Lower Pilarcitos Creek Groundwater Basin Study - Coastside ...

Figure 4 illustrates the areal distribution of rainfall in the Half Moon Bay area
(SCVWD, 1989). The pattern of precipitation is complex, with significant variations
depending on elevation and topographic setting. In general, rainfall increases from a low
of about 26 inches at the coast to highs of 42 inches near Pilarcitos Lake on the
northeast and 40 inches along the Kings Mountain crest on the southeast. The
intervening area (crossed by Highway 92) has lower rainfall, averaging 30 inches.
Evapotranspiration. Evapotranspiration (ET), the loss of water to the
atmosphere through evaporation and plant transpiration, is relatively low in the study
area. The California Department of Water Resources (DWR) includes the Half Moon Bay
area within its Coastal Plains Heavy Fog Belt zone (DWR, no date). Reference
evapotranspiration is approximately 33 inches. This ET rate refers to a well-watered turf
and is applied to an individual crop type through use of a coefficient that accounts for the
typical growing season of the crop and crop water consumption. Actual
evapotranspiration rates from a field also are influenced by cropping and irrigation
practices. The study area is marked by a variety of crops (for example, brussel sprouts,
daisies, and pumpkins) and differences in cropping and irrigation practices, all of which
have changed over time. Accordingly, the water balance for this report applies the
reference ET rate to all crops and landscaping. This results in some overestimation of
groundwater consumption.
The Lower Pilarcitos Creek groundwater basin also includes extensive areas of
natural vegetation, including areas of grass and shrubs along the coastal bluffs and a
corridor of water-loving trees and shrubs (phreatophytes) along Pilarcitos Creek.
Evapotranspiration for the natural grass and shrubs was estimated by applying monthly
ET coefficients for sagebrush-grass (Stephens, 1996), which reduced the ET rate to
about 22 inches per year. Phreatophytes are assumed to consume the full potential ET
(Stephens, 1996).
Land Use
Land use in the Half Moon Bay area was mapped by the California DWR in 1977
and 1987 (DWR, 1977 and 1987). For this study, the 1987 land use map was updated to
current conditions (autumn 2002) through field reconnaissance. DWR land use maps
provide information on the full range of land uses including natural vegetation, recreation
areas, urban land uses (e.g., urban, urban residential, schools, and vacant lands), and
agricultural land uses (specific crop types, irrigation status, fallow and idle lands).
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