Activity 4.1: Your Water Budget

Activity 4.1: Your Water Budget

Just like any budget, you can calculate your water budget to determine if you are living within your means. Are you using as much water as is falling on your site? This is a useful gauge even if you are not living off rainwater, to get a sense of the impact of your purchases and practices.

In this activity you will:

1) Calculate water usage.

It is not as simple as adding up a few numbers. Many of our purchases (e.g. food, clothing, and gas) take large amounts of water to produce and it's hard to get an accurate measure. Agriculture is the largest user of water in the US (source: FAO), so our food choices might, in the end, have the most effect on our water usage.

a. Online water use calculators.

There are numerous ones out there and they provide a ballpark estimate at best. Pay attention to assumptions of each.

Try this to start;

Conduct an Internet search using terms like "water usage calculator" and for comparison gather your numbers from at least two other water use calculators.  Note your observations and conclusions and compare to the estimate you make in part b of this assignment.

b. Estimate household usage.

Use the following chart to note an estimate of usage:





 ___ showers/week x ____ minutes x 4 gallons/minute =



 ____ baths/week x 36 gallons/bath =



 ___ people x 20 flushes/week x 4 gallons/flush =



 ___ people x ____minutes/week x 3 gallons/minute =



 ___ loads/week x 40 gallons/load =



 ____ loads/week x 13 gallons/load =


 Hand &
 Dish Washing

 ___ times/weekly x ____minutes x 3 gallons/min.= 


 Lawn &

 ___ times/week x ____total minutes x 3 gallons/minute =















2) Calculate potential capture.

a. Rainwater catchment. 

To calculate the annual rainwater catchment potential coming off your roof surface you will need the square footage of all the roof structures on your home site. You can simply measure the "footprint" or square footage of your buildings to get an estimate. You will also need to find out what your annual rainfall is estimated to be (easy enough to find out with a quick Internet search). Also know that there are 7.48 gallons per cubic foot of water.

Then use this formula: 

(square footage (ft2) X annual rainfall (ft) X 7.48 gallons/ft3 = maximum runoff (gallons)

For example:

Yurt = 625 sq feet

Shed = 100 sq feet

Duck house = 20 square feet

TOTAL = 745 square feet

36 inches (3 feet) of rain annually 

745 (ft2) X 3 (ft) X 7.48 gal/ft3 = 16, 717 gallons of water per year. 


b. Site Runoff.

To calculate site runoff you need to know the square footage (catchment area) of your site, your annual rainfall estimate, and your estimate runoff coefficient (some provided below). Runoff coefficient relates the amount of runoff to the amount of precipitation received. It is a larger value for areas with low infiltration and high runoff (pavement, steep gradient), and lower for permeable, well vegetated areas (forest, flat land).

Then use this formula: 

catchment area (ft2) X annual rainfall (ft) X 7.48 gal/ft3 X runoff coefficient = net runoff (gal)

For example - a six acre site: 

261,360 ft2 X 3 ft X 7.48 gal/ft3 X .2 = 1,172,984 gallons of water per year.

*Note: 1 acre = 43,560 sq ft

Some runoff coefficients:

   Urban areas                  0.70 - 0.95

   Sandy soil, flat, 2%              0.05 - .10
   Sandy soil, avg., 2-7%          .10 - .15
   Sandy soil, steep, 7%           .13 - .17
   Heavy soil, flat, 2%              .15 - .20
   Heavy soil, avg., 2-7%          .18 - .22
   Heavy soil, steep, 7%            .25 - .35

Agricultural land  
  Bare packed soil              0.30 - 0.60  
  Cultivated rows              0.20 - 0.50
  Pasture                          0.15 - 0.45
  Woodlands                      0.05 - 0.25

With both of these formulas, you can substitute annual rainfall amounts for large and small rain events, to get a sense of how much water would be coming onto the site during these times. Don't forget to convert inches of rain to feet, to get the formula to work.

3) Compare water usage and potential capture.

Are you within your budget? Note though rainwater could be substituted for many of your current water uses, but runoff most likely only for use in the landscape.

4) Submit assignment.