Climate Change Impacts on the Water Resources of the Cooper Creek Catchment

Climate Change Impacts on the Water Resources of the Cooper Creek Catchment

The likelihood of changes to stream flow and flooding was assessed for the upper reach of the Cooper Creek (at Currareva) by perturbing input data to the Sacramento (rainfall-runoff model) and Integrated Quality-Quantity Model (IQQM) models according to quantified ranges of climate change for 2030. These ranges incorporate the range of global warming (IPCC 2001) and regional changes in temperature, rainfall and potential evaporation encompassing the results from seven different climate models. The methods used were primarily designed to manage uncertainty and its impact on natural and productive processes. The wettest, driest and average climate scenarios for the region were used in hydrological models to assess changes in water flow for the Thomson River. Resulting changes in flood inundation downstream of Currareva were assessed and potential changes in vegetation identified. Changes in climate and water flow were measured against a base period from 1961-1990. The dry scenario for 2030 was associated with mean temperature increase of 1.7o C, 4% lower annual rainfall and 9% higher evaporation. The wet scenario for 2030 was associated with mean temperature increase of 1.0o C, 1% higher annual rainfall and 3% higher evaporation. The driest and wettest extremes indicate a range of change in mean annual flow of -7.1% to +1.5% by 2030. The median and dry scenarios were associated with a reduced frequency of low daily flows (<1000 ML/d) compared to base. The impact is likely to be associated with reduced waterhole persistence and connectivity during droughts. Climate change was associated with extended lengths of periods of no flow. The longest simulated period of no flow was 280 days for the base scenario and 361 days for the average scenario, an increase of nearly 30%. These estimates assume that there is no major abstraction from waterholes, and that pumping for stock, irrigation and domestic supply will further reduce persistence times. The mean number of days per year of no flow at Currareva was nearly 2 weeks longer for the average and dry scenarios compared to the base scenario. The longer periods of no flow associated with the average and dry scenarios may have an adverse impact on the natural and human systems downstream of Currareva. The 100 percentile flow under the dry scenario was 11% lower than the base scenario. A reduction in maximum flows may also result in decreases in inundation on the borders of floodplains, which may result in decreases in biodiversity in these areas, shrinking the floodplain. Annual and shortlived grass species may also be replaced by perennial grass species from neighbouring communities. The average and dry scenarios were also associated with a small reduction (2-9%) in high daily flows (99, 95, 90 and 88 percentile) and the wet scenario a small increase (3-4%) in high daily flows (99, 95, 90 and 88 percentile) compared to the base scenario. The relationship between recorded peak discharge at Currareva and recorded area of inundation shows beneficial flooding downstream started at a flow of 8370 ML/day equivalent to a height of 2.9 metres (9 feet 6 inches). The inundation area downstream from Currareva was very sensitive to small increases in flow volume and height around this level (equivalent to the 87 percentile of flow). Within the range of small event floods (88-92 percentile flows) the wet scenario was associated with an increased inundation area of up to 32% and the dry scenario a decreased inundation area of down to 75%. This change in inundation area of small event floods may have an impact on the production of herbage, natural resources and biodiversity near the main channels. Less inundation of small flood events on the floodplains may also mean that pastures in the outer country are used more. The increased grazing pressure on the outside country may lead to the degeneration of perennial grasses due to the decrease in available recovery time.