Lower Pilarcitos Creek Groundwater Basin Study - Coastside ...

Watershed Water Balance
Two water balance approaches are provided for the Pilarcitos Creek drainage
basin. One approach is regional, and accounts for the entire surface watershed
area (17,069 acres) above the Pilarcitos Creek at Half Moon Bay gauge station.
The second approach, summarized in the next section, applies the soil moisture
balance methodology to the Lower Pilarcitos Creek groundwater basin (1,765
acres) below the Pilarcitos Creek at Half Moon Bay gauge.
Methodology and Data
A water balance study for a watershed accounts for all water (surface water,
evapotranspiration [ET], and groundwater) that occurs in a watershed. If the estimates
and calculations are realistic, then the final water balance should be zero (i.e., Inflow –
Outflow ± Change in Storage = 0). The first methodology determines the regional water
balance based on a lumped sum parameter approach, average annual rainfall, and
streamflow measurements. The second methodology determines the local water balance
for the Lower Pilarcitos Creek groundwater basin based on monthly soil moisture, ET,
and water yield calculations during a selected time interval (1986 to 2001). For
comparison, the first methodology determines the relative volume or magnitude of key
water balance components (e.g., ET), while the second methodology provides estimates
of specific components for the groundwater basin.
The following summarizes pertinent data from previous sections for the water
balance calculations. Figure 1 shows the watershed areas, divided into sub-basins
based on the location of stream gauge stations, while Table 3a summarizes the acreage
of each area contributing to the stream gauge station and average record discharges.
Table 8 summarizes the stream gauge station and precipitation data for each watershed.
The drainage areas reported in Tables 3 and 8 differ by about two percent, reflecting
differences in measurement by the USGS and Todd Engineers, respectively.
Table 8 also shows the estimated average precipitation upstream and
contributory to each streamflow gauge station. Precipitation ranges from 32.53 to 38.98
inches in the Pilarcitos Creek watershed above the Half Moon Bay gauge and 36.66
inches above the Purisima Creek gauge. These average values were estimated by
measuring the area between isohyets (Figure 4) for each area and then calculating a
weighted rainfall average for the drainage area.
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