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

GROUNDWATER FLOW MODELLING
For sand under conditions of a sin variation in the applied pressure head, cumulative inflow
across the base of the column is seen (Figure 6.22) to vary non-uniformly and displays a
phase difference with the forcing head function. A greater inflow occurs during the first cycle
as initial conditions require wetting before more stable conditions are reached in the following
cycles, with cumulative inflow remaining above zero. The minimum level of the cumulative
inflow is seen to rise between each cycle indicating a continued slow draw of water by
capillary action. The cumulative inflow is less when a lower saturated conductivity (Ksat) is
assigned to the sand - a 75% reduction of Ksat from 200 cmd -1 to 50 cmd -1 produces a 30%
reduction in the range of the cycle of cumulative flow. This indicates that both saturated and
unsaturated flow processes are occurring. The cumulative inflow curve displays asymmetry in
the downward half of each cycle where unsaturated flow and gravity drainage dominate.
The moisture content profile of the sand and clay columns vary during the course of a cycle
(Figure 6.23 a and b). The formation of drainage fronts increases the variability of the
available storage during the cycle. The full 25 cm rise in the water table occurs in both the
sand and the clay moisture profiles.
The range of variation (Figure 6.24) in apparent specific yield (aSy) was greatest in the sand
and least in the clay. The variations in Sy are non-linear and are heavily influenced by the
phase difference between the forcing head function and the cumulative flow. The falling limb
of the head function produces a generally lower Sy than the rising limb. The lowest values of
aSy in any one cycle occur during the initial head increase and the first phase of drainage. The
aSy values calculated for the first cycle are considerably higher than those of the following
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