The impact of urban groundwater upon surface water - eTheses ...

GROUNDWATER FLOW MODELLING
cycles owing to the initial ‘wetting up’, and highlight the importance of the initial conditions
and the drying history.
The apparent specific yield can not be directly related to the actual specific yield as aSy
values of > 0.5 are obviously incorrect. Global estimates of specific yield calculated using the
total change in head and cumulative inflow over the quarter cycle were sand 0.19 (max
possible 0.43), loam 0.11(max possible 0.25), clay 0.005 (max possible 0.38). The apparent
specific yield and global specific yield are both dependent on the forcing head function
applied. An increase in wavelength or a decrease in amplitude both result in increased
estimates of aSy and global specific yield (Figure 6.25). The apparent specific yield is time
dependent and governed by flow rates in the unsaturated zone. These flow rates determine the
time taken to saturate/desaturate the column by a height equivalent to the step change in the
forcing head. This time may be greater than the time interval over which the change in forcing
head occurs which would lead to a decrease in the apparent storage. Downward flow rates of
unsaturated gravity drainage determine the rate of change of storage prior to the next re-
saturation. Lower frequency head cycles increase drainage time and lower amplitudes
decrease the vertical extent to be drained between maximum and minimum forcing head.
The storage capacity and conductivity of the unsaturated zone is controlled by the vertical
distribution of the moisture content and this is dependent on the geology and the wetting and
drying history. The response of the water table to a forcing head event is dependent on the
unsaturated storage capacity and conductivity, which will vary according to the relative
position of the water table within the vertical moisture content profile. The water table
response will vary between events and will be dependent on the rate of change in the river
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