Bulletin 166-4, Urban Water Use in California, August 1994

Chapter 3. Factors Influencing Urban Water Use

This chapter discusses climatic, socioeconomic, and water conservation factors that influence per capita water use. More information on climatic factors is found in Appendix E.

Climatic Factors

Climate has a great overall influence on annual and seasonal changes in per capita water use. Climatic factors (temperature, precipitation, solar radiation, humidity, wind) operate in combination to influence the amount of water consumed by plants. Their effects differ with locality and fluctuate seasonally and from year to year.

The sun is the original source of all energy involved in the transformation of water from liquid to vapor. Solar radiation provides the energy for photosynthesis and plant growth. Factors such as humidity, wind, and temperature originate as a result of solar radiation. For this reason, California's sunny inland valleys have much higher rates of water use than coastal areas.

Temperature is the climatic factor that most influences the water use of landscape vegetation. Seasonal per capita water use values reflect the changes in urban landscape water demands as users respond to changes in temperature.

Precipitation most effectively reduces the need for supplemental landscape irrigation water when it occurs during late spring through early fall when vegetation is actively growing. In winter, landscape water requirements are sharply reduced when dormancy, or near dormancy, of warm weather turf species such as Bermuda grass and ornamental shrubs and trees lowers or eliminates the need for irrigation.

High humidity suppresses plant water use. The evaporating power of the atmosphere increases under drier conditions, causing water to be used at a greater rate by plants. In coastal areas, the humidity remains uniformly high because of proximity to the ocean and prevailing westerly winds.

Wind accelerates water use. Air movement removes saturated air from contact with moist soil, water, and leaf surfaces and replaces it with warmer and drier air from adjacent areas. Plants exercise partial control over water loss from their leaves, while moisture loss from water and soil surfaces increases in relation to wind speed.

Evaporative Demand

The evaporative demand is the cumulative influence of these climatic variables on the rate of water evaporation. Numerous studies have shown that water evaporation from a standard water surface resolves the climatic influences into one easily measured variable that closely correlates to plant water use. The standard water surface most commonly used in California is the U.S. Weather Bureau Class A evaporation pan.


The climatic factors mentioned previously drive the processes of evapotranspiration (ET). The components of ET are transpiration, the movement of water through the leaves into the air as vapor to cool the plant, and evaporation of water from the plant surface or soil adjacent to the plant.

Evaporative demand based on evaporation of water from a standard U.S. Weather Bureau Class A evaporation pan in an irrigated pasture or equivalent environment can be used as an index for evapotranspiration.

The evapotranspiration of cool-season grass, or reference evapotranspiration (ETo) can be used as an index of landscape water requirements. The map in Figure 3-1 (previously published in DWR Bulletin 113-3) shows average annual ETo in California. These values range from 30 inches in the extreme northwest in Del Norte County to more than 90 inches in the desert of southeast California.

The lines of equal average annual ETo extend generally from north to south, with higher values in the south. The 60-inch iso-line is characteristic of the Central Valley and also extends through Central Los Angeles County. The hot summers in the Central Valley are offset by the warmer winter climate in Los Angeles, resulting in about the same average annual ETo for both areas. The effect of seasonal variations in ETo on plant water use is reflected in per capita water use values.

As determined by the Department, variations in per capita water use and ETo, precipitation, and July temperatures for 20 communities in California are displayed in Figure 3-1, Figure E-1, and Figure E-2 (Appendix E) respectively. These figures indicate that climatic factors alone cannot explain all differences in per capita use

Seasonal Water Use

The variation in seasonal water use in any water year is directly related to the variation in the climatic and meteorological factors. Previous issues in the Bulletin 166 series focused primarily on outdoor use and its relationship to evapotranspiration. The seasonality of water use is represented in Figure 3-1. This graph also illustrates how the seasonality of water use varies depending on the type of water year. The water use between October and April increases in drought years versus average water years because less precipitation means more landscape irrigation.

A thorough discussion of the seasonality of water use is found in Seasonal Components of Urban Water Use in Southern California (PMCL, 1990). The seasonal use was derived by the minimum month method of analyzing water use records. Although most of the residential seasonal water use is due to landscape irrigation, the seasonal variation in commercial, industrial, and governmental uses is the result of space cooling and process cooling needs as well. The seasonal water uses related to the categories of use in Southern California are displayed in Table 3-1.

For this bulletin, the Department analyzed water production data from 1980-87 for 68 cities in various regions of California using the minimum month method. Transects were followed from the coast to the interior. The percentage of seasonal use is displayed in Table 3-2.

Figure 3-1 illustrates the annual variation in seasonal water use for Los Angeles and Fresno based on the minimum month method. While baseline water use for both cities is essentially the same, the seasonal use in Fresno is greater due to the large landscape irrigation requirement during summer in the Central Valley. The large seasonal component reflects in Fresno's annual per capita water use. Table 3-3 compares baseline and peak seasonal use, percentage seasonal use, and annual per capita use for Los Angeles and Fresno.

Seasonal Versus Outdoor Use

The difference between seasonal water use and outdoor use may need further explanation. Since landscape irrigation, car washing, etc., may occur in any month of the year, the minimum month method may underestimate outdoor use and, therefore, overestimate indoor use. Indoor water use may also increase during the summer.

The PMCL seasonal use study (February 1990) indicated that outdoor use was almost 9 percent higher than that estimated by seasonal use in Southern California. Of the outdoor use, 90 percent in the residential sector and 84 percent in the non-residential sector was due to irrigation.

For the statewide analysis of urban water use in this bulletin, it is assumed that the minimum month determined over the 7-year period (1980-87) approximates indoor use. This is considered a valid assumption because the difference in volume of water between seasonal and outdoor use is small and affects the statewide analysis of total urban water use very little.

Socioeconomic Factors

Climatological factors help explain much of the water use in various regions. Other factors affecting water use range from economic to esthetic factors and cover a wide range of activities.


A previous Departmental study found that personal income correlates with unit water use in various areas of the South Coast study area (DWR, 1959). Cities with a large per capita income generally used more water per capita than cities with lower per capita income. A recent update of that report shows a similar relationship of consumption of water to household income that is illustrated in Figure 3-1.

Large residential lots in higher income areas require more irrigation water and water costs tend to constitute a smaller fraction of the household financial budget. Also, higher income families may own more water-using appliances. However, this trend may not be as obvious currently because most families have major appliances.

Swimming pools are normally associated with higher income families in single-family residences. One method estimates that swimming pools, compared to ordinary landscaping, add about 40 gallons per housing unit per day to single-family residential use in Southern California (PMCL, 1990).

Evaluating Urban Water Conservation Programs: A Procedures Manual (February 1992), a report prepared for California Urban Water Agencies supports the presumption of higher water consumption with higher income levels. It states, "Empirical studies clearly show that increasing income (in real terms) will cause an increase in water use in the residential sector." The elasticity of income in single-family sectors varies from 0.3 to 0.6. This means that a 10 percent increase in income may cause a 3 to 6 percent increase in water use. Further, a number of new urban water-use-forecasting methods include income as one explanatory variable.

Water Price

Water pricing structures generally serve as a means for water suppliers to recover costs of delivery to the end users. When California's developed water supply was deemed more than adequate to serve the existing population, urban water suppliers used a declining block pricing structure, where the unit cost varied inversely to the quantity of water used. As water shortages have become more common, suppliers have tended to discontinue such rate structures.

Pricing can work to influence demand by providing an incentive for customers to manage water use more carefully. The customer's sensitivity to pricing may be enhanced by rate structures that increase the cost of water at a constant or increasing rate as use increases.

Chapter 4 contains a detailed discussion of water pricing that is continued in Appendix F.

Water Meters

Today, virtually all large communities in California are at least partially metered. In 1991, Senate Bill No. 229 enacted the Water Measurement Law (Section 110 of Chapter 8 of Division 1 to the Water Code) requiring all new potable water service connections be metered after January 1, 1992. Generally, commercial and manufacturing businesses (other than self-supplied) have been metered for many years. Some unmetered or partially metered communities are still found in the Central Valley, foothills, and mountains, primarily involving residential accounts.

Bulletin 166-1 (DWR, 1968) documented the effects of metering in the San Joaquin Valley (southern portion of the Central Valley). It reported a 42 percent lower per capita water use in metered cities compared to flat rate communities. This study compared metering of all water users rather than restricting the study to only residential users. A Study of the Proposed Requirement for Mandatory Water Metering for Municipalities or for Domestic Water Systems (Arthur D. Little, Inc., 1974) showed that flat-rate services in 1970 averaged 330 gpcd in the Central Valley of California compared with 230 gpcd for metered cities, a 43 percent difference. However, this report also concluded that (1) the cost of metering may outweigh its benefits, (2) non-metered cities normally are close to inexpensive supplies, and (3) effective metering could cause landscaping to bear the brunt of water savings. Currently, metering would not have as dramatic an effect because of conservation activities causing a "hardening" of demand.

Generally, metering caused a reduction in water use and provided useful planning data for establishing a rate structure. While there is still opposition to metering in parts of the Central Valley and other outlying areas, the trend toward metering all deliveries in California continues. In some agencies, metering and pricing are linked to indoor versus outdoor water usage.

Water-Using Appliances and Other Household Uses

More water-using household appliances came into use during this century. A variety of these water-using or water-operated fixtures were added to homes and early models were not designed to save water. This contributed to increasing per capita water use trends. Most newer models on the market now require far less water than their predecessors. For example, new horizontal axis washing machines are being introduced that reduce indoor water use further. The introduction of new water-using devices is also diminishing.

Community Make-up

A number of sizeable communities surrounding large metropolitan areas and within commuting distance of city centers have developed as bedroom communities for people working in industrial and commercial city core areas. Bedroom communities generally expand rapidly, but initially have little growth in job-supporting industries.

As bedroom communities enlarge, they sometimes develop their own manufacturing and services industries and become more self-sufficient. For example, in recent decades communities in San Mateo County that were formerly small residential satellites of San Francisco have matured into predominantly industrialized areas. This increased industrialization in San Mateo County and in neighboring Santa Clara County has led to higher local per capita water use rates.

Water use trends are affected by changes in the patterns of land use. As cities age and enlarge, the centers are progressively rezoned for higher population density uses under urban renewal or similar programs. Single-family dwellings are replaced by multiple-unit structures with some land set aside for commercial strips and other industrial development.

In a small community, the addition or removal of a single high-water-using entity, such as food processing, of significant size can noticeably increase or decrease the per capita water use. The effect can be less pronounced for a major city where one addition or removal of a water-intensive industry may have minimal impact.

Many communities have an influx of non-residents (daily, on weekends, or seasonally) who are probably counted as permanent residents of another community and are not considered in per capita water use calculations. Relying solely on gross per-capita use values in these communities can be misleading.

Residential Density

Population density is higher in apartments than in single family dwellings. Per capita interior household water use is slightly higher in single family homes than apartments. Also, the rate of exterior water use to irrigate landscape for apartment dwellers is somewhat less than for persons living in single family dwellings, due in part to more persons sharing the landscaped area. For example, in the Metropolitan Water District service area, the average outdoor use was 92 gpcd for single family dwellings and 79 gpcd in multifamily units (MWD, 1990).

Between 1970 and 1980, there was a decrease in the percentage of single-family dwellings to multiple-family households. This trend leveled off in the 1980s. Table 3-1 shows the percentages of single family and multiple family dwellings in California between 1970 and 1990. The reduction in proportion of single-family dwellings led to the leveling in the statewide per capita water use.

Persons per Household

In 1970, an average of 3.3 persons lived in single family dwellings and 2.4 persons occupied the average apartment unit (Table 3-5). The overall effect of the decrease in persons per household during the 1970s was to increase per capita water use by increasing landscape water requirements per person. Conversely, increasing persons per household lowers per capita residential water use. An increase in persons per household, which occurred during the 1980s, contributed to a leveling of the statewide gpcd.

Water Conservation

In recent years, urban water conservation has contributed to the leveling off of the historically upward trend of per capita water use, even before the droughts of 1976-77 and 1987-92. In the early 1970s, a number of California water suppliers established conservation programs and the State increased its efforts to disseminate information and help local agencies implement programs to reduce water consumption. The need to stretch existing water supplies to meet the needs of a growing population was recognized.

The 1976-77 drought helped prompt the incorporation of water demand management into the State's laws and institutions. California was the first state to establish efficiency standards for toilets in 1978. The same year, the California Energy Commission established flow rate standards for shower heads and faucets, conserving both water and the energy used to heat it.

A report prepared for the California Urban Water Agencies by the Planning and Management Consultants, Ltd. (February 1992), states that ". . . an important change in water supply planning involves the use of water demand management (or water conservation alternatives). . . Comprehensive water supply management requires the evaluation of two basic sets of alternatives: (1) those alternatives that reduce water use and/or loss, and (2) those alternatives that augment supplies. Taken together, these alternatives provide the basis for efficient water management."

Urban Best Management Practices

As a result of hearings held by the State Water Resources Control Board on water rights affecting the San Francisco Bay and Delta, a coalition of urban and environmental water interests formed the California Urban Water Conservation Council. In 1991, the Council developed a voluntary Memorandum of Understanding that will lead to implementation of 16 Best Management Practices for urban water users and requires signatories to report on their status of implementation of the BMPs. The Council is continuing its efforts toward quantifying the effects of these practices and developing guidelines to help water suppliers determine the effectiveness of BMPs for their systems.

As of May 1994, 111 water suppliers, serving at least 80 percent of the State's population, plus 16 public advocacy groups and 47 other interested parties, had signed the MOU.

Initial Best Management Practices. As stated in the MOU, the initial BMPs that signatory water suppliers have committed to implement are:

  1. Interior and exterior water audits and incentive programs for single family residential, multifamily residential, and governmental/institutional customers.
  2. Plumbing, New and Retrofit.

    a. Enforcement of water-conserving plumbing fixture standards including requirements for ultra low flush (ULF) toilets in all new construction beginning January 1, 1992.

    b. Support of State and federal legislation prohibiting sale of toilets using more than 1.6 gallons per flush.

    c. Plumbing retrofit.

  3. Distribution system water audits, leak detection, and repair.
  4. Metering with commodity rates for all new connections and retrofitting existing connections.
  5. Large landscape water audits and incentives.
  6. Landscape water conservation requirements for new and existing commercial, industrial, institutional, governmental, and multifamily developments.
  7. Public information.
  8. School education.
  9. Commercial and industrial water conservation.
  10. New commercial and industrial water use review.
  11. Conservation pricing.
  12. Landscape water conservation for new and existing single family homes.
  13. Water waste prohibition.
  14. Water conservation coordinator.
  15. Financial incentives.
  16. Ultra low flush toilet replacement.

Potential Best Management Practices. The MOU commits the California Urban Water Conservation Council to study potential measures to determine whether they meet the criteria to be designated as BMPs. These include:

  1. Rate structures and other economic incentives and disincentives to encourage water conservation.
  2. Efficiency standards for water using appliances and irrigation devices.
  3. Replacement of existing water using appliances (except toilets and showerheads whose replacements are incorporated as Best Management Practices) and irrigation devices.
  4. Retrofit of existing car washes.
  5. Gray water use.
  6. Distribution system pressure regulation.
  7. Water supplier billing records broken down by customer class (e.g., residential, commercial, industrial).
  8. Swimming pool and spa conservation including covers to reduce evaporation.
  9. Restrictions or prohibitions on devices that use evaporation to cool exterior spaces.
  10. Point-of-use water heaters, recirculating hot water systems and hot water pipe insulation.
  11. Efficiency standards for new industrial and commercial processes.

Over time, additional cost-effective measures will probably be identified and implemented as technology improves and alternative supplies become more expensive.

Urban Water Management Planning Act

Assembly Bill 797 enacted the Urban Water Management Planning Act in 1983. As an amendment to the Water Code, this act requires urban water suppliers serving more than 3,000 customers or more than 3,000 acre-feet annually to prepare and file with the Department a comprehensive water management plan including specified elements. The first plans were due in 1985, with updates required at least every 5 years. Currently over 300 public and privately owned water suppliers are reporting.

Landscape Water Use and Conservation

Landscape water use is directly related to a community's climate, makeup, housing mix, population density, and income.

In recent years, the Department's Agricultural Mobile Lab Program has focused on landscape irrigation system evaluations. During 1992, the Department contracted with UC Davis (Bowers and Hanson, 1993) to analyze 363 landscape irrigation evaluation reports in San Diego, Riverside, and Ventura counties. The mobile lab program was developed primarily to provide management direction and not to gather basic data. However, the data on distribution uniformity and irrigation efficiency are certainly useful when only a few landscape water-use data sets are available. The distribution uniformity and irrigation efficiency statistics are presented in Table E-1 (in Appendix E) and Table 3-1 respectively.

Although difficult to apply to landscapes, the methods developed to estimate landscape water use are based on concepts developed for agriculture. Landscapes include a wide variety of species, planted in different densities and in varying microclimates (Table E-2). UC Cooperative Extension Leaflet 21493 (1991) suggests an approach for developing an overall landscape coefficient based on a species factor, density factor, and microclimate factor. This coefficient is determined by multiplying the three factors found in Table E-2.

Probably the most comprehensive publication on landscape water efficiency was published by the American Water Works Association (Bennett and Hazinski, 1993). This publication discusses the water-budget approach, where landscapes are metered, and the checklist approach for unmetered landscapes.

Water demands for landscaping is expected to increase as population growth moves inland. In examining the data for 1980 from Bulletin 198-84 (DWR, 1984) about 2 million acre-feet were allocated to exterior landscape uses in California. By 1990, this value had risen to over 3 million acre-feet.

PMCL (1991) analyzed residential landscape irrigation in Southern California. In this study, households with deficit irrigations exceeded households with surplus irrigations 56 percent to 44 percent. The actual percentage of deficit irrigation is probably higher than reported because all rainfall was considered effective at reducing the irrigation requirement.

In North Marin Water District, a study on water savings from irrigation scheduling on large turf sites at schools and parks (classified as governmental) showed that a 16 percent reduction in water usage could be expected (NEOS Corporation, 1991). However, a similarly scheduled private park showed a water savings of only 7.7 percent.

A study carried out for East Bay Municipal Utility District (1992) compared daily water consumption for single family homes having water-conserving landscapes with traditional turf-oriented landscapes. The water savings were estimated at 42 percent for the front yards of homes using water-conserving landscapes.

In September 1990, Assembly Bill 325 was passed and added Article 10.8 to Chapter 3, Division 1 of Title 7 of the Government Code. This legislation required the Department to develop a model landscape ordinance. The ordinance used a water budget approach relying on ETo data in the required landscape design. The water budget specified an average landscape coefficient of 0.5, divided by a minimum irrigation efficiency of 0.625, to yield an ET Adjustment Factor of 0.8. This factor was then used to adjust the average annual ETo.

Water conservation will undoubtedly continue to play a significant role in balancing California's water supply with urban, agricultural, and environmental water needs. Proven conservation measures will be implemented by more agencies and new measures will gain acceptance. Current measures for landscape conservation have yet to affect statewide landscape water use noticeably. However, BMPs and ordinances for landscape water use have only been adopted since 1991 and 1992. Agencies will need to manage water in new ways and prepare differently for shortages as water use continues to become more efficient.

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