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In California, delivering water is the major use of electricity in the state and consumes about 10% of all generated electricity.
The required energy can vary from 500 KW-HR/acre-foot for rain water pumped from the ground to 3,500 KW-HR/acre-foot for water moved from Northern California, stored underground in Southern California, recovered and delivered to homes in Southern California. Ocean water desalination requires about 4,425KW-HR/acre-foot.
Conservation offsets the need for the most expensive water. Using 3,500KW-HR for 1 acre-foot implies that each 1 HCF (100 cubic feet) of water requires 8 KW-HR of electricity. 1 HCF is called a "billing unit" and equals 748 gallons.
One gallon of gasoline has an equivalent energy of 31KW-HR, ignoring conversion inefficiencies. One gallon of gasoline weighs 6.2 pounds. When it burns, each carbon atom combines with 2 heavier oxygen atoms and adds about 19.2 pounds of carbon dioxide (CO2) into the atmosphere. So, 1 gallon of gasoline = 31KW-HR = 19.2 pounds of CO2.
19.2 pounds of CO2 per 31 KW-HR equal about 0.62 pounds/KW-HR. Since it takes about 8 KW-HR to deliver 1 HCF of water from Northern California to homes in Southern California, then each HCF of water delivered requires the addition of 5 pounds of carbon dioxide into the air. Reducing water consumption reduces carbon dioxide emissions.
Another way to look at this process is that 1 gallon of gasoline is the energy equivalent of delivering 4 HCF of water from Northern California to the homes of Southern California. For other parts of the US where groundwater from rainfall or river water is pumped to homes, the cost to the environment will be about 1/4 that of the most expensive water in California. Remember that this ratio ignores conversion efficiencies which may double, triple or quadruple the CO2 emission ratio.