Lower Pilarcitos Creek Groundwater Basin Study - Coastside ...

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for the Lower Pilarcitos Creek groundwater basin, which is characterized by a relatively thin sequence of alluvial sediments and shallow fine-grained sediments. 29
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. 30
Page 1 and 2: Coastside County Water District Hal
Page 3 and 4: LIST OF FIGURES Figure 1 Figure 2 F
Page 5 and 6: Executive Summary This report summa
Page 7 and 8: criteria to a monthly evaluation of
Page 9 and 10: top of well screens. Pumping at hig
Page 11 and 12: 1997 (Nelson, 1997 and 1998). This
Page 13 and 14: Figure 4 illustrates the areal dist
Page 15 and 16: • Urban: urban, urban residential
Page 17 and 18: should be noted that Geoconsultants
Page 19 and 20: slopes. In addition, the Pilarcitos
Page 21 and 22: The bedrock has been affected by fo
Page 23 and 24: Cross section A-A’ (Figure 16) wa
Page 25 and 26: Lower Pilarcitos Creek Test Well An
Page 27 and 28: well construction. The drilling flu
Page 29 and 30: drawdown is defined as the distance
Page 31 and 32: Because drilling depths are shallow
Page 33 and 34: Marine Terrace Hydraulic Properties
Page 35 and 36: 1993. Review of these maps indicate
Page 37: (1.000 g/cm 3 ), allowing the fresh
Page 41 and 42: ET can be expressed in terms of per
Page 43 and 44: groundwater inflow to the Lower Pil
Page 45 and 46: This is a simplified accounting of
Page 47 and 48: 3,000 homes exist within the basin.
Page 49 and 50: (Geoconsultants, June 1987 and Augu
Page 51 and 52: To estimate consumption of this wat
Page 53 and 54: echarge is one of the largest singl
Page 55 and 56: 1987 land use conditions to 822 AFY
Page 57 and 58: An example was provided involving p
Page 59 and 60: Water Quality Groundwater quality i
Page 61 and 62: colloidal form. Accordingly, the al
Page 63 and 64: Economic Feasibility This section a
Page 65 and 66: Overall, groundwater is characteriz
Page 67 and 68: The overall capacity of the five we
Page 69 and 70: Maintenance costs are estimated to
Page 71 and 72: Conclusions This study indicates th
Page 73 and 74: Earth Sciences Associates, February
Page 75: Todd, David K., 1980, Groundwater H

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