Lower Pilarcitos Creek Groundwater Basin Study - Coastside ...

Area above Pilarcitos Creek at Half Moon Bay Gauge
A water balance was conducted for the area above the Pilarcitos Creek at Half
Moon Bay gauge (Table 9, Column C). This balance includes the upper Pilarcitos Creek
watershed and provides an estimate of the groundwater outflow to the Lower Pilarcitos
Creek groundwater basin.
Based on the isohyetal map (Figure 4), the average annual rainfall on the 17,069
acre watershed is 34.08 inches (about 2.84 feet). No surface or subsurface inflow occurs
in the watershed. After treatment at the Nunes Water Treatment Plant (WTP), minor
amounts of imported water (77 AFY) are brought into the watershed from Crystal
Springs Reservoir (see Appendix 6). Therefore, the total inflow to the Pilarcitos Creek
watershed is about 48,552 AFY or approximately 365 percent greater than the upper
Pilarcitos Creek watershed.
Because of the proximity to the coast (fog belt), the ET for the entire Pilarcitos
Creek watershed was assumed to be approximately five percent less than the average
ET of the upper Pilarcitos Creek (76.5 percent) and Purisima Creek (74.8 percent)
watersheds or about 71.5 percent. Accordingly, ET is estimated to be 34,660 AFY.
Average streamflow for the Pilarcitos Creek at Half Moon Bay gauge station is
11,543 AFY for the record between 1967 and 2001. Subsurface outflow cannot be
readily measured and is estimated from the residual of the water balance. Based on
records provided by the City of San Francisco (see Appendix 5), exported water from the
watershed amounts to about 760 AFY. Other diversions include the Pilarcitos Creek
wellfield operated by CCWD (155 AFY) and Pilarcitos Lake diversions (913 AFY) (see
Appendix 6). Therefore, the difference between the inflow (48,552 AFY) and outflow
(48,031 AFY) suggests that 521 AFY is groundwater outflow. The undetected
subsurface outflow for the Pilarcitos Creek watershed can also be estimated using
Darcy’s Law (254 to 508 AFY). In terms of the surface water runoff coefficient for the
Pilarcitos Creek watershed, calculations show that 0.68 AF/acre of water drains from the
watershed.
Precipitation at the Half Moon Bay Station ranges between 13.03 inches in 1972
to 52.92 inches in 1983; the long-term average is 26.40 inches. Therefore, total inflow to
the Pilarcitos Creek watershed could range between 23,985 (49.4 percent of average) in
dry years to 97,347 (200.5 percent of average) in wet years. The total estimated
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