Lower Pilarcitos Creek Groundwater Basin Study - Coastside ...

uses. The computed decline in recharge is the result of increased impervious surfaces in
the groundwater basin over the study period.
Deep Percolation from Irrigation. Irrigation in the groundwater basin includes
irrigation of crops and irrigation of landscaping. A portion of the water applied for
irrigation of crops or landscaping percolates through the soil downward to recharge the
groundwater basin. If the source of the irrigation is water imported from beyond the limits
of the groundwater basin, then the deep percolation represents an inflow to the
groundwater basin. If the irrigation source is local groundwater, then the deep
percolation is not an inflow, but rather a return flow that simply recycles groundwater
back into the basin.
Two areas of cropland are irrigated primarily with sources of water beyond the
groundwater basin. These include fields irrigated by diversions from Frenchmans Creek
on the north edge of the basin and fields that have been irrigated from Arroyo Leon on
the south edge of the basin. These fields encompass an estimated 180 acres. Review of
the soil moisture balance for landscaping and cropland (Appendix 7) indicates that the
irrigation water required to supplement rainfall and fulfill the full potential ET is 16.70
inches or 1.39 feet/year. It is assumed that irrigation efficiency is 85 percent; in other
words, more water is applied than is actually needed by the crops. In addition, it is
assumed that the surplus water percolates downward and does not run off the fields as
surface water. Accordingly, the applied water amounts to 1.64 feet/year and the amount
of deep percolation is 0.25 feet/year (1.64 -1.39). Application of this deep percolation
rate to the 180 acres of irrigated cropland results in a deep percolation of 45 AFY.
Landscape irrigation in the groundwater basin includes the irrigation of parks,
playing fields, and landscaping around public buildings, businesses, apartment
complexes, and homes. Some of this irrigation is provided by local groundwater;
however, most of the landscape irrigation currently is supplied through the importation of
CCWD water (primarily its Crystal Springs, Pilarcitos Lake, and upper Pilarcitos Creek
wellfield supplies).
Information on the amount of water imported to the groundwater basin per se is
not readily available. Accordingly, water imported to the groundwater basin has been
estimated using available information on housing and water sales. First, the United
States Census 2000 reports the existence of 4,004 occupied housing units in the City of
Half Moon Bay, which extends to the north and south beyond the basin. With an
estimated 1,000 homes beyond the groundwater basin boundaries, then approximately
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