Urban Water Needs Analysis
When compared to agricultural water needs analysis, urban needs analysis is a newly emerging field. It is based on the same basic premises as the agricultural model in terms of landscape water needs, but other factors, such as per capita water needs and industrial water needs are based primarily on estimates or averages of past years, rather than strict mathematical determinations. While there is a history of districts using agricultural water needs analysis, no districts in California have been asked or required to do these calculations.
During 1996, Reclamation staff worked with the California Department of Water Resources to develop a methodology for urban needs analysis and tested it at two sites. Further testing and evaluation would be helpful before proceeding.
The urban water needs analysis process is a step-by-step approach for wholesale and retail contractors using municipal and industrial water to take stock of their water resource needs. The process is based on this formula:
| urban water needs = residential needs + non-residential needs + distribution system needs |
Residential needs equals the number of residents in the district times a baseline per capita factor for indoor needs plus the acreage of residential landscape times an ET factor.
Non-residential needs (other direct uses of urban water, including commercial, institutional and industrial) equals a baseline need determined by the average amount of water used over the past 5 years plus the acreage of non-residential landscape times an ET factor.
Distribution system needs equals unaccounted for beneficial uses (authorized, unmetered water uses such as fire fighting, main flushing, storm drain flushing, sewer and street cleaning, construction site use, water quality and other testing, and water for processing at treatment plants) plus distribution system losses (leaks in storage and distribution system, evaporation, illegal connections, theft).
Following is a worksheet outlining the calculations and a description of each component, including documentation for the assumptions used and methods for obtaining the necessary data.
Urban Water Needs Analysis Worksheet
- Residential
Needs
- a. interior needs:
number of residents x 70 gpcd x 365 (days) / 325,851 gallons per acre foot total acre feet per year - b. landscape needs:
total acreage of residential irrigated landscape (acres) x annual ETo (inches per year) x - 1.0 (ET factor)
/ - 12 (inches per foot)
total acre feet per year - c. total residential interior & landscape:
a + b = total acre feet per year - Non-Residential Needs - Commercial, Industrial, Institutional
- a. interior needs
- average amount of
- February water use for previous 5 years
x 12 total acre feet per year - b. landscape needs
- total acreage of non-residential irrigated
- landscape (acres)
x - annual ETo (inches/year)
x - 1.0 (ET factor)
/ - 12 inches per foot
total acre feet per year - c. total non-residential interior & landscape:
a + b = total acre feet per year - Distribution System Needs
- a. unaccounted for beneficial uses
- acre feet per year unmetered water for fire, line flushing etc. (1-2 percent of average total metered use for previous 5 years)
- b. distribution system losses
- acre feet per year slow meters, pipe breaks, leaks, etc. (less than 7 percent of average total metered use for previous 5 years)
- c. total distribution system needs:
a + b = total acre feet per year total residential interior & landscape + total non-residential interior & landscape + total distribution system needs TOTAL URBAN WATER NEEDS (acre feet per year) - a. interior needs:
Residential Indoor Needs
Residential indoor needs equals the number of residents in the district times a baseline per capita factor of 70 gpcd.
In most cases, districts have good information about the number of people within their service area. In particular, districts will know the number of residential connections they have and can extrapolate the number of people from that figure. The statewide average number of people per household is 2.7, based on the 1990 census.
More specific information is available from city and county census data centers. Also, the California Department of Finance can provide assistance with population data. The Demographic Research Unit can be reached at (916) 322-4651.
Most of the information on per capita water use is estimated rather than actually measured. Here is a summary of the average per capita water data from four sources:
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For purposes of assessing residential indoor water needs, it is proposed that the baseline per capita factor be established at 70 gpcd. This is almost twice the minimum of the water use estimated in the AWWA End Use Study and CUWA Manual. It is less than the water use estimated for non-conserving households in the AWWA End Use Study. It falls between the HUD Study's gallons per capita per day for non-conserving and conserving households and would establish the need at 5 gallons less than the DWR's latest residential indoor use estimates.
MWD found that their multi-family residents used less water than their single family residents. If a district has collected such data on their customers, or other information related to the age of their housing stock or other pertinent data, this information can be used to modify the baseline per capita factor.
Another consideration is that houses built since 1994 must comply with state and federal water efficiency laws which will reduce indoor use without behavior changes. The baseline per capita factor for residents of homes built since 1994 could be expected to be reduced significantly.
Landscape Needs
Landscape needs equals the acreage of irrigated landscape times an ET factor (100 percent)
Landscape acreage. A few districts have surveyed their customers and have substantial data on the amount of irrigated landscapes in their service areas. Most have no information at all. There are several approaches a district can take to determine landscaped acreage.
First, a simple survey form can be mailed to all customers, asking them to measure their landscapes and to return the information to the district.
Second, the district staff can measure the large landscape customers such as parks and golf courses, or target a survey to only those sites. Then an estimate of other landscape areas could be made with information from local land use planning agencies or other sources. For example, if District A has 350 single family residences, each with an average of 1,000 square feet of landscape, the total acreage for this sector would be approximately 350,000 square feet.
Another alternative would be to employ aerial photography or satellite technology to determine the landscaped area of the district.
Reclamation's Water Conservation group has recently convened a meeting of urban agencies and others to discuss these different options and to initiate a new publication that will provide details to interested districts.
Evapotranspiration data. Evapotranspiration (ET) is the quantity of water evaporated from the soil and transpired by plants. It is an indicator of different climatic conditions. The historic average annual evapotranspiration (ETo) in California is 40 inches per year.
Example. District A has 350,000 square feet of irrigated landscape. The ETo is 45 inches per year.
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The Water Conservation in Landscaping Act of 1990 (Assembly Bill 325) required that the California Department of Water Resources develop a Model Water Efficient Landscape Ordinance that could be used by jurisdictions throughout the State. With the assistance of an advisory task force representing the landscape industry, water agencies, cities, counties, developers, and environmental interests, the Department adopted the Model Ordinance in 1992.
Through this process a consensus was reached that designing, installing, and maintaining California landscapes at a level of water use that would not exceed 80 percent of ET would be the standard established to determine the efficiency of landscape water use.
It is proposed that the ET factor be set at 100 percent for purposes of the urban water needs analysis process. At this level, even the most water intensive landscapes should have ample amounts of irrigation. A lower ET factor, such as 80 percent of ET, could be considered as more information becomes available.
Non-Residential Needs (Interior)
Non-residential needs (other direct uses of urban water, including commercial, institutional and industrial) equals a baseline need determined by the average amount of water used over the past 5 years, plus the amount dedicated to landscape irrigation. In the formula, it is suggested that the average amount of February water use for the previous five years be used as the base (multiplied by 12 and divided by 5). This is suggested because it would be a good indicator of interior use in cases where landscape water use is not separately metered.
The commercial, institutional and industrial water needs component is probably the most obscure and unpredictable part of this calculation. An inventory of these customers would most likely be necessary.
Distribution System Needs
Unaccounted for beneficial uses includes unmetered water for fire, and line flushing. It is based on a percentage of the average total metered use for previous 5 years. One to two percent is the usual amount estimated for these purposes.
Distribution system losses includes losses due to slow meters, pipe breaks and leaks. It is also based on a percentage of the total metered use for previous 5 years. This should not normally exceed seven percent.