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HYDC02 12/5/05 5:38 PM Page 54

54 Chapter Two

BO X
Continued
2.8

Fig. 2 Map showing land subsidence in the Central Valley of Fig. 3 Volumes of land subsidence in the major subsiding areas of
California. After Williamson et al. (1989). the San Joaquin Valley and groundwater abstraction in the Los
Banos–Kettleman City area, 1925–1977. After Williamson et al.
(1989).
reduced the demand for groundwater. However, subsidence
recurred during the drought of 1976–1977 owing to an increase
in groundwater abstraction. The correlation between ground- subsidence have been controlled by importing surface water and
water abstraction and the volume of subsidence in the Los decreasing groundwater abstractions. In these areas, the recovery of
Banos–Kettleman City area is good, indicating that about 43% of the lower pumped zone water levels to nearly their predevelopment
the water pumped from the lower pumped zone (at least 75–80% elevation may lead to future overestimation of the available
of the total) was derived from compaction of the fine-grained sedi- groundwater resources. If groundwater pumping increases again,
ments in the aquifer system. water levels will drop rapidly towards the previous lows because of
Land subsidence continues to be a problem in some areas of the the loss of aquifer storage capacity that resulted from the previous
Sacramento and San Joaquin Valleys, although the areas of greater compaction of fine-grained sediments.

saturated thickness of the aquifer or the height of the shows lateral variation as a result of an irregular lower
water table above the top of a lower aquitard bound- aquitard boundary or differences between recharge
ary. The transmissivity will, therefore, vary if there and discharge areas in the same aquifer.
are large seasonal fluctuations in the elevation of the The storage term for an unconfined aquifer is known
water table or if the saturated thickness of the aquifer as the specific yield, S , (or the unconfined storativity)
y