Lower Pilarcitos Creek Groundwater Basin Study - Coastside ...

ET can be expressed in terms of percent precipitation. For the Purisima Creek
watershed, 74.8 percent of the precipitation on the upper Purisima Creek watershed is
removed from the watershed by ET. The surface water runoff coefficient for the Purisima
Creek watershed is 0.76 AF/acre, consistent with the bedrock geology of the watershed.
Area above the Pilarcitos Creek below Stone Dam Gauge
A second regional water balance was conducted for the watershed above the
Pilarcitos Creek below Stone Dam gauge (Table 9, Column B). The results of the water
balance for this area are similar to the results estimated for the Purisima Creek
watershed.
The average annual precipitation on the 4,090 acre watershed is 38.98 inches
(about 3.25 feet). No surface or subsurface inflow occurs in the watershed and no water
is imported into the watershed. Therefore, the total inflow to the upper Pilarcitos Creek
watershed is about 13,284 AFY or approximately 27 percent greater than the upper
Purisima Creek watershed, comparable to the difference in areas (23 percent).
Precipitation that falls on the upper Pilarcitos Creek watershed drains toward the
Pacific Ocean and eventually passes the gauge stations on Pilarcitos Creek as surface
water or groundwater. ET becomes an unknown quantity to be derived as the residual or
remaining quantity of water after all other water balance components are itemized and
accounted. Change in storage is assumed to be zero.
Average streamflow for the Pilarcitos Creek below Stone Dam gauge station is
about 2,339 AFY for the record between 1997 and 2001. The undetected subsurface
outflow for the upper Pilarcitos Creek watershed is about 23 AFY (1 percent). Based on
records provided by the City of San Francisco (see Appendix 5), diversions from
Pilarcitos Creek amount to about 760 AFY. Therefore, the difference between the inflow
(13,284 AFY) and outflow (3,122 AFY) suggests that 10,162 AFY is removed by ET and
deep percolation.
ET as a percent of precipitation is 76.5 percent, similar to the 74.8 percent ET
estimated for the upper Purisima Creek watershed. The surface water runoff coefficient
for the upper Pilarcitos Creek watershed is 0.76 AF/acre, consistent with the bedrock
geology of the watershed.
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