Per Capita Water Consumption Calculations Need Refinements

LeRoy W. Hooton, Jr.

December 1, 2006

(Excerpts from a letter written to Utah Division of Water Resources division director Dennis Strong)

In recent years there has been greater emphasis placed on per capita consumption data as a means of setting and measuring water conservation goals.  Per capita consumption figures have been used by M&I water suppliers for many years, but with this new emphasis, per capita data and its meaning have taken on greater importance.

Per capita water consumption is affected by a number of variables including climate differences; household and lot sizes; building densities; the mix of domestic, commercial and industrial users; and demographics.  These indices can result in distorted per capita values when comparing one community to another, or accurately reporting water usage. 

With the emphasis on water conservation, per capita consumption figures have become extremely important in the following ways:

• In Utah as part of the State Water Plan, there is a goal to reduce per capita water consumption by 25 percent. The per capita number was calculated by using statewide M&I water consumption divided by the state’s population.  Because per capita consumption defines the state’s conservation goal, it has become the primary measurement of M&I water usage in Utah.  This has resulted in a fixation on the per capita number as a means of comparing one state’s per capita consumption to another; one region to another; and one local community to another.

• Under the Central Utah Project Completion Act, Section 207 and the Utah Lake System (ULS) water petition agreements between the Central Utah Water Conservancy District and various water agencies, water conservation goals as measured by per capita consumption data are established, including penalties.

Salt Lake City is disproportionately impacted by non-resident daytime population, including commercial, institutional, seats of government and visitor populations.  According to the Census Bureau, Salt Lake City ranks 3rd highest for commuter-to-residents of cities of its size or larger nationwide (Deseret News, April 25, 2006), with 131,180 workers added to the City’s 181,743 resident population every work day. Some examples of the City’s non-residential water consumption establishments are the University of Utah, Research Park, downtown hotels and businesses and the City’s third largest water user an oil refinery. Much of the City’s Northwest Quadrant is non-residents. According to the Salt Lake City Department of Economic Development, west-side commercial establishments and the airport host approximately 74,705 workers.  A quick drive through the City’s west-side reveals a tremendous growth in new commercial buildings, which will continue to attract new non-resident workers in large numbers, which will further distort the City’s per capita figures.

The current calculation method, in some cases, creates inequity between residents of different communities based on the level of non-resident water consumption occurring in a particular community.  It is recognized that it is impossible to take into account all of the factors that affect per capita consumption without adding a level of unacceptable complexity; however, where legitimate and documented non-resident water uses drastically distort a community’s per capita consumption calculation, there needs to be an acceptable process to more accurately measure and report the per capita consumption of that community.