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

GROUNDWATER FLOW MODELLING
amplitude 5-100cm) and the function varied over time steps of 1/50 days. Initial moisture
conditions for the model were set, based upon the moisture profile after 10 days wetting.
Cumulative inflow to the model was sensitive to the initial conditions, and the model was run
through at least 10 cycles of the sin function until stable conditions of flow across the
boundary were reached.
6.11.2.3 Calculation of apparent and global specific yield
In order to investigate the variations in storage that occur during a forcing head cycle it was
necessary to calculate a value representative of storage (apparent storage) for each time step
in the forcing cycle. The apparent storage was calculated based upon the definition of specific
yield, that is the volume of water released over a given time period per unit horizontal area
per unit head change. The flow across the lower boundary of the model was divided by the
change in the forcing head on a per time step basis. A global specific yield was calculated
equal to the cumulative inflow divided by the total head change per quarter of the sin forcing
head cycle.
6.11.2.4 Model Parameters
The geology was varied between runs and the column was set to represent a single geological
type on each occasion. Van Genuchten parameters for unsaturated flow were obtained
(Anderson, 1990, Digges la Touche, 1998) for the soil types clay, sand and loam to simulate
the range of conditions that are likely to occur in the Tame Valley. Details of the results and
parameters used are given in Appendix 25.
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