Public Water Systems (PWS) whose water service area substantially matches a municipal area are directed to use the California Department of Finance population estimates for that municipality (Method No. 1). If the water service area does not match a municipal area, the “persons per residential connection” method should be used to extrapolate population estimates from 2010 census data (Method No. 2).
The service area match to the municipal area can be determined using Geographic Information Systems (GIS) or other mapping tools. If there are areas within the water service area that are not served by the water system (i.e., private wells, other water supplier) the map of the water service area shall exclude that area. For water systems that do not have mapping capabilities, the California Environmental Health Tracking Program of the California Department of Public Health has a Drinking Water Systems Geographic Reporting Tool. Water systems using the tool can map water system boundaries online and then download the mapped file. The tool is available at http://www.ehib.org/. Other entities, such as water wholesalers, local council of governments, or consultants, may be able to provide assistance with GIS mapping and service area population calculations.
Water Service Area Matches Municipal Area
Method No. 1: If the water service area substantially matches the municipal area (95% or greater overlap for both the service area and municipal area), the water system may use the California Department of Finance (DOF) population estimates, available at the following link: http://www.dof.ca.gov/research/demographic/reports/estimates/e-4/2011-20/view.php.
NOTE: Water systems may choose to modify DOF population estimates if there are small areas (less than 5%) within the city (or census designated place) that are not served by the system or small areas outside the city (less than 5%) served by the water system.
Water Service Area Does Not Match the Municipal Area
If the water service area does not substantially match the municipal area (less than 95% overlap for both the water service area and municipal area), the water system should estimate service area population using either:
- Private vendor population software
- “Persons per Residential Connection” (detailed below).
Method No. 2: “PERSONS PER RESIDENTIAL CONNECTION” POPULATION METHOD
STEP 1: Determine 2010 Census Year Population
- Calculate the 2010 service area population using GIS by overlaying the water service area map on the census block map for that area. Water service area population must be calculated using the census block data, the smallest census unit.
- For census blocks split by the water service area boundary, water systems may determine whether or not the blocks’ population should be included based upon either the area or the population distribution within the boundary. This determination should be based on a detailed examination of each individual block.
- Area: If half or more of the census block area is within the water service area, the water system should include it in its population calculations. Otherwise, the block should be excluded from the population calculations.
- Population Distribution: If half or more of a census block’s population is within the service area, the water system should include it in its population calculations. Otherwise, the block should be excluded from the population calculations.
- Water systems whose service area matches a census designated place may use the census data for its 2010 population.
STEP 2: Calculate “Persons per Residential Connection” in the 2010 Census Year.
- Using water system billing records, determine the number of active residential connections that were served by the water system in 2010 (multi- and single family combined).
- Divide the 2010 population by the 2010 number of active residential connections.
- The result is “Persons -per-residential- connection” for the 2010 census year.
STEP 3: Estimate Population for the Non-Census Year
- Calculate the number of residential connections (multi- and single family combined) for the non-census year, i.e., 2014.
- Multiply the number of residential connections for the non-census year by the “Persons per Residential Connection” for the 2010 census year.
- The result is the estimated population for the non-census year. See example table below.
A B C D Year # Residential
Connections for
Non-Census Year
(from agency records)Persons per
Residential Connection
for the Census Year
(from Step 2)Estimated
Population
for the Non-Census Year
(B) X (C) = (D)2014 15300 3.7 56610