For the purposes of the USGS data, domestic water use encompasses everything we do with water at home. “Common indoor water uses are drinking, food preparation, washing clothes and dishes, bathing, and flushing toilets,” the report explains. “Common outdoor uses are watering lawns and gardens or maintaining pools, ponds, or other landscape features in a domestic environment.”
The report credits a number of federal policy interventions with reducing home water use. The National Energy Policy Act of 1992 is a big one. It established efficiency standards for toilets (the now ubiquitous 1.6 gallons per flush), bathroom faucets (2.2 gallons per minute at 60 psi) and shower heads (2.5 gallons per minute at 80 psi). The legislation passed with overwhelming bipartisan support and was signed into law by President George H.W. Bush. Further amendments to the bill, passed in 2005, improved efficiencies for water-using appliances, such as dishwashers and washing machines.
The EPA estimates that 70 percent of our total home water consumption happens indoors, with the remainder going to outdoor uses. But these numbers vary considerably by region, with households in arid Western states devoting 50 percent or more of their annual water budget to maintaining lawns and landscaping. As a result, per capita domestic water use varies greatly at the state level, ranging from 35 gallons per day in Connecticut to 184 gallons in Idaho.
It's worth pointing out that domestic water use accounts for just a tiny fraction of overall American water consumption, on the order of about 1 percent. Thermoelectric power (e.g., steam-driven electric turbines) accounts for 41 percent of all water use, while irrigation for crops eats up another 37 percent.
As part of its report, the USGS published this nifty map, breaking down the different water use categories at the county level.
While our average per capita home water use is declining, it remains much higher than in other wealthy nations, including the United Kingdom (37 gallons per day) and Germany (32 gallons). Part of the issue is that water in the United States remains very cheap relative to other countries, which has led some economists to call for smarter, more flexible pricing schemes in America's water markets.
While higher prices may hurt in the short term, there's good reason to take a long view on the issue. A 2014 Government Accountability Office report found, for instance, that water managers in 25 states expected to deal with regional or statewide water shortages over the coming decade, along the lines of what much of California experienced during its recent drought.
The era of cheap water, in other words, may be coming to an end, whether we plan for it or not.