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Physical hydrogeology 51

(a) (b)

Fig. 2.21 (a) Hydrogeological map of the north-east of Salisbury Plain showing the intermittent and perennial sections of the River
Bourne. Position A is the site of the photograph shown in (b) located at Collingbourne Kingston and looking north in December 2002.
The river bed is covered by lesser water parsnip Berula erecta, a plant that proliferates in still to medium flows of base-rich water.
Hydrogeological information based on the British Geological Survey (1978); plant species identification using Haslam et al. (1975).

deformations are likely in gravel or indurated sedi- 2.11.3 Transmissivity and specific yield of
mentary aquifers. A possible consequence of ground- unconfined aquifers
water abstraction from confined aquifers is ground
subsidence following aquifer compaction, especially For an unconfined aquifer, the transmissivity is not
in sand–clay aquifer–aquitard systems. A notable as well defined as in a confined aquifer, but equa-
example is the Central Valley in California (Box 2.8). tion 2.23 can be applied with b now representing the