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300 Chapter Eight

Fig. 8.13 Catchment resource model
results illustrating the predicted effects of
water resources management on flows
in the River Wensum at Costessey Mill
in Norfolk, eastern England. (a) The
percentage depletion of mean weekly
flows are shown due to the net effect
of surface water and groundwater
abstractions and discharges for the period
1971–1992. A negative percentage
depletion indicates a gain in flow. (b) The
7-day flow duration curves are shown for
both the naturalized and observed flows
for the period 1971–1992. The upper curve
shows natural flows, the lower curve
observed flows. After Hiscock et al. (2001).

between the historic net abstractions and nil net ab- higher percentage flow depletion occurring when
stractions produces a series of net effects of abstrac- river flows are very low.
tions and discharges. An example is shown in Fig. The catchment resource model was also used to
8.13a for the River Wensum in Norfolk, presented predict the impact of an increase in spray irrigation to
as percentage depletions of mean weekly flows. The the total licensed amount from both surface water
predicted maximum depletion in the mean weekly and groundwater sources (an important water use
flows of 14% occurred in the drought of 1989–1992. in the predominantly agricultural area of eastern
In Fig. 8.13b the modelled historic and naturalized England). For this scenario, the maximum potential
surface flow characteristics are shown as flow dura- depletion of the long-term (1971–1992) 95 percentile
tion curves. Again, in the case of the River Wensum, 7-day flow in the River Wensum is predicted to
the impacts of net abstractions are small, with the be 17%. Compared with flows during the drought