Calculating how much water is available
The formula for calculating how much water my roof will collect is:
[ Collection area (in square feet) x 144 (square inches per foot) x Rainfall (inches of rain water depth) ] / 231 (cubic inches per gallon) = Gallons of water
Those square feet are based on flat land, so the slope of my roof doesn't matter. With a house that's about 50 feet by 30 feet, that's 216,000 square inches. If I capture all of a one-inch rainfall that's 931 gallons. Okay, so one inch of rain is a lot of water.
How much rain can I expect?
I'm in Minnesota, and the average rainfall during each month of the growing season (April - October) is:
2.06 3.06 4.17 3.95 3.69 2.91 2.29That's 22.13 inches total, divided over 7 months is a little over 3 inches a month, so there's about one inch of rainfall every 10 days.
931 gallons every 10 days. That does seem useful. At a minimum I can keep the dry flower box damp.
Average rainfall for each state
You can find your state's average rainfall in the table below, which came from the NOAA. This is a rough estimate, as many states have wetter and drier areas.
Total Precipitation in inches by month Climatology by state based on climate division data: 1971-2000 State Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Total Alabama 5.91 5.18 6.65 4.77 4.69 4.59 5.39 3.90 4.26 3.16 4.79 5.01 58.28 Arizona 1.30 1.25 1.35 0.54 0.42 0.31 1.74 2.09 1.30 1.20 0.97 1.13 13.61 Arkansas 3.65 3.61 4.93 4.82 5.20 4.31 3.52 2.87 3.72 4.08 5.34 4.72 50.78 California 4.14 3.95 3.55 1.40 0.83 0.32 0.19 0.30 0.58 1.20 2.63 3.11 22.20 Colorado 0.79 0.75 1.29 1.47 1.92 1.44 2.01 1.95 1.35 1.20 1.03 0.77 15.97 Connecticut 4.30 3.23 4.42 4.30 4.33 4.05 4.19 4.39 4.36 4.31 4.42 4.08 50.39 Delaware 3.92 3.10 4.34 3.51 4.18 3.52 4.05 4.63 4.19 3.38 3.31 3.55 45.68 Florida 3.61 3.17 4.13 2.84 3.92 6.94 7.19 7.26 6.39 3.46 2.88 2.76 54.57 Georgia 5.20 4.49 5.28 3.62 3.58 4.44 5.05 4.66 3.89 3.01 3.55 3.94 50.72 Idaho 2.12 1.72 1.73 1.56 1.97 1.51 0.92 0.84 1.08 1.30 2.07 2.14 18.96 Illinois 1.97 1.99 3.22 3.83 4.31 4.12 3.94 3.69 3.24 2.87 3.41 2.74 39.32 Indiana 2.44 2.31 3.40 3.96 4.47 4.25 4.20 3.88 3.18 2.92 3.62 3.09 41.72 Iowa 0.95 0.98 2.20 3.33 4.23 4.62 4.27 4.18 3.40 2.51 2.13 1.24 34.05 Kansas 0.77 0.91 2.25 2.59 4.18 3.88 3.61 3.16 2.68 2.18 1.73 0.98 28.92 Kentucky 3.77 3.77 4.60 4.19 5.05 4.34 4.47 3.71 3.49 3.13 4.09 4.38 48.98 Louisiana 5.91 4.66 5.32 4.89 5.46 5.20 5.15 4.47 4.55 4.05 5.06 5.38 60.09 Maine 3.46 2.52 3.37 3.46 3.64 3.77 3.74 3.61 3.68 3.70 3.82 3.51 42.28 Maryland 3.66 3.03 4.09 3.43 4.27 3.80 4.11 4.01 4.04 3.38 3.41 3.41 44.64 Massachusetts 4.17 3.34 4.16 4.11 3.91 3.84 3.83 4.02 3.94 4.16 4.35 4.05 47.88 Michigan 2.02 1.45 2.22 2.69 2.94 3.23 3.15 3.58 3.65 2.85 2.77 2.28 32.84 Minnesota 0.86 0.64 1.45 2.06 3.06 4.17 3.95 3.69 2.91 2.29 1.58 0.77 27.44 Mississippi 5.92 4.96 6.30 5.61 5.39 4.47 4.80 3.67 3.81 3.45 5.19 5.64 59.23 Missouri 1.95 2.15 3.51 4.04 4.82 4.26 3.96 3.58 3.97 3.36 3.82 2.81 42.23 Montana 0.80 0.60 0.90 1.30 2.34 2.40 1.65 1.35 1.33 1.05 0.86 0.80 15.37 Nebraska 0.51 0.60 1.66 2.38 3.75 3.45 3.27 2.63 2.10 1.56 1.16 0.57 23.63 Nevada 0.96 0.92 1.09 0.76 1.01 0.63 0.51 0.63 0.71 0.75 0.81 0.76 9.54 New Hampshire 3.42 2.62 3.37 3.50 3.76 3.85 3.94 3.97 3.66 3.95 3.93 3.44 43.42 New Jersey 3.94 2.96 4.18 3.93 4.28 3.77 4.46 4.57 4.14 3.51 3.70 3.71 47.15 New Mexico 0.67 0.60 0.71 0.63 1.11 1.23 2.27 2.64 1.82 1.34 0.83 0.78 14.63 New York 2.99 2.41 3.12 3.42 3.68 4.00 3.81 3.86 4.14 3.48 3.74 3.24 41.90 North Carolina 4.51 3.71 4.67 3.47 4.28 4.40 5.04 4.97 4.78 3.61 3.45 3.57 50.45 North Dakota 0.50 0.45 0.80 1.40 2.31 3.19 2.75 2.10 1.74 1.41 0.73 0.44 17.82 Ohio 2.51 2.27 3.06 3.47 4.06 4.10 4.09 3.77 3.10 2.62 3.16 2.93 39.16 Oklahoma 1.48 1.78 3.07 3.32 5.13 4.24 2.73 2.75 3.80 3.39 2.79 2.05 36.55 Oregon 3.79 3.16 2.93 2.18 1.88 1.27 0.62 0.73 1.07 1.90 3.97 4.06 27.55 Pennsylvania 3.04 2.58 3.41 3.55 4.07 4.45 4.15 3.78 4.08 3.18 3.58 3.16 43.02 Rhode Island 4.45 3.63 4.65 4.32 3.72 3.52 3.20 3.99 3.80 3.79 4.55 4.36 47.98 South Carolina 4.70 3.79 4.64 3.16 3.69 4.72 5.05 5.29 4.51 3.42 3.21 3.67 49.84 South Dakota 0.45 0.53 1.30 2.11 3.10 3.24 2.75 2.13 1.66 1.62 0.82 0.44 20.14 Tennessee 4.65 4.31 5.60 4.51 5.33 4.48 4.67 3.52 3.88 3.32 4.77 5.18 54.22 Texas 1.59 1.67 1.85 2.14 3.52 3.35 2.22 2.54 3.19 2.95 2.02 1.84 28.87 Utah 1.05 0.99 1.21 1.07 1.20 0.64 0.88 1.01 1.07 1.30 0.99 0.82 12.26 Vermont 3.11 2.33 3.07 3.28 3.84 3.93 4.21 4.49 3.97 3.67 3.73 3.19 42.82 Virginia 3.64 3.13 4.04 3.42 4.26 3.79 4.34 3.84 4.00 3.50 3.29 3.15 44.39 Washington 5.38 4.37 3.76 2.71 2.23 1.77 1.02 1.05 1.65 3.05 5.85 5.93 38.78 West Virginia 3.46 3.11 3.97 3.62 4.57 4.23 4.75 4.13 3.51 3.05 3.46 3.43 45.30 Wisconsin 1.22 1.00 1.96 2.86 3.37 4.02 4.07 4.27 3.74 2.50 2.29 1.35 32.64 Wyoming 0.63 0.57 0.86 1.36 2.06 1.58 1.33 1.02 1.14 1.04 0.76 0.62 12.97 Calculated at NOAA-CIRES CDC based on data obtained from NCDC
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