The climate of the subregion is characterised by hot, dry summers and cool winters. Maximum temperatures average 32 to 34 °C through the summer and 16 to 18 °C through the winter, while minimum temperatures are typically about 17 to 19 °C in summer and 5 °C through winter (Figure 11). Using the Köppen climate classification, the north-west is classed as hot and persistently dry. There is a narrow subtropical belt with no dry season through the centre of the subregion (Nyngan to Coonamble), and in the south and east the climate is temperate with no dry season and hot summers.
The mean annual rainfall of the subregion is 500 mm. Across the subregion, it ranges from 1070 mm/year in the Warrumbungle Ranges to 440 mm/year in the north-west. Higher mean rainfall also occurs in the hills east of Coolah. At Dubbo, mean rainfall is 575 mm/year and at Nyngan in the west it is 450 mm/year (Figure 13). The headwaters of the Macquarie River have been included in Figure 13 to show rainfall in the higher runoff yielding areas of the surface water basin. Figure 12 shows the annual time series of rainfall from 1900 to 2011 for the subregion. The annual mean for this period is 520 mm, but it can be seen that there is considerable variability with a low of 260 mm in 1902 during the federation drought and a maximum of 1160 mm in 1950. The orange line shows the low frequency trend for the record. The years between 1900 and 1946 were on average drier (455 mm/year) than the latter half of the 20th century (565 mm/year). The millennium drought is weakly evident in the subregion with the rainfall mean of 490 mm/year for 2002 to 2009 being 30 mm/year less than for the full record. Rainfall is summer-dominant, varying from 175 mm (60 mm/month) for the December to February period to 115 mm (40 mm/month) between June and August (Figure 11).
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Source data: derived from (i) Mean monthly and mean annual rainfall data (BoM, 2013a) and (ii) Mean monthly and mean annual maximum, minimum and mean temperature data (Bureau of Meteorology, 2013b)
Potential evapotranspiration (PET) was calculated on a daily time step using the Penman method with a wind spline for the period 1981 to 2012. The spatial pattern of mean annual PET for this period is shown in Figure 14, including the headwaters of the Macquarie River which are outside the subregion. Mean annual PET ranges from 1640 mm in the south-east around Coolah to 2150 mm in the north-west and east of Coonamble. Higher rates occur to the west of the subregion and lower rates in the higher elevation, higher rainfall areas to the south-east. Mean monthly PET ranges from 60 mm in June to 270 mm in December and January, reflecting the annual temperature pattern (Figure 11). On average, potential evaporation is limited by water availability throughout the year (Figure 11), so actual ET rates are not as high as the potential.
Source data: Bureau of Meteorology (2012)
Figure 13 Mean annual rainfall distribution
Source data: Bureau of Meteorology (2013c)
Figure 14 Mean annual potential evapotranspiration distribution
Source data: Bureau of Meteorology (2014)
In a recent report by the South Eastern Australian Climate Initiative (SEACI) (CSIRO, 2010), the significance of the period from 1997 to 2009 was assessed relative to other recorded droughts since 1900 and as an indicator of what the future climate might look like. While it was found that this period was the driest in the 110+ year record for the south-eastern corner of Australia with rainfall 5 to 30% less than the 1895 to 2008 mean, in the Central West subregion, the rainfall was either within 5% (southern margins of subregion) or 5 to 20% greater (north and western subregion) than the long-term mean. However, modelled streamflow was estimated to be 5 to 30% lower in those areas where rainfall showed little change from the long-term mean and 5 to 50% greater for the more northerly areas where the 1997 to 2009 rainfall was above the long-term mean. In general, analysis has shown that the impact on streamflow of a small percentage change in rainfall is enhanced with a 5% reduction in mean rainfall leading to a 10 to 15% reduction in streamflow and a 5% increase in rainfall leading to a 10 to 15% increase in streamflow in south-eastern Australia (Chiew, 2006).
However, the most recent analysis of likely future climate for the region suggests that the probability of runoff reductions is greater than 50%. Post et al. (2012) modelled future runoff at a 5 km grid resolution for south-eastern Australia. The climate series was informed by simulations from 15 global climate models used in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report, taking into account changes in daily rainfall distributions and seasonal rainfall (and potential evapotranspiration) amounts, for an increase in global mean surface air temperature of 1.0 °C (2030 relative to 1990), and 2.0 °C (2070 relative to 1990). In the Macquarie-Castlereagh river basin, the best estimates under 1.0 °C of warming are for a 1% reduction in mean rainfall (–5 mm) (Table 6) but with small increases likely in the summer months and decreases in the winter months (Figure 15). Across all models, the potential evaporation was projected to increase 2 to 5% (26–66 mm). The net result under 1.0 °C of warming is a projected 8% reduction (–3 mm) in runoff (Table 7), again with small increases in the summer months and more significant reductions through the winter (Figure 15).
Table 6 Summary of projected impacts of climate change on rainfall for the Macquarie-Castlereagh river basin
Source: Post et al. (2012; their Table 2)
GCM is global climate model
Table 7 Summary of projected impacts of climate change on runoff for the Macquarie-Castlereagh river basin
Source: Post et al. (2012; their Table 3)
GCM is global climate model
Figure 15 shows the distribution of rainfall and runoff changes across the year for the historical record and projected changes at Dubbo. Projected decreases in winter rains will likely result in reduced runoff, while small increases in January to March rain will increase runoff. The wider range of projections for runoff than rainfall indicates greater uncertainty in these across the full year.
Source: Figure 18 in Post et al. (2010)
The implications of a changing climate were considered in the sustainable yield report for the Macquarie-Castlereagh river basin in 2030 based on IPCC Fourth Assessment climate change projections and assuming current levels of development (CSIRO, 2008). The mean runoff reduction forecast for this river basin in this report was 6%, slightly less than the 8% reduction projected in the more recent SEACI report, but the assessment of impacts remains relevant.
Table 8 summarises impacts on the water resource for the Macquarie-Castlereagh river basin and wetland inundation events for the Macquarie Marshes. For a 6% reduction in runoff and assuming no new development in the catchments (for example, no construction of new water storages or significant land cover change), the CSIRO estimated an 8% reduction in water availability, a 9% reduction in end-of-system flows and a 4% reduction in diversions, with the impact on diversions depending on the class of water. Under median and drier climate futures, the frequency of large inundation events will reduce and the overall flood volumes will also decrease. These reductions would reduce the scale of bird breeding events. At the dry extreme of the climate projections, there would be serious impacts on the wetland ecology and potential loss of species (CSIRO, 2008).
Table 8 Projected impacts of climate change on water resources and wetland inundation in 2030 relative to the historical average (1895 to 2006)
Source: CSIRO (2008, their tables 4-6, 4-7, 7-2 and p.86)
In the lower Macquarie Alluvium groundwater management unit, river losses under the median development pathway would not change, but for the dry development pathway losses are projected to increase by 0.3 GL/year. Recharge is not projected to change significantly under the median development pathway.
It is important to remember that there are considerable uncertainties in the climate change projections and this makes definitive conclusions about the direction and magnitude of changes on water resources difficult to make. However, the SEACI modelling suggests even drier ‘best’ estimates than the earlier studies, so the weight of evidence is for a shift to drier conditions.
