The Hydrologic Cycle and Water Use in America

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When I talk about the possibility of running out of fresh potable water, my students often ask me "but doesn't the water just keep running through the hydrologic cycle?". So I decided to look up the annual precipitation in the US, and compare it with our water use. I found some useful numbers, but you need to use dimensional analysis to figure out how they're related.

Precipitation

First I googled "US annual preciptitation", and found a lot of pretty maps showing precipitation in different regions. After a great deal of searching on the web and a college textbook, I found three different measures of precipitation. The problem is they all have different units. How can we compare them?

Precipitation Area Amount Source
Global, over Land 100 Trillion (100 x 1012) m3/year Textbook
North America 18,300 km3/year Global Environmental Outlook
Continental US 5.6 x 1012 m3/ year Textbook

Exercise 1: convert all three precipitation values into liters/year (use dimensional analysis). Hint: 1 m3 = 1000 L, and 1 km = 1000 m. How many m3 in 1 km3?

Exercise 2: What fraction of the total rainfall on land occurs over the North American continent? Think about your answer - does it make sense?

Water Use in America

Americans use a lot of water, but all uses fall into one of two categories:

Sources for US Water Withdrawals
U. Michigan Center for Sustainable Systems 		Uses for Withdrawn Water
USGS Report, 2000
  • 64% Surface Fresh Water
  • 20% Ground Water
  • 15% Salt Water (Surface Ocean)
  • 47% cooling thermoelectric power plants
  • 34% irrigation
  • 11% public supply
  • 5% industrial
  • 3% domestic, livestock, aquaculture & mining

Power plants generate a lot of heat, and use water to cool their equipment. They tend to use surface water, and are located near rivers, lakes or the ocean. Almost all the salt water withdrawn in the US is used to cool power plants. You're probably familiar with the iconic shape of nuclear cooling towers. According the USGS Q&A page, those towers are for cooling the water after the water was used to cool the equipment. This cooling water is returned to the environment, so it's not considered "consumed", only "withdrawn".

Public supply includes all municipal water use, including your house (if you don't have a well) and swimming pools, water for city gardens and the like. About 2/3 of the fresh water withdrawn goes to irrigation.

Year Total Withdrawal Fresh Withdrawal Fresh Consumption Source
1993 407 Billion gal/day 339 Billon gal/day 94 Billion gal/day EPA
2000 408 Billion gal/day 346 Billon gal/day USGS

Exercise 3: Convert all values in this table to L/year. Hint: use 1 gal = 4 L.

Exercise 4: What fraction of the total rainfall in the continental US was withdrawn as fresh water in 2000?

annual rainfall on each continent, GEO US total water use EPA, 1993 water allocation USGS, 2000

Individual Water Use

We tend to talk about how much water it takes to flush toilets and water lawns. The town of Cary has some good water saving suggestions for individuals. All household water use adds up to 50-200 gallons/person/day.

Exercise 5: If we all saved water, we could reduce American household consumption by 100 gallons/person/day. There are 300 million people in the US. How many liters/year could we save?

Exercise 6: Divide the answer from Exercise 5 by our water consumption in 1993 (exercise 3). What fraction of our total water use could we save by reducing household consumption?

Exercise 7: People in sub-saharan Africa use about 25 m3/person/year. Convert this number to liters/person/year. How many Africans could be supported by the water we could save in Exercise 5?

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