The impact of climate change on rainfall and
streamflow in the Denmark River catchment
Summary
Climate projections indicate that Western Australia's south-west region is likely to
become drier in the next century. At the same time development is expected to
expand. An increasing water demand with a diminishing water supply poses a
serious challenge for water resource managers. One of the first steps in assessing
this risk is to establish an estimate of water availability under a drying climate. This
report details the findings of an investigation to assess surface-water availability in
the Denmark River catchment under a range of projected future climate scenarios.
The results of this study were derived by analysing the response of a hydrologic
model (LUCICAT) of the Denmark River catchment to general circulation model
(GCM MK3.5) downscaled rainfall for three emissions scenarios defined by the
Intergovernmental Panel on Climate Change (IPCC): high - A2; moderate - A1B;
and low - B1. The hydrologic model was driven with 40 downscaled rainfall
sequences from 1975 to 2100 for each emissions scenario.
It was found that under a high greenhouse gas emissions scenario (A2), a rainfall
decline of 3 per cent by 2030 could lead to a streamflow reduction of 13.5 per cent,
while an 8 per cent reduction in rainfall by 2085 could result in a 32.5 per cent
reduction in streamflow. Rainfall and streamflow reductions for a low emissions
scenario (B1) would be similar to the high emissions scenario in 2030, at 2.5 and 10
per cent respectively. However, by 2085 the effect of the low emissions scenario
would be more apparent, with smaller rainfall and streamflow reductions at 2 and 8
per cent respectively.
There was a slight shift in seasonality between projected and observed rainfall
towards more rainfall later in the year. The shift in seasonality was amplified in the
streamflow results. This is a similar result to previous climate change studies in
south-west Western Australia (Berti et al. 2004; Kitsios et al. 2008). However, unlike
the two previous studies, there was no shift in the peak rainfall month in the Denmark
River catchment.
The northern portion of the catchment historically experiences less rainfall and was
found to be more sensitive to future rainfall decline than the south. For instance, by
2085 (A2 scenario) streamflow was projected to reduce by 57 per cent at Kompup,
compared with a 32.5 per cent reduction at Mt Lindesay due to projected rainfall
alone.
Land-use change may have a greater impact on streamflow than projected climate
change in the Denmark River catchment. A simulation, using repetition of observed
rainfall (1971-2005), was undertaken to assess the sensitivity of the catchment to
land-use change. Plantations, found mostly in the northern portion of the catchment,
have a large impact on streamflow. After setting plantation conditions to 'fully grown'
in the model in 2006, there was a sharp decline in streamflow to 2030. The northern
portion of the catchment was most sensitive to land use with a streamflow decline of
63 per cent at Kompup, compared with 30 per cent at Mt Lindesay.
Rainfall and streamflow reductions for the end-of-century timeframe were much lower
for the B1 scenario than for the A2 and A1B scenarios. These results suggest that
streamflow reduction in the Denmark River can be minimised through conscious
global efforts to improve technology, social equity and to protect the environment (all
aspects represented in the B1 emissions scenario). However, any changes would
need to start now given they take a long time to produce an effect.
The greenhouse gas emissions scenarios used in this study are not predictions of the
future. They are only projections of possible outcomes of a range of social, economic
and environmental developments. While reducing emissions to a low emissions
pathway (such as the B1 scenario) appears to be the most beneficial option, it is also
the most challenging given we are currently following an emissions growth pathway
higher than the A2 scenario.
Even though this study provides evidence that regional impacts of future climate
change can be reduced by making large (global) efforts to reduce greenhouse gas
emissions now; some effects of a drying climate are unavoidable and need to be
incorporated into future water planning.
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