May - June 2010
Vol. 73 No. 3

  

Who uses how much? California water by the numbers

Summary of California Urban water use and the potential for cost-effective conservation improvemnts.

200 million acre-feet (MAF) - the volume of water California gets in an average rainfall year (22.9 inches) from snow and rain, plus water imports from Oregon, Colorado, and Mexico. About 50 - 60% of that evaporates, is absorbed and then "exhaled" by plants (evapotranspiration), infiltrates the soil, or flows across state lines, into inland salt sinks or the Pacific Ocean. The year 2000 was about average for rainfall (97% of typical precipitation), making it a good reference year; the following figures are from the Department of Water Resources (DWR) for that year.

82 MAF - amount of water available to planners at DWR to allocate between human uses and the environment:

39 MAF - used to meet environmental mandates for rivers, the Delta, and wetlands;

43 MAF - amount of water withdrawn from imports, surface water, groundwater, and reused and recycled water for all human activities:

34 MAF - went to irrigated agriculture. Agricultural use varies considerably from year to year. Generally more is needed in low rainfall years when less total water is available;

9 MAF - went to "urban water use" - the water consumed by all residential, institutional, commercial, and industrial activities - all human uses other than agriculture. Total urban use has remained fairly steady. In 2000 urban water use was further broken down as follows:

0.7 MAF - the industrial sector and energy production;

1.6 MAF - the commercial sector;

0.7 MAF - large landscape (commercial and institutional irrigation);

5.6 MAF - the residential sector, comprised of:

3.3 MAF - indoor residential (toilets, clotheswashers, and showers comprise the largest volumes);

2.3 MAF - outdoor residential (landscape irrigation).

Per-capita urban water use

Water wasters are not found only in Southern California. Up and down the state, per-capita water use is lower along the coast and dramatically higher inland. This is directly related to climate differences. Inland residents may see this as a justification for higher water consumption, but it should be remembered that for many people, living in warmer, drier inland zones is a deliberate choice. Living on large properties and cultivating water-needy landscaping are yet further choices. These are based partly on culture and habit - the suburban California vision of emerald-green lawns and swimming pools epitomizing the good life that was cultivated and sold to us in the early 1900s.

It is time we stop chasing that outdated mirage. In other parts of the West, the transformation is well under way. New Mexico has reduced residential water consumption to 107 gallons per capita per day (gpcd) according to 2005 data from the U.S. Geological Survey. That achievement - in a state that receives 33% less precipitation than California - puts us to shame with our statewide average of 124 gpcd! By USGS calculations, overall 2005 U.S. average residential water consumption was 98 gpcd.

Urban water use by county

California water balance by year.

Huge variations are found in per-capita-consumption numbers in different source documents, depending on the definitions used. Nevertheless, comparisons of entries within one source can provide interesting insights. The county averages in the following table are taken from an on-line California map provided by the Sacramento Bee, based on a 2008 data set: "Figures show water usage from public water supply, minus use for industry and irrigation, divided by population taking public water (no well water included)."

County     Gallons per capita per day

San Francisco     108.4 (the lowest, by far)
San Mateo         133.4
Marin                154.8
Contra Costa     172.0
Alameda           173.9
Napa                184.5
Los Angeles      185.0
Sacramento       260.9
Mono                471.6 (highest)

See more counties on the interactive state map.

Water units and equivalents

1 acre-foot = 43,560 cubic feet = 325,851 gallons (the volume of water that would cover the area of an acre one foot deep). Different sources describe average California households as using annual amounts that range between one-half and one acre-foot; it is unclear why such a range of values is cited.

MAF - million acre-feet. The unit used to describe yearly water supply and consumption at the state and regional level.

1 cubic foot = 7.48 gallons (weighs 62.4 pounds).

1 CCF = 100 cubic feet (imagine a volume 5' long x 5' wide x 4' high) = 748 gallons - this is the "unit of water" that Bay Area utilities use for metering and billing.

MGD - million gallons per day - a little more than three acre-feet per day. This is the unit used by the East Bay Municipal Utility District to express future water demand and supply projections, as in the Water Supply Management Plan 2040.