Lower Pilarcitos Creek Groundwater Basin Study - Coastside ...

the annual recharge. This results in perennial yield estimates of 1,813 AFY and 1,507
AFY in 1987 and 1991, respectively.
Perennial (Safe) Yield
The perennial yield of a groundwater basin is defined as the rate at which water
can be withdrawn perennially under specified operating conditions without incurring
adverse impacts (Todd, 1980). The term “perennial yield” is used here instead of safe
yield, which has been widely misinterpreted to be a fixed quantity of water (often
equated to recharge) that could be extracted. Perennial yield embodies the concepts
and intent of City of Half Moon Bay Local Coastal Plan policies (Policy 10-14) which
state that new or increased well production shall be limited to a safe yield factor that will
not impact water-dependent sensitive habitat, riparian habitats, marshes, and
agricultural water use. Perennial yield also accounts for adverse impacts of saltwater
intrusion, land subsidence, and decreased creek flow.
Perennial Yield and Recharge. Perennial yield and safe yield have been
equated with the total amount of recharge entering a groundwater system. For the Lower
Pilarcitos Creek Groundwater Basin, the total average annual recharge is about 2,200
AFY. However, the total recharge can far exceed the amount of water that can be
withdrawn without undesirable impacts. Accordingly, the total amount of recharge may
provide little if any guidance in developing a groundwater basin. This is the situation in
the Lower Pilarcitos Creek Groundwater Basin. Instead, the amount of groundwater that
can be safely developed is defined by the hydrologic impacts that can be tolerated and
generally depends on the amount of natural discharge that can be captured (Bredehoeft,
1982).
Perennial Yield and Discharge. For the Lower Pilarcitos Creek groundwater
basin, the single most important feature of the water balance is the predominance of
subsurface discharge to the ocean. Unlike some groundwater basins, the aquifer of the
Lower Pilarcitos Creek groundwater basin does not discharge significantly to a stream.
Accordingly, development of the proposed Lower Pilarcitos wellfield would not capture
groundwater that would have supported stream baseflow and riparian habitat. Instead,
future wells must capture a portion of the subsurface discharge to the ocean.
The most significant potential adverse impact of such capture would be saltwater
intrusion from the ocean. As explained in the section on saltwater intrusion, additional
pumping of groundwater would result in inland migration of the fresh-salt water interface.
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