San Benito County Water District San Benito County Water District

WATER BALANCE
Annual reports prepared prior to 2006 included annual water balances to illustrate the state of
the groundwater basin. Because water balances often do not change significantly from year to
year, the District decided to include a special section on the groundwater balance in the Annual
Report every three years. The water balances were prepared for the past three water years
(2006, 2007, and 2008), employing a similar methodology as past annual reports for
consistency. Detailed information about the methodologies used to estimate each water balance
element is presented in Appendix E.
The water balance provides a quantitative view of the state of the basin, providing estimates for
the specific inflows and outflows of the basin or subbasins. While many of the values presented
in the table are estimates, the relative magnitude of each water balance element helps illustrate
the mechanisms at work in the basin. This detailed understanding of the groundwater system
can serve as a basis to compare changes in the basin over time and provide valuable tools for
groundwater basin management. The basic concept of the water balance is to show the
difference between inflows and outflows and estimated change in storage. In addition to
computing the net difference between the estimated total inflows and outflows, change in
storage is estimated independently based on water level changes and aquifer storativity values.
The water balance is prepared for each water year and for each subbasin. The results of the
subbasin water balances are shown in Tables 6 through 8 and a summary of the 2006, 2007,
and 2008 Zone 6 water balances is shown on Figure 18. Figure 18 also shows the water
balance for 2005, the most recent wet year, for comparative purposes.
INFLOWS
There are six major sources of inflow to the subbasins in Zone 6 and surrounding areas. These
include natural stream percolation, percolation of reservoir releases, percolation of CVP water,
deep percolation (from rainfall and/or irrigation), percolation of reclaimed water, and
subsurface groundwater inflow. Many of the basin inflows are controlled by hydrologic
conditions. Natural stream percolation and deep percolation from rainfall are directly related to
the volume and distribution of rainfall. Inflow to reservoirs is controlled by stream flow and
releases are partly a function of current inflow and storage. Because they are related to rainfall,
these three inflows are generally higher in wet years and lower in dry years. While water year
2006 had average rainfall, both 2007 and 2008 were dry.
Natural stream percolation occurs in every subbasin except the Bolsa Southeast subbasin and is
most significant in subbasins with larger streams, Pacheco, Hollister West and San Juan.
Stream percolation can vary considerably from year to year depending on rainfall and
groundwater levels. For example, stream percolation in Zone 6 was an order of magnitude less
in water year 2007 (a dry year) than in water year 2005(the most recent wet year): 1,241 af
to11,540 af respectively (see Figure 18). In addition, relatively high groundwater levels in
2007 and 2008 may have also contributed to “rejected recharge” or the decrease in percolation
due to insufficient subsurface storage capacity.
Annual Groundwater Report for Water Year 2008 December 2008
San Benito County Water District 34