11.1: How Much Water Do We Really Use?

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Community water use is often expressed as gallons per capita per day (gpcd). The term “per capita” is the same as per person. How much does one person use each day? In general, an average person uses water daily to take a shower, use the restroom, cook, drink, wash dishes and clothes, brush teeth, wash hands, etc. The amount of water consumed/used can be estimated if certain assumptions are made. For example, it can be assumed that a typical shower head has a flow rate of 5 gallons per minute and each individual takes one shower per day. A toilet might use 2 gallons per flush and each person is assumed to use the restroom three times per day. An efficient dish and clothes washer might use 7 gallons and 20 gallons respectively. Brushing teeth, washing hands, drinking, and cooking might add up to 2 gallons. You can see that the typical amount of water usage can then be calculated rather quickly. These values are typical for efficient usage. Older toilets and appliances can use several times the amount of water use. The table below gives some examples of efficient and non‐efficient indoor water usage by appliance or device. The units are in gallons.

Toilet
Conventional	Low-Flow		Ultra Low-Flow
5.0	3.5		1.6
Washing Machine
Conventional	Efficient			Front Load
37.0	26.0			21.0
Dishwasher
Conventional	Efficient
18.0	6.0
Shower Heads
Conventional	Low-Flow
5.0	2.5
Faucets
Conventional		Low-Flow
3.0		2.5

Although these values can add up to significant daily usage, the primary household water use can be attributed to landscape irrigation. Due to a combination of climate and lifestyle, Californians can use up to 70% of their household water consumption on landscape irrigation. Depending on the size of the actual landscaped area the water usage will vary dramatically and the gpcd will closely follow. Conversely, the national average water use for landscape irrigation is much lower at approximately 30%.

Utilities don’t usually go into this much detail in calculating gpcd. Some utilities look at the entire water used in one year and compare it to the total population served. This can provide an adequate answer, but if the utility provides water to large commercial or industrial customers the gpcd can be skewed. Usually, a utility will only look at residential water usage and estimate the total population. Regardless of the process, the per capita water use is expressed as the amount of water used per person per day as indicated in the following formula:

gpcd = water used (gpd)/total number of people

Remember, the total number of people represents the population you are observing. For example, if it is a house of 5 people then the total number of people would equal five. In addition, if you are looking at the total amount of water used in one year, it is probably represented as acre‐feet per year (AFY), in which case it would need to be converted to gallons per day.

In 2009 Senate Bill SBx7‐7 was signed into law. It requires California water utilities to reduce water consumption in urban water use by 20% by the year 2020. This has prompted utilities to implement water conservation strategies such as high-efficiency appliance rebate programs, various other incentives, tiered water rates, and water budget-based water rates. All of these programs are designed to help reduce water use. Calculating gpcd is extremely important for water utilities, especially in California, or any other area where water conservation is needed or more importantly mandated.

Exercises

Solve the following problems.

What is the average gpcd of a small community of 3,200 people that use approximately 500,000 gallons per day?
A water utility produced 15,000 AF of water last year that supplied a population of 50,000. What was the averaged gpcd for this community?
A water utility in northern California has an average gpcd of 155. If the total population of this community is 42,300, how much AF of water must this utility supply in one year?
A large water utility has 10 wells that can produce a combined flow of 15,500 gpm. The average gpcd for this community of 175,000 residents is 235. How many AF of water must this utility purchase per year to make up the supply needed for this community?
A small water system has one well that pumps 250 gpm. This well serves a population of 1,575 with an average gpcd of 195. How many hours per day must this well run to meet the demand?
What is the gpcd of a community with 1,250,000 people if the annual water used is 245,000 AF?
A water district has a goal of 145 gpcd and an annual water projection of 21,250 AF. What is the population that can be served?
A house of 4 people used 36 CCF of water in 31 days. What is their gpcd?
In question 8, what would the gpcd be if you took out 65% of the usage and classified it as outdoor usage?