Lower Pilarcitos Creek Groundwater Basin Study - Coastside ...

This is a simplified accounting of what happens to rainfall. However, it can be
tailored to a specific area by taking into account local rainfall amounts, soil conditions,
and ET for different land use conditions (e.g., native vegetation, crops, or landscaping).
This method also can yield insights into the variability of groundwater recharge in
average, wet, and dry years.
Land uses in 1987 and 2002 (representing conditions at the beginning and the
end of the study period) were subdivided into three major categories: urban, open space,
and irrigated agriculture. As described in the Land Use section of this report, urban
areas are estimated to be 32 percent impervious. For the purposes of the water balance,
the remaining 68 percent of the area is assumed to be irrigated landscaping. Similarly,
agricultural areas are estimated to be 96 percent irrigated cropland and 4 percent
impervious. Open space is considered to have no effective impervious area and to be
entirely non-irrigated native vegetation (grass and shrubs). As shown in Table 10, the
land uses can be subdivided according to these percentages and then regrouped
relative to their impervious, irrigated or non-irrigated condition.
Impervious areas provide no significant recharge, so water balance spreadsheets
(see Appendix 7) were developed for two land use conditions: open space/native
vegetation and irrigated landscaping/cropland. Water balances were computed monthly
for the 15-year study period, water years 1987 through 2001. The water balances used
monthly rainfall values from the Half Moon Bay airport climate station, average monthly
ET values for the coastal fog belt zone, and a soil water holding capacity of 6.7 inches.
Full potential ET was applied both to cropland and landscaping, while ET from natural
vegetation was reduced using a coefficient for grassland/sagebrush. Phreatophyte
consumption (estimated as the full potential ET) is addressed separately as a water
balance outflow item.
Table 11 summarizes rainfall and recharge for each year in the study period. As
indicated, the average rainfall recharge rate is greater on the natural vegetation/open
space areas (7.98 inches/year) than on landscaping/cropland areas (5.32 inches/year),
reflecting the lower ET of natural vegetation. For both land use types, annual recharge
ranged from a low of zero in the drought year of 1990 to more than 20 inches in 1998.
Table 12 applies the recharge rates from Table 11 to total acreage values from
Table 10 for 1987 and 2002 land use conditions. As indicated, average recharge from
rainfall amounts to 885 AFY with 1987 land use conditions and 822 AFY given 2002 land
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