Lower Pilarcitos Creek Groundwater Basin Study - Coastside ...

Although numerous wells were drilled in the 1980s for domestic use, most of these were
subsequently abandoned. While the pumping for the Balboa wellfield has increased
since the mid-1980s, the cropland irrigated with local groundwater has declined slightly
since 1987.
Water Balance Equation
The water balance equation combines the components of inflow, outflow, and
change in storage. In this water balance evaluation, each component of inflow, outflow,
and change in storage has been evaluated independently. Over a representative period
of time, the equation should balance; any residual represents the net error inherent in
estimations and assumptions.
Table 13 summarizes the water balance for the Lower Pilarcitos Creek
groundwater basin. Inflows amount to an estimated 2,162 AFY, outflows total 2,284
AFY, and change in storage is zero. Combining the components results in a residual of
-122 AFY, which represents a net error of about 5 percent. For discussion purposes, the
average annual inflows and outflows are balanced over the study period at 2,200 AFY.
With regard to inflows, rainfall recharge and stream recharge apparently are the
most significant inflows, each accounting for about one-third of inflows. Subsurface
inflow from the upper Pilarcitos Creek valley also is significant. Pipeline leakage,
although known to occur, is a very small component.
Stream recharge is an important component of the water balance equation. As
noted in the section on stream recharge, no data exist to evaluate the amount of stream
recharge. However, the independent evaluation of all the other components and their
subsequent combination indicate that stream recharge is a substantial source of inflow
to the groundwater basin.
Of the outflows, subsurface outflow to the ocean is predominant, accounting for
an estimated 77 percent of the outflows. Groundwater export and consumption account
for the remaining outflows. Phreatophyte consumption, while crucial to the maintenance
of the riparian corridor, is not a significant part of the water balance equation.
Change in storage has been deemed zero, although available data (i.e., water
levels in well 29F04) suggest a slight increase in groundwater storage over the study
period. The increased groundwater storage and the net residual error of –122 AFY
suggests that inflows are underestimated and/or outflows are overestimated. Stream
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