Lower Pilarcitos Creek Groundwater Basin Study - Coastside ...

Area above the Purisima Creek Gauge
A water balance for the Purisima Creek gauge was conducted to verify and
compare values estimated for the Pilarcitos Creek watershed. Unlike the Pilarcitos Creek
watershed, the Purisima Creek watershed is a nearly native and non-urbanized
watershed. Table 9 (Column A) shows an accounting of the water in the upper Purisima
Creek. Note that the altitude of the Purisima Creek gauge (about 380 feet above msl)
and the average annual precipitation (36.66 inches) are similar to the altitude of the
Pilarcitos Creek below Stone Dam gauge (500 feet above msl) and precipitation in the
Pilarcitos Creek watershed (34.08 to 38.98 inches). Both watersheds experience the
same basic weather patterns.
Based on the isohyetal map (Figure 4), approximately 36.66 inches (about 3 feet)
of precipitation falls on the 3,168 acres during an average year. The area for the
Purisima Creek watershed is approximately 25 percent smaller than the area for the
Pilarcitos Creek below Stone Dam drainage basin.
For all regional watershed analyses presented, it can be assumed that the
surface water drainage divides also represent the groundwater divides. This reasonable
assumption implies that surface water and groundwater inflow to the watershed does not
occur. In addition, no water is imported into the Purisima Creek watershed. Therefore,
the total inflow to the Purisima Creek watershed is 9,678 AFY.
Precipitation that falls on the Purisima Creek watershed drains toward the Pacific
Ocean and passes the gauge station on Purisima Creek as surface water or
groundwater, or is consumed by plants and evaporation. ET cannot be directly or easily
measured. Therefore, ET becomes an unknown quantity to be estimated 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 Purisima Creek gauge station is about 2,418 AFY for
the record between 1959 and 1969. Subsurface outflow cannot be readily measured but
can be assumed to be about 1 percent of the surface water discharge in the upper
reaches of a watershed. Subsurface flow can also be estimated, where appropriate,
using Darcy’s Law. The undetected subsurface outflow for the upper Purisima Creek
watershed is about 24 AFY (1 percent). Exported water to the Purisima Creek watershed
is zero. Therefore, the difference between the inflow (9,678 AFY) and outflow (2,442
AFY) suggests that ET consumes 7,236 AFY.
31