9                                       17                               9,532
    2,000                          .623                                               .9                                       17                             19,064
    3,000                          .623                                               .9                                       17                             28,596
    4,000                          .623                                               .9                                       17                             38,128
    5,000                          .623                                               .9                                       17                             47,660
100,000                          .623                                               .9                                       17                             95,319         
Definitions-
Catchment = The square footage of the area rain falls on that can be harvested
Gallons / sqft / inch of rain = .623 gallons can be collected per 1 inch of rain on a clean metal roof
Efficiency Factor = Most roofs are not metal and other roofing materials may absorb or evaporate rainfall
Note: The actual efficiency factor will vary based on the type of roof material on your home and .9 is used as an average.
MP avg inches / Year = The average inches of rainfall for the Monterey Peninsula during recent years
Gallons / Year = Estimated gallons harvestable based on 17 inches of average rainfall on the Monterey Peninsula.