In addition to assessing current water use, a detailed forecast of water needs is crucial to proper planning and the evaluation of whether conservation will be beneficial in meeting part of the future demand. There are many reasons why water use can increase or decrease. The principal reasons include:
(a) Growth in population;
(b) Migration away from the service area;
(c) Declining household size or increases in the number of households;
(d) Increased or decreased employment;
(e) Increased or decreased industrial production;
(f) Economic growth or downturn; and/or (g) Increased or decreased personal income.
Water use can also decrease, or increase at a slower rate, due to changes in the relative numbers of different types of customers (e.g., new homes with more or less irrigated landscaping) increasing numbers of persons living in a typical house, and conservation activities such as more water-efficient fixtures and appliances.
For example, constructing more multi-family building units versus single-family homes will result in future population growth with lower water-use patterns, lower per capita demand and lower per account water use. Multi-family homes typically use less water than single family dwellings.
Figure 4.3 presents an example water demand forecast with and without effects of the United States plumbing code for a community in California with an estimated population growth of 183,100 in 2010 to 212,000 in 2035.
Two methods for forecasting water use, total per capita use and water use per account, are presented in Table 4.3. Other more sophisticated methods are available but the second method described here is usually adequate for conservation planning purposes. Table 4.3 includes the extension of the sample data given in Tables 4.1 and 4.3 in order to demonstrate the two methods.
38,000 40,000 42,000 44,000 46,000 48,000 50,000 52,000 54,000
Water Demands (ML/Yr)
Year Water Demand without estimated water savings
Water Demand with water savings estimated from higher efficiency fixtures naturally replaced due to the effect of United States National Plumbing Code (and state and local codes)
Figure 4.3 Example water demand forecast with and without water savings from United States plumbing codes.Adapted from: Maddaus Water Management (2011).
Table 4.3 An example of forecasting future water demand.
Methods/////parameters Value
Method 1: Per Capita Water Use
Average annual water use (production) 50 MLD
Current population served (persons) 100,000
Current per capita use 500 lcd
Population In 5 years 110,000
In 10 years 120,000
In 20 years 135,000
Future water use In 5 years 55 MLD
In 10 years 60 MLD
In 20 years 67.5 MLD
Peak day ratio 1.4
Peak day usage in 20 years 94.5 MLD
Method 2: Projection by Customer Class Step 1: Develop unit water use values
Domestic Total use 20 MLD
Population (persons) 100,000
Per capita use 200 lcd
Non-domestic use Total use 20 MLD
No. of employees (jobs) 60,000
Per employee use 333 led
Step 2: Project future use Domestic
Future population In 5 years 110,000
In 10 years 120,000
In 20 years 135,000
Future domestic water use In 5 years 22 MLD
In 10 years 24 MLD
In 20 years 27 MLD
Non-domestic use
Future employment In 5 years 70,000
In 10 years 80,000
In 20 years 100,000
Future non-domestic use In 5 years 23.3 MLD
In 10 years 26.6 MLD
In 20 years 33.3 MLD
(Continued)
4.2.1 Method 1
–total per capita water use
The simplest forecasting method assumes that growth in total water production will be directly proportional to population growth and that per capita water use will not change in the future. It directly links future demand to future population as:
Future water use (MLD)=Current per capita water use (lcd)×future population
The experience of Singapore with declining per capita use is presented in Case Study 5. A projected future decline in per capita use may be considered when reviewing water demand forecasts. It is not customary to use a planned declined in per capita, and most common to use the current per capita demand for a baseline without conservation forecast. But as is shown in Figure 4.4 conservation impacts on demand can be factored in at the end of the planning process.
Table 4.3 An example of forecasting future water demand (Continued).
Methods/////parameters Value
Total future water use In 5 years 45.3 MLD
In 10 years 50.6 MLD
In 20 years 60.3 MLD
Water losses in 20 years (as the current level) 20 percent 15.1 MLD
Total future water use in 20 years 75.4 MLD
Peak day ratio 1.4
Peak day usage in 20 years 105.6 MLD
Figure 4.4 Overview of water-use projection process.Source: Maddaus, W. and Maddaus, L. (2006).
4.2.2 Method 2
–projection by customer class
Figure 4.4 gives an overview of demand forecasting by category of use. This method allows for different growth rates in different water-use categories. For example, if employment is growing faster than population, non-domestic water use may grow faster than domestic water use. This method is more sensitive than method 1; however, it does assume that per account water use does not change over time.
The second part of Table 4.3 can be used as a worksheet to develop a forecast by customer category. Per capita and per employee water-use values are developed for domestic and non-domestic use. If data are available for additional classes, such as single-family homes and multi-family dwellings, additional details can be included.
Projected future water use by class=Use factor×future population or employment
The amount of water loss is added to total use by all the categories in order to find the total amount of water needed to be produced in future years. Water use can be further subdivided into interior use and exterior use (using seasonal distribution), which is helpful in analysing the water savings potential.
Peak day use can be computed by applying the overall peak day factor, which is helpful to understanding the need for new or expanded water treatment facilities that apply peak day water use as a design criteria.
As illustrated in Table 4.3, where employment is growing faster than population, method 2 results in a significantly higher projected water use after 20 years than in method 1, that is, 75.4 MLD compared with 67.5 MLD. The difference in peak day use is even greater. Hence, method 2 is preferable where the data required to complete calculations are available.