Assessing current water use involves determining the characteristics of the water service area and defining current water use by type of demand.
4.1.1 Describing the service area
Table 4.1 can be used as a worksheet to characterize the water service area in ways that are useful to forecasting water use. The table includes sample numbers in order to demonstrate methodology.
Population projections are usually available from local governments and/or regional planning agencies.
Employment projections (i.e., the number of jobs and not employed residents) are usually available from the same planning agencies or from transportation planning agencies. Because transportation agencies forecast trips from home to work, they may have useful databases on the location and number of current and future jobs. A forecast of 20–30 years is usually adequate for water-use efficiency planning. Longer range demand forecasts may be useful for analyzing climate change scenarios.
4.1.2 Describing water use
Table 4.2 presents a worksheet that can be used to characterize existing water use. Sample numbers are shown in order to demonstrate the methods. Characterization includes:
(a) Average annual water production, which is the current total amount of water produced or withdrawn from a source and imported or pumped into the service area. If growth in water use has been low, the amount of water produced over the previous few years can be averaged; if not, water-use data for the last complete year on record should be used;
(b) Estimation of unmetered water use. If the water comes from a pumped source, use the following formula:
Pumping volume=Pumping rate×time of operation
The time of operation can be estimated from the electricity meter readings.
(c) Water losses, which can be estimated by conducting a system water audit. Chapter 6 describes the assessment of water losses. The value entered in Table 4.2 should be the amount of water for which customers are not billed, whether or not it is metered. The water losses can be less than 10 percent in a relatively new, well-managed system, but more than 50 percent in a poorly-maintained or older system;
(d) Peak day ratio, which is the volume of the water produced on the day of highest water use divided by the amount of the water used on an average day (annual water use in million litres/365 days). Alternatively, the peak month ratio can be computed from production and/or billing data:
Peak Day Ratio= Maximum Day Production Annual Average Daily Production Table 4.1 An example of service area description.
Service area characteristic Value
Current population (persons) 100,000
Future population (persons)
In 5 years 110,000
In 10 years 120,000
In 20 years 135,000
Current number of domestic service connections
Single-family 20,000
Multi-family 5,000
Total 25,000
Current number of non-domestic connections
Commercial 3,000
Industrial 500
Institutional (public) 500
Total 4,000
Current employment (number of jobs) 60,000 Future employment (number of jobs)
In 5 years 70,000
In 10 years 80,000
In 20 years 100,000
(e) Estimated seasonal water use, which is the amount of water use that exceeds interior use.
Interior or indoor water use is generally taken to be the lowest monthly (2 months if bimonthly billing cycle) water use pro-rated over a year. The formula for calculating seasonal use based on billing cycle, which is typically associated with outdoor use such as landscape (garden) watering, is:
Seasonal water use percentage (Monthly)=Lowest month water use×12×100 percent Average annual water use
Table 4.2 An example of water use description.
Water-use characteristic Value
Average annual water production 18,250 ML/yr 50 MLD Extent of metering
Domestic 90 percent
Non-domestic 100 percent
Estimated unmetered use 2 MLD
Total metered water use 38 MLD
Water losses 10 MLD
as a share of water production 20 percent
Peak day water use 70 MLD
Peak day water use to average day water use ratio 1.4 Estimated seasonal use
Month with lowest demand February
Average demand in month with lowest demand 40 MLD
Non-seasonal water use 80 percent
Seasonal use 20 percent
Average water use by customer category
Single-family domestic 15 MLD
Multi-family domestic 5 MLD
Commercial 10 MLD
Industrial 7.5 MLD
Institutional (public) 2.5 MLD
Water losses 10 MLD
Total 50 MLD
System supply safe yield 80 MLD
System capacity 60 MLD
Seasonal water use percentage=Lowest consecutive 2−month water use ×6×100 percent Average annual water use
(f) Average water use by customer class. This information may or may not be available from customer billing records. Depending upon the categories used by the utility, complete Table 4.2 by expressing the results in million litres per day (MLD). In some cases, water-use data are only available in terms of meter size. The smallest meters are usually reserved for single-family homes (and some small businesses). Larger meters are used in apartment complexes, commercial establishments, schools and industries. Meter-size data can normally be utilized to categorize water use into domestic and non-domestic, unless multi-family units are the predominant type of domestic dwelling.
(g) Checking the accuracy of data. The following guidelines can be used to evaluate the distribution of water use for piped water systems:
(i) Interior per capita domestic water use may be between 50 and 200 L per capita per day (lcd); per capita use is commonly higher in a single-family dwelling than in a multi-family unit;
(ii) Exterior or outdoor per capita domestic water use varies from a small value (10–30 lcd) in multi-family buildings to an always larger value in single-family buildings (40–80 lcd);
(iii) Commercial water use per employee can vary considerably but is often comparable to per capita interior domestic water use. However, it is expressed on a litre per employee/day (led) basis.
4.1.3 Analysing historical water use
In addition to completing Table 4.2, fluctuations in water use over the previous 3–5 years or an even longer period should be analysed. Changes occur in water use due to:
(a) Growth (or decline) in water accounts, industrial production or dwelling units of the population served;
(b) Number, value, and type of housing units that are constructed;
(c) Condition or health of the economy (unemployment rate);
(d) Cost of water supply;
(e) Climatic and weather conditions; and/or (f) Conservation activities.
If historical annual water production, population, water accounts and rainfall are available for past years, then a trend graph can be constructed as presented in Figure 4.1. Population in the United States historically trended closely with production, which increased until the 1970s when water conservation technologies improved (i.e., lower volume for flushing toilets) and regulations were adopted. In California, three historic droughts were also experienced where restrictions drastically cut demand and then water use patterns returned to normal, though not a 100% return to levels observed prior to the droughts.
If monthly or bi-monthly water-use data are available, those data should be recorded on a table or spreadsheet, and a chart of water use versus time should be prepared. Next, divide total monthly water use by the number of accounts billed for each month. This will represent water use by a typical customer and show changes over time due to growth, climate (weather), water-use efficiency programmes or other reasons. Figure 4.2 provides an example of the seasonal fluctuations of water use by an average single-family dwelling in the United States in an area with a humid climate. Note that the water use almost doubles between the wet and dry seasons. A 12-month moving average (the average of the prior 12 months, computed each month) will show water-use trends.
91 cm 48 cm 97 cm 46 cm 99 cm 43 cm 89 cm 107 cm 114 cm 41 cm 91 cm 152 cm 58 cm 38 cm 91 cm 122 cm 69 cm 38 cm
City of Santa Cruz, California, United States 1951-2011
Figure 4.1 Historical annual water use in Santa Cruz, California, United States. Source: Personal communication with Toby Goddard (2013).
Litres per Day per Account (LPD/A)
Single Family Domestic Accounts - after Weather Normalization
ACTUAL Weighted Moving Average after Weather Normalization
Forecast Pre-Drought BASELINE Pre-drought Base (1984-87)
Post-drought Base (1999-04) Projection Base 2007-08
Figure 4.2 Seasonal water use by single-family residences in Santa Cruz, California, United States.Source:
Personal communication with Toby Goddard (2013).