THE COMMONWEALTH OF MASSACHUSETTS
WATER RESOURCES COMMISSION
River Basin Planning Program
Generic Water Needs Forecasting Methodology
Municipal Water Needs Forecast
The methodology for forecasting public water supply needs for each basin uses a disaggregated
approach, based on the most recent three to five years' water use (called Base Demand) for each
community in the study area. The three main water use categories for each community are
residential, non-residential (including commercial, industrial, agricultural, municipal), and
unaccounted-for water (UAW).
The main assumptions of the disaggregated approach and the calculations used to arrive at the
disaggregated 20-year forecast shown are detailed below.
"Method 1" is used to arrive at water use forecasts for those communities which
are able to provide sufficient disaggregated water use data
show a residential gallons per capita daily use (gpcd) of 80 or less
have an unaccounted-for water factor of 15% or less
For communities that have insufficient data to develop a disaggregated water needs forecast, have a
residential gpcd over 80, or a high percentage of unaccounted-for water, Method #2 has been used.
Disaggregated Water Needs Forecast - Method #1
1. Columns A through G show most recent census estimate and Base municipal water use data (base
average day demand). Column D, seasonal population factor was calculated by multiplying the
summer population by 3 (for the 3 summer months), dividing this number by 12 (for the year).
Column E, the base service population, is calculated by multiplying Column A, the current
population estimate by Column B, the percent of the population served by the water supplier, then
adding Column C, the out of town population and Column D, the seasonal population factor, to this
product. Column F, base ADD shows the average of the average day demand of the most recent
three to five year water use. The water demand information was obtained from the water supplier
for each community and from Department of Environmental Protection (DEP). Column G, base
gpcd is calculated by dividing column G, base ADD by column E, base service population.
2. Columns H through J show the residential portion of the base average day demand (ADD). The
percentage of residential water use is shown in column H. The residential gpcd, shown in column I,
is calculated by dividing column J, residental ADD by column E, base service population.
3. Columns K and L, non-residential water use, are based on information provided by the
community or DEP.
4. Columns M and N, unaccounted-for water (UAW), are obtained by subtracting the residential
and non-residential water use from the total water use in F; thus N=(F-[J+L]). UAW consists of
domestic and non-domestic meter underregistration, potentially recoverable and unavoidable
leakage, and unmetered use, especially public unmetered use. The data were obtained either from
DEP statistical sheets or from personal interviews with the water supplier for each community.
5. Columns P through R show how residential water use is projected to change through the end date.
The projected service population, column R, is calculated from the population projection, column P,
and the estimated increase percentage of the population served in the future, column Q. The future
service population is multiplied by the existing residential gpcd, column I, to obtain the future
residential water use shown in column S. All the communities in Method 1 have existing residential
gpcd at or below 80, so their existing gpcd is carried forward.
6. Column T, the future non-residential portion of projected demand, is calculated by adding two
factors to existing non-residential water use:
(a) Additional non-residential ADD due to population change
Projected population change between present and future is multiplied by the existing "non-
residential gpcd" which results in the new non-residential ADD for the additional population. (In
cases where population is projected to decline, this non-residential water use also will decline.)
(b) Allowance for new non-residential uses independent of population change
Communities often experience changes in commercial or industrial sectors which is not directly
linked to population change. In order to plan for changes in non-residential water use not tied to
population change, and in lieu of specific economic projections for each community, a growth
factor based on past economic trends is included.
The average non-residential water use was estimated over the past 10 year period. The volume
change between the first three years of the period and the last three years was used to estimate the
change expected to occur in succeeding decades, unless the community could provide specific
information. Where non-residential water use has shown a decline between the two periods, the
non-residential water use is held constant.
7. In calculating Column U, unaccounted-for water, the methodology assumes that if existing UAW
is 10 percent or lower, the existing percentage will continue in the future. If the UAW is greater
than 10%, it is assumed that there will be a reduction in UAW during the planning period to reach
8. Column V, the forcasted ADD, was calculated by adding columns S, T, and U.
Water Needs Forecast - Method #2
Method #1 could be applied in those communities where it was possible to obtain disaggregated data
on residential, non-residential, and UAW in order to project those components. Due to a low
percentage of metering and/or inaccurate metering, a number of communities do not have adequate
data to permit a disaggregated water use forecast.
The method assumes that the gpcd of any additional population will have a gpcd of no more than 70,
and makes no allowance for unaccounted-for water. Therefore, towns which have a high residential
gpcd (greater than 80) and/or a high unaccounted-for water component (greater than 15%) are
subject to Method 2.
The main assumptions of Method #2 and the calculations used to arrive at the demand projections
are detailed below.
1. Columns A through G are calculated as in Method #1.
2. Columns H through N are shown where possible, or otherwise shown as unknowns. Until these
communities are either more fully metered or are able to reliably distinguish their UAW, a
disaggregated methodology cannot be used.
3. Columns P through S show the projected service population .
4. Column T shows the increase in the residential service population between base service
population and the projected service population, and is derived by subtracting S from E.
5. Column U shows the additional increase in residential demand based solely on population
change, and is derived by multiplying I by T. If column I is greater than 70, the future residential
gpcd is kept at 70.
7. One part of Column U, the increase in the non-residential portion of projected demand, is
calculated as shown in Step 6(a) of Method #1. For the portion of growth in non-residential sectors
independent of population change, a method similar to step 6(b) was used with a slight variation. As
in Method 1, the average non-residential water use was estimated for the first and last three years of
the previous decade. The volume change in non-residential water use for this time period was then
determined. However, for Method #2 communities, the growth rate was slowed to reflect the
inadequacy of the information and the assumption that additional water is available in the water
8. The future ADD, column X, was calculated by adding the base ADD, column W, to the increase
in residential demand, column U, and column V, the increase in non-residential demand.
Department of Environmental Management, Office of Water Resources