A Groundwater Extraction

Total Groundwater Extractions (AF)

Water Use Sectors

Rural Residential

B Groundwater Extraction Methods

Meters

Metered Municipal Wells

Metered connection maintained by California
Water Service and Durham Irrigation District.

Electrical Records

Land Use

Land use estimates were derived from crop mapping and CropScape survey results

Typical uncertainty for water balance calculation

Groundwater Model

Other

Rural residential groundwater extraction is estimated based on California Water Service Company's 2020 Urban Water Management Plan 2020 usage of an average per capita water use of 181 gallons per capita per day. Population data from the 2020 census was coupled with water district boundary data to identify total population not serviced by municipal supplies

Uncertainties are from population estimates and gallon per capita per day estimates

C Surface Water Supply

Total Surface Water Supply (AF)

Methods Used to Determine

Diversions for local supplies are
estimated based on historic State
Water Resource Control Board
eWRIMS (Electronic Water Rights
Information Management System)
data for total diversions. Surface
water delivery estimates are based on
historic deliveries in the area that
have occurred in dry and critical years

Water Source Types

D Total Water Use

Water Source Types

Water Use Sectors

Rural Residential

E Change in Storage

Method used to calculate change in storage

The spatial distribution of estimated changes in groundwater storage for the period from spring 2023 to
spring 2024 are shown in Figure 4-2 for the subbasin. Since groundwater storage is closely related to
groundwater levels, measured changes in groundwater levels can serve as a proxy for and be utilized to
estimate changes in groundwater storage. The change in groundwater storage was estimated based on
the change in measured spring-to-spring groundwater levels at each RMS well, multiplied by the area of
a Thiessen polygon surrounding that RMS well (defining a representative area for each RMS well) and a
representative storage coefficient of 0.1 for the subbasin.
Spring measurements used to calculate the change in groundwater storage were computed as the average
of all available groundwater level measurements from March and April of the respective year. The
representative storage coefficient was established by roughly calibrating the estimated change in storage
based on changes in observed groundwater levels (i.e., calculated using groundwater level data,
representative area, and a storage coefficient parameter) with estimated change in storage outputs from
the BBGM, as reported in the GSP to aggregate characteristics across all zones of the subbasin system. A total of 20 pairs of concurrent annual storage changes assembled from both methods over the period
from WY 1999 through WY 2018 were used for calibration. Determination of a representative storage
coefficient allows for estimating the change in volume of groundwater storage based on the measured
change in groundwater levels and known representative area (i.e., Thiessen polygon) associated with each
groundwater level measurement.
Negative changes in storage values indicate lowering groundwater levels and depletion of groundwater
storage, whereas a positive change in storage values represents rising groundwater levels and accretion
of groundwater in storage. As shown in Figure 4-2, the change in storage for each representative area
(i.e., Thiessen polygon) in the subbasin over the previous year ranged from roughly zero to 10,000 AF. The
representative areas around the northwestern and central portions of the Subbasin had a larger positive
change in storage. Total groundwater storage change in the subbasin was estimated to be approximately
104,500 AF between spring 2023 and spring 2024.

F Monitoring Network Module

G PMA Module