4.1: Usage

Rainwater harvesting systems are typically sized based upon supply or demand. In either case, it is important to calculate, and maybe even conserve, demand. Many sources of demand can exist for a system with varying methods to determine their relative demands. For example,

• Sinks
• Showers
• Dishwashing machines
• Clothes washing machines
• Toilets
• Garden hoses
• Farm irrigation
• Livestock
• Leaks

Washing machines and toilets are measured in volume per use. Here are some typical values:

• Dishwashing machines: Many older machines are 10-15 $$\frac{gal}{cycle}$$. Many Energy Star machines use 4-6 $$\frac{gal}{cycle}$$.
• Clothes washing machines: Many older machines are 40-45 $$\frac{gal}{cycle}$$. A full-size Energy Star machine uses approximately 13 $$\frac{gal}{cycle}$$.
• Toilets: Many older toilets are 3.5-7 $$\frac{gal}{flush}$$ (GPF). A Watersense High-Efficiency Toilet uses just 1.28 GPF.

After determining the water usage of your washing and flushing machines, you can count or estimate the number of usages per a certain length of time (e.g. per month) to determine your demand from those machines.

Sinks, showers, garden hoses, and various household leaks, among other items, are determined by their volumetric flow and time used. Volumetric flow rate “Q” is the volume of water passing through a cross-sectional area in a given amount of time. Two common examples are gallons per minute (GPM) and liters per second (LPS). Volumetric flow is defined in the following formula:

$Q = \frac{V}{t}$

• V = volume
• Q = volumetric flow
• t = time

Therefore, volume can be written as:

$V=Q*t$

For example, a low flow showerhead is 1.5 GPM. If you shower for a leisurely 11 minutes, the volume is:

$V=Q*t=1.5\;\frac{gal}{min}*11\;\frac{min}{use}=16.5\;\frac{gal}{use}$

If you do that daily:

$16.5\;\frac{gal}{use}*1\;\frac{use}{day}*30\;\frac{days}{month}=495\;\frac{gal}{month}$

Farm irrigation and livestock demands are dependent on many environmental and technical factors. Determining those demands are outside the scope of this book. Looking at existing use for each month in the past year (or even better, longer-term monthly averages) at your location is a great place to start.