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Risk of illness percentages were calculated at various discharge percentiles for each
loading reduction. The differences between risk of illness at full FS loading and the
other loadings were determined. This accounts for the risk reduction expected by
reducing FS loading.
Benefits accruing to the population were calculated for each discharge percentile.
These values were aggregated across all percentile flows and corrected for the
frequency of occurrence to determine total annual benefits (TB). Because the
entire Scottish population was used, the total benefit value had to be corrected to
elicit benefits occurring only at Irvine Beach. A 2004 study indicated that 3% of all
Scottish beach visits were made to Irvine Beach (Scottish Executive Social Research,
2004). The total benefits to Scotland were therefore multiplied by 3% to calculate
the benefits at Irvine Beach. To calculate future benefits, discounting was done to
give the present value. This was done on a 25-year timescale with a 6% interest
rate in this study, as they are similar to values used in evaluating costs of agricultural
infrastructure and machinery.
Using the above framework, benefits from reducing faecal loading were calculated
for the River Irvine catchment. Irvine Beach is located in Ayrshire in the west of
Scotland (Grid Reference: NS305375). Discharge data from SEPA for the River Irvine
for 1989–2003 were used to determine summer flow percentiles.
RESULTS AND DISCUSSION
FS concentrations at Irvine beach simulated with the modifications of the Vinten
et al. (2004) model are shown in Figure 2. Risk of illness was determined for all
loadings at various levels of discharge (Figure 3). Table 4 shows the annual benefit,
and aggregated benefit over 25 years of improved bathing water quality at Irvine
Beach.
100000
75 th 80 th 85 th 90 th Percentile Flow
95 th
BW Faecal Streptococc
(cfu/100 mL)
10000
1000
100
10
1
C urrent loading
25% reduction
50% reduction
75% reduction
80% reduction
85% reduction
90% reduction
95% reduction
0 1 2 3 4 5
Daily C atchment Discharge (mm/day)
Figure 2: Predictions of faecal streptococci concentrations in bathing water
at the Irvine Beach at various river Irvine scaled discharges
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