Water: Harvesting Rain Water

Water & Resilience.

Through a case-study of Troy Gardens Community Co-housing.

Related post: Food; Filtering Water


  • With 101,920 square feet of impermeable surface area, Troy can harvest about 1.8 M gallons of water (63% of the water we use).
  • NOW: How much of the harvestable water can we actually store and use? Use depends on storage capacity and ability to purify the water for drinking and cooking (see #5 below)


To understand Troy’s water resilience, we might start with knowing how much water we can harvest and how much we use. For those interested, those calculations are below. However, after doing this work, I realized that it’s probably not all that relevant since without a lake, or huge cisterns, we are unlikely to be able to store the water that runs off the property. Thus, storage becomes the central issue.

(1) Calculate catchment area as length x width of roofs, walks, parking, and other non-permeable surfaces.

  • South Courtyard impermeable surface area = 44,080 square feet
  • North Couryard = 57,840 square feet
  • Total = 101,920 square feet (approx)

(2) Calculate amount of runoff. “The general rule of thumb is for 1,000 square feet of catchment surface, each inch of rain produces 600 gallons of harvestable water” (source), but the formula for more precise calculation is:

Gallons of rainwater harvested per year = Inches Annual Rainfall / 12 * (Square Feet from #1) * 7.48 * (% Runoff)

For example, 90% runoff rate, 48 inches of annual rainfall, 100 sq ft of catchment area = 48/12 * 100 * 7.48 * 90% = 2693 gallons of rainwater. [source is spreadsheet downloadable here].

The table below presents annual precipitation statistics for Madison, WI (based on precipitation data from 1869-2003 from this table), the calculated annual gallons of rainwater Troy can harvest based on 101,920 square feet of impermeable surface area, and the amount of money that water is worth based on a rate of $2.81 per 1,000 gallons currently quoted by Madison Water (see below).

Annual Precipitation (in) Gallons Harvestable/yr at Troy Troy Savings Per Year
Average: 31.42 1,796,505 $5,048
Median: 31.58 1,805,653 $5,074
SD: 6.13 350,496 $985
Min: 13.54 774,178 $2,175
Max: 52.94 3,026,957 $8,506
95% range: 19.15 – 43.68 1,094,942 – 2,497,497 $3,077 – $7,018


(3) Cost of water & average water usage in Madison, WI: “The cost for 1,000 gallons of water [in Madison, WI] is $2.81 and for sewer $2.33. Overall use in the city in 2010 was 105.12 gallons per person per day, though less for residential accounts—65 gallons per person per day.” (source)

(4) Apply to Troy. Assume 65 gallons of water per person per day. Assume 120 people.

Used Per Year: 2,847,000
Harvested Avg (from above): 1,796,505
% Harvested of Total Used: 63%
Net needed: 1,050,495
Cost: $2,952


(5) See this post on water filters.

(6) Logistical: How do you get the water to where it’s needed (e.g., flushing your toilet, bath)

(7) How much water can we actually store?

6 thoughts on “Water: Harvesting Rain Water”

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