potential-impacts-of-climate-change-on-the-swan-and-canning-rivers

might not (pessimistic end <strong>of</strong> range) ‘go away’ in the period <strong>of</strong> interest being projected; and
the A2# and B1# rainfall and stream-fl ow ranges were formed into an optimistic and pessimistic
range by the optimistic assumption that the multi-decadal-variability factor goes away or the pessimistic
assumption that it persists.
Post 1990 GHG Ensembles for A2 and B1 Scenarios
The most satisfactory available modelling <strong>of</strong> regional rainfall applies statistical downscaling methodologies,
developed for IOCI to achieve regional resolution from global modelling. In practical terms,
interpretations <strong>of</strong> rainfall <strong>change</strong> from model evaluations <strong>of</strong> GHG effects are essentially relative rather
than absolute. Rainfall scenarios Tables 2-1 and 2-2 below, are drawn from multi-model (CSIRO
Mk3 and CCAM models only) trends for A2 and B1 scenarios inferred from ensembles (mostly from
a 1990 <strong>climate</strong> base) analysed by Charles et al (2004) and reported in unpublished unabridged
reports <strong>of</strong> IOCI Stage 2 (Bates pers. comm). Streamfl ow scenario development refl ects associated
studies applied to the Stirling Catchment (Department <strong>of</strong> Environment 2004).
Some interpolations on a rainfall decrease per degree Celsius global warming were used,
where necessary, to adjust these data to 2070 and 2100 horizons used in this report.
Tables 2-3 and 2-4 then apply the base-line adjustments discussed above to broadly represent:
the median ensemble estimate;
the 50 percentile range <strong>of</strong> (chaos affected) ensemble runs for the A2 and B1 scenarios; and
with the underlying stated assumptions <strong>of</strong> baseline adjustment (on the role <strong>of</strong> other anthropogenic
factors and chaos in the actual <strong>climate</strong> outcomes realised at 1990).
Assumptions for Baseline adjustment
Greenhouse Gas driven (GHG) rainfall decrease coeffi cients (50 percentile range <strong>of</strong> ensemble runs)
beyond 1990, are derived from multi-model studies by Charles et al. (2004). Their 1990 baseline
required adjustment to accommodate GHG and other attributed causes in the observed post 1970s
decrease. This adjustment involved the following assumptions.
At 1990 the gross observed shift in winter rainfall reduced it to 90 per cent <strong>of</strong> 1925-75 mean (IOCI
Climate Note No. 5/2004)
GHG was assumed to have reduced it to 95 %, other anthropogenic (OA) effects to 98 per
cent <strong>of</strong> 95 per cent and multi decadal variability (MDV) combined with OA to 94.5 per cent <strong>of</strong>
95 per cent (equivalent to MDV causing 30 per cent <strong>of</strong> observed reduction)
As the GHG-affected rainfall further decreases, the OA and MDV contributions were assumed to
remain as a fi xed proportion <strong>of</strong> the base rainfall. In some scenarios the MDV effect might disappear
with time and this possibility was allowed to widen the projected uncertainty range.
Streamflow
Streamfl ow projections in this Appendix (Tables 2-3 and 2-4) aim to refl ect streamfl ows <strong>of</strong> the Intermediate
and West Agricultural Zone (using analyses for Stirling). They are built as a simple geometric
progression (graphed below) assuming that each 10 per cent reduction <strong>of</strong> rainfall produces a 1/3
decrease <strong>of</strong> fl ow. As observed elsewhere in this report, stream-fl ow decreases in the high rainfall
zone <strong>of</strong> the scarp-lands are more marked than in the intermediate/agricultural hinterland.
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