Lower Pilarcitos Creek Groundwater Basin Study - Coastside ...

The amount of subsurface inflow from the upper Pilarcitos Creek valley has been
quantified using two methodologies. First, Darcy’s Law was applied to the cross section
where the Pilarcitos Creek valley enters the coastal plain. As a result of variable water
table gradients, the resulting subsurface flow varies between about 254 and 508 AFY.
Second, the subsurface flow was computed as the residual of the water balance
at the groundwater basin boundary. This accounting of the difference between inflows
(48,475 AFY) and outflows (48,026 AFY) over the 35-year period of record indicates an
average subsurface flow of 449 AFY.
Stream Recharge. Available hydrogeologic information show that the Pilarcitos
Creek channel is situated above the regional water table over much of its 6,000-foot
length across the Lower Pilarcitos Creek groundwater basin. Accordingly, a downward
hydraulic gradient exists to the water table, indicating that streamflow percolates
downward. This downward percolation may be slow and subject to temporary storage in
perched water bodies, but eventually provides inflow to the underlying aquifer.
No data exist to demonstrate what happens to streamflow that passes the gauge
station at Highway 1. Such data could be obtained through a synoptic survey of the
stream. A synoptic survey involves measurement of streamflow at several stations along
the channel, accounting for any inflows (e.g., from storm drains) and any discharges
(e.g., diversions). The increase or decrease in streamflow along the channel length then
would reflect inflow from groundwater or outflow (recharge) to groundwater, respectively.
For example, a survey of Denniston Creek on March 16, 1998 indicated recharge of 29
AF per day (14.60 cubic feet per second [cfs]) along the 6,000-foot-long reach crossing
the coastal plain (Balance Hydrologics, April 2002, p. A-28).
Although data are lacking, anecdotal information suggests that the lower reach of
the creek provides flow to groundwater recharge. For example, a survey on July 25,
1985 noted streamflow of 0.65 cfs at Highway 1 and zero flow at the mouth of the stream
(Zatkin, 2002). However, assessment of the amount of recharge is complicated by
irrigation diversions from the creek and riparian vegetation consumption.
For this study, no adequate data are available to provide a firm estimate of
stream recharge. However, this source of inflow is probably significant. In the absence of
data, recharge often is estimated as a percentage of rainfall or runoff. For example,
estimates of the El Granada area (Kleinfelder, April 1988) suggested recharge at five
percent of runoff. Previous estimates for the Lower Pilarcitos Creek groundwater basin
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