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

GROUNDWATER FLOW MODELLING
general, the difference between modelled and observed heads was less than 0.5m and model
head contours reflected the shape of the contoured field piezometric data.
The groundwater discharge to the river is strongly dependent on the close proximity of the
outer boundary conditions (Section 6.7), and sensitive to changes in these (Section 6.9). In
some places groundwater flows may be maintained at above natural levels by the fixed head
boundaries. Significant groundwater abstraction is thought to occur within the model area but
limited information is available on the quantity of abstraction and the pumping regime
(intermittent). Therefore the calibration did not include the influence of the abstraction wells
which are considered later (Section 6.8). For the preceding reasons the calibration was aimed
at obtaining the minimum likely transmissivity. The parameters which yielded an acceptable
discharge to the river of 9.7 MLd -1 were transmissivity 400 m 2 d -1 , recharge 0.00069 md -1 ,
river conductance 720 m 2 d -1 .
6.3.2.4 Sensitivity of parameters
The initial assessment of the sensitivity of the model was carried out indirectly during the
calibration process based primarily upon discharge to the river and to a lesser degree on the
deviation between the modelled and observed heads. The parameters of transmissivity and
river bed conductance were selected for specific sensitivity analyses. Aquifer transmissivity
values in the range 50 to 750 m 2 d -1 were used and riverbed conductance values in the range
100 to 1500 m 2 d -1 . The model discharge to the river was most sensitive to transmissivity.
Riverbed conductance was of secondary importance. Riverbed conductance is of increased
significance at low values (