San Benito County Water District San Benito County Water District

to damage the local walnut groves and apple orchards, respectively. High water levels can also
cause a threat to water quality as the water table is closer to the ground surface and more
susceptible to contamination.
Note that two areas of low groundwater elevations surround major pumping centers: the
southern portion of Hollister East where the City of Hollister and Sunnyslope CWD have wells
and the southern part of the Bolsa where intensive agricultural pumping occurs.
CHANGE IN GROUNDWATER LEVELS
Figure 16 documents the change in water levels from October 2007 to October 2008. In order
to compute the change in water elevations, the difference between the October 2008 and the
October 2007 maps was calculated and contoured. The accuracy of this map was checked by
examining water level changes in individual wells that were monitored in both October 2008
and October 2007.
Overall, water levels rose slightly during water year 2008. The largest change occurred near the
southern edge of the Hollister East subbasin where October 2008 water levels were as much as
80 feet higher than in October 2007. This area has historically experienced large year-to-year
fluctuations in water levels, presumably reflecting variable pumping at nearby wells.
Additionally, there was a small decrease of water levels near the City of Hollister (around 10
feet).
Groundwater elevation changes from year to year can be used to determine the change in
storage, which is the net volume of water added or removed from the basin over the water year.
The change in storage was calculated by using the change in water levels (ft) and multiplying
by the total area (ft 2 ) to determine the total volume change. This volume change is then
multiplied by the average storativity of the subbasin (shown in the table below) to represent the
amount of water that a given volume of aquifer will produce. The storativity values for each
subbasin were derived from a numerical model of the basin developed by Yates and Zhang
(2000).
Table 5 shows the calculated change in storage for each subbasin in Zone 6. The total change
for Zone 6 was an increase of 2,807 af. The greatest change occurred in the Bolsa Southeast
subbasin where 2,483 af more water was stored at the end of water year 2008 compared to
water year 2007. Outside Zone 6, groundwater storage in the Bolsa subbasin increased by 1,796
af.
Annual Groundwater Report for Water Year 2008 December 2008
San Benito County Water District 27