Future impacts of climate variability, climate change ...

Future impacts of climate variability, climate change and land use change on water resources in the Murray Darling Basin

Overview and Draft Program of Research

Roger Jones, Peter Whetton, Kevin Walsh and Cher Page CSIRO Atmospheric Research

Final: 10/07/02


Table of Contents Table of Contents .................................................................................................................................................... 2 Executive Summary................................................................................................................................................. 3 Projected climate change ..................................................................................................................................... 3 Macquarie River water resources ........................................................................................................................ 3 Climate variability and land-use change.............................................................................................................. 3 Catchment-wide implications .............................................................................................................................. 4 Future directions.................................................................................................................................................. 4 Part One................................................................................................................................................................... 5 Introduction ............................................................................................................................................................. 5 Policy background ................................................................................................................................................... 5 Climate change projections for the Murray-Darling Basin...................................................................................... 6 Previous assessments............................................................................................................................................... 7 Future impacts on the Murray-Darling Basin .......................................................................................................... 8 Projected regional climate ................................................................................................................................... 8 Macquarie River risk assessment implications for the MDB .............................................................................. 8 Climate variability and land-use change.......................................................................................................... 9 Critical thresholds.......................................................................................................................................... 10 Part Two ................................................................................................................................................................ 11 The Macquarie Study ............................................................................................................................................ 11 The Region ........................................................................................................................................................ 11 Model structure.................................................................................................................................................. 12 Baseline climate and results .............................................................................................................................. 12 Climate change scenarios .................................................................................................................................. 13 Climate model runs............................................................................................................................................ 15 Probability distributions .................................................................................................................................... 15 Uncertainty analysis .......................................................................................................................................... 16 Bayesian analysis............................................................................................................................................... 17 Critical thresholds.............................................................................................................................................. 18 Risk assessment ................................................................................................................................................. 19 Potential impact on water resources of reforestation......................................................................................... 20 Summary ........................................................................................................................................................... 21 Draft research plan ................................................................................................................................................ 21 Program aim ...................................................................................................................................................... 21 Method .............................................................................................................................................................. 21 Modelling system .............................................................................................................................................. 22 Attribution ......................................................................................................................................................... 22 Resources........................................................................................................................................................... 22 Appendix A ........................................................................................................................................................... 24 Transfer functions.............................................................................................................................................. 24 Appendix B ........................................................................................................................................................... 24 Artificial scenarios for sensitivity analysis of climate change on environmental flows .................................... 24 References ............................................................................................................................................................. 26 IMPORTANT DISCLAIMER This report relates to climate simulations based on computer modelling. Models involve simplifications of real physical processes that are not fully understood. Accordingly, no responsibility will be accepted by CSIRO or the clients (the South Pacific Regional Environment Programme) for the accuracy of forecasts or predictions inferred from this report or for any person's interpretations, deductions, conclusions or actions in reliance of this report. Address for correspondence: Dr Roger Jones CSIRO Atmospheric Research PB No.1 Aspendale Victoria 3195 Phone: 61 3 9239 4555 Fax: 61 3 9239 4688 Email: [email protected]

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Executive Summary The Intergovernmental Panel on Climate Change has nominated the impacts of climate change on water as a key issue for Australia. Under current climate, the Murray-Darling Basin (MDB) is affected by large climate variability, high water-use as a proportion of total streamflow and competing demands for water. Climate change is expected to impact on runoff and streamflow, but large uncertainties have hampered planning of practical steps to manage these impacts. The possibility that already limited environmental flows and plans to recover some of these flows in the MDB may be threatened by climate change is investigated through three avenues: • Climate change projections for Australia, recently released by CSIRO, are re-investigated for the MDB. • Conclusions from recent impact modelling and risk assessment on flows and water use in the Macquarie River catchment are used to infer possible basin-wide changes. • The impacts of climate variability and land-use change in combination with climate change are described. Projected climate change Recent projections of rainfall change for the MDB suggest a decline in winter and spring rainfall by the year 2030. In summer, rainfall may either decrease or increase, with increases slightly more likely, while in autumn the direction of rainfall change is uncertain. Possible rainfall increases are largest towards the north of the MDB and decreases are largest to the south. Temperature is expected to increase in all areas. Potential evaporation is also highly likely to increase in all areas due to higher temperatures. These increases will be larger in regions and seasons in which rainfall decreases. Increases in open water evaporation will affect wetlands and water storages. Macquarie River water resources Water resources in the Macquarie River system were investigated by coupling CSIRO’s climate scenario generator to the IQQM river management model of the NSW Department of Land and Water Conservation. The study investigated possible changes to flows, irrigation allocation and environmental flow allocations using the full quantifiable range of possible precipitation and potential evaporation change from nine climate models. The baseline climate period was 1980–1996. The results indicate decreases in streamflow in a warmer world into the Burrendong Dam, the main storage for the catchment. The simulated change in storage range from +1% to –30% in 2030 and from +6 to –55% in 2070. A risk assessment suggests that the most likely outcomes in flow are from about 0 to –15% in 2030 and 0 % to – 35% in 2070. Uncertainty analysis showed that about two thirds of the uncertainty in water resources impacts was due to uncertainty in projected rainfall change as a function of global warming and about one quarter was due to uncertainty regarding the rate of global warming itself. This demonstrates the importance of understanding how future rainfall may change, and in correctly attributing observed changes over time. The Macquarie study identified two critical water management thresholds. A critical threshold marks the point at which an activity or system faces an unacceptable level of harm. The two thresholds were constructed: the minimum amount of flow required to ensure the continued breeding of water birds in the Macquarie Marshes, and irrigation allocations falling below a level of 50% for five consecutive years. The results suggest that the likelihood of exceeding both thresholds is about 1% in 2030, and 30–40% in 2070. Climate variability and land-use change Simulated flow for the Macquarie River based on 1890–1947 input was much less than that for the period 1948– 1996. The rainfall climate was therefore classified as a “drought-dominated” regime before 1948 and a “flooddominated” regime after 1948. The shift between regimes was abrupt, having a significant affect on the simulation of flows, shifting from about 25% less than the long-term mean to about 25% greater after 1948. This is supported by observed flows elsewhere in the MDB. These shifts in rainfall regime also considerably affected the risk of critical threshold exceedance. In 2030, the risk of exceeding both thresholds under a drought-dominated regime increases from 1% to about 30%. In 2070, these probabilities are 60–70% in a drought-dominated regime and 10–20% in a flood-dominated regime. However, these regimes of decadal rainfall variability are poorly understood and cannot be predicted, nor can changes in regime be diagnosed in the short term. Reforestation of the upper Macquarie catchment was also considered. Three scenarios of reforestation covering from 2% to 10% of the upper catchment reduced flows by 4% to 17%. Under climate change, these reductions

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were mainly additive, suggesting that the joint effect of reforestation and climate change further increases the risk to streamflow. This highlights the need for tree-planting programs to be carefully targeted to maximise salinity benefits while minimising streamflow losses. Catchment-wide implications Broader conclusions applicable to the wider basin can be drawn from the Macquarie study. Projected changes in rainfall and evaporation are broadly similar. Two factors are important: in the south of the MDB, rainfall reductions appear to be slightly more likely, and streamflow is more reliant on winter-spring rainfall, which is likely to decrease. Therefore, it is very likely that streamflow throughout the MDB will be reduced under enhanced greenhouse conditions. Reductions in flow in southern catchments may be larger than those in the Macquarie, while flow reductions in northern catchments may be more moderate. The water resources in the Basin have been developed and operated in the flood-dominated climate of the latter 20th century. Most changes in rainfall variability are likely to reduce those resources. The risk to water resources from climate change is far greater under a drought-dominated climate than it would be under a normal or flooddominated climate. Further decreases from reforestation and afforestation are probable but cannot be quantified because future planting and regrowth rates and patterns have not been investigated on a large scale. Increases in future flows from climate change and variability cannot be discounted but appear unlikely. Future directions Risk assessment in the Macquarie catchment, and climate projections for the MDB show that streamflow in the Basin is under threat from three sources: climate change, decadal-scale rainfall variability and land-use change. The Macquarie assessment needs to be extended across the Basin so that planning to ameliorate the impacts of these reductions can commence. The modelling system developed for the Macquarie study can be applied to the other catchments of the MDB in a new program of research. This program would aim to incorporate as many climatic and hydrological uncertainties as possible to estimate likely future changes in flow for the Murray and Darling Rivers. Avenues for changing water management strategies, involving management, legislative and economic options, would be investigated. Critical thresholds for a number of key systems would be constructed and linked to climate (e.g. critical environmental flow thresholds) and subject to a risk assessment. The aim of this program would be to produce options for the sustainable long-term management of environmental flows in the Murray-Darling Basin. This program would require a consortium of partners involving Federal and State government agencies, research organisations and the Tertiary research sector. The co-operation of each State government through their interest in individual catchments and current role in water management is critical.

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Part One Introduction This paper provides an overview of how climate change, land-use change and climatic variability may affect water resources and the allocation of environmental flows in the Murray-Darling Basin (MDB). It was prepared for the Murray-Darling Basin Commission as part of the wider project Environmental Flows and Water Quality Objectives for the River Murray. The paper is based on recent research conducted by CSIRO and collaborators and is divided into two parts. Part One discusses Australia’s international treaty obligations towards climate change and the resulting national policy response, previous research into water resources in Australia, climate projections for the MDB and likely outcomes in flows based on the research to date. Part Two summarises the results of a recent study of climate change impacts on water resources in the Macquarie River Catchment that contribute to the wider conclusions for the MDB, and draft proposals for a future research program.

Policy background Australia has international obligations under the United Nations Framework Convention on Climate Change, ratified in December 1992. The main goal of the convention is as follows: “The ultimate objective … is to achieve … stabilization of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system. Such a level should be achieved within a time-frame sufficient to allow ecosystems to adapt naturally to climate change, to ensure that food production is not threatened and to enable economic development to proceed in a sustainable manner.” The convention indicates that this requires a precautionary approach: “The Parties should take precautionary measures to anticipate, prevent or minimize the causes of climate change and mitigate its adverse effects. Where there are threats of serious or irreversible damage, lack of full scientific certainty should not be used as a reason for postponing such measures, taking into account that policies and measures to deal with climate change should be cost-effective so as to ensure global benefits at the lowest possible cost.” Of relevance to the management of water resources are the following: “The Parties have a right to, and should, promote sustainable development. Policies and measures to protect the climate system against human-induced change should be appropriate for the specific conditions of each Party and should be integrated with national development programmes, taking into account that economic development is essential for adopting measures to address climate change.” “[The Parties should] cooperate in preparing for adaptation to the impacts of climate change; develop and elaborate appropriate and integrated plans for coastal zone management, water resources and agriculture, and for the protection and rehabilitation of areas … affected by drought and desertification, as well as floods…[and] take climate change considerations into account, to the extent feasible, in their relevant social, economic and environmental policies and actions, … employ appropriate methods, for example impact assessments, formulated and determined nationally, with a view to minimizing adverse effects on the economy, on public health and on the quality of the environment, of projects or measures undertaken by them to mitigate or adapt to climate change…” This convention entered into force in March 1994. It is currently being addressed through the National Greenhouse Strategy (AGO, 1998). Module Eight of the NGS addresses adaptation strategies for climate change but to date, limited progress has been made (AGO, 2000). In many sectors a number of scientific issues are unresolved. Here, we address the issues relevant to the management of water resources in the Murray-Darling Basin in a warmer world.

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Climate change projections for the Murray-Darling Basin Recently, CSIRO prepared ranges of projected future climate change for the Australian region (CSIRO, 2001). Ranges of temperature and rainfall change were based on the full range of projected global warming as given by Intergovernmental Panel on Climate Change (IPCC, 2001) in combination with projected regional changes obtained from nine climate models. The ranges of change allow for uncertainty in human behaviour (uncertainty in future emissions of greenhouse gases), as well as climate science uncertainty (differences in the response of climate models). Ranges of annual average warmings across the MDB are 0.4 to 2.0°C by 2030 and 1.0 to 6.0°C by 2070 relative to 1990. Variations in the warming range across the Basin and over the four seasons are minor, although there is a tendency for the warming to be weaker in the south of the basin in winter (e.g. around 0.8 to 5.0°C in 2070). Ranges of rainfall change given in CSIRO (2001) have been re-analysed to provide more detailed information over the MDB (Figure 1). Uncertainty in projected regional rainfall change is large. In summer, ranges of change are biased slightly toward increase over most of the basin (–8% to +13% by 2030 and –25% to +40% by 2070). In autumn the direction of change is uncertain over most of the basin (–8% to +8% in 2030 and –25% to +25% in 2070). However in winter and spring there is a strong bias toward rainfall decrease. In winter, most of the basin is in the range is –8% to +3% in 2030 and –25% to +8% in 2070, and in spring –13% to +3% and – 40% to +8%. Projected rainfall decreases are more evident in the southern parts of the basin. Regional climate modelling simulating changes typical of the above patterns strongly indicate an increase in dry springs. Model results also indicate that extreme daily rainfall events are likely to become more extreme, where average rainfall increases, stays the same or decreases slightly. Seasonal changes in extreme daily rainfall have not been investigated.

Annual

Summer

2030

Winter

-40

-20 0 20 Rainfall Change (%)

40

2070

Autumn

Spring

-40

-20 0 20 Rainfall Change (%)

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Figure 1: Ranges of change in average rainfall (%) for around 2030 and 2070 relative to 1990. The coloured bars show ranges of change for areas with corresponding colours in the seasonal and annual maps.

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Annual average potential evaporation increases by 0% to 8% per degree of global warming (pdgw) over most of Australia and up to 12% pdgw over the upper catchment areas of the MDB. All models simulated increases in potential evaporation over the MDB. Increases are highest where rainfall decreases and is least in regions and seasons where rainfall increases.

Previous assessments Previous impact assessments on water resources in the MDB were undertaken by Chiew et al. (1995), Schreider et al. (1996, 1997) and Wang et al. (1999). An earlier study on the Macquarie catchment integrating water supply, environmental flows and economic outcomes for agriculture is described in Hassall and Associates (1998). These studies utilised earlier climate change scenarios (CSIRO 1992, 1996) which contained both substantial increases and decreases in rainfall, or single model outputs. The CSIRO (1992) scenarios were based on an earlier generation of climate models that utilised simplified “mixed-layer” oceans and lacked ocean currents and dynamic phenomena such as the El Niño – Southern Oscillation (e.g. Allan et al., 1996). The 1996 regional climate change scenarios (CSIRO, 1996) summarised both mixed layer and more realistic coupled ocean atmosphere-models. These scenarios also encompassed a broad range of rainfall uncertainty. The resulting impact studies listed above produced conflicting outcomes. Using rainfall changes based on CSIRO (1992) and estimating potential evaporation (Ep) from changes in temperature, Chiew et al. (1995) simulated both substantial increases and decreases in runoff and streamflow for several small, ungauged catchments in the upper Murray region. Schreider et al. (1996, 1997) investigated changes in the snow-free Goulburn and Ovens catchments and the snow-affected Mitta Mitta and Kiewa catchments using “most wet” (temperature increase of +1.5°C, rainfall increase of +20% in summer, +10% winter) and “most dry” (+2°C, no summer rainfall change, –10% winter rainfall change) scenarios for 2030, based on CSIRO (1992). These assumptions produced neutral to negative changes in streamflow in the snow-free catchments and slightly positive to negative changes in the snow-affected catchments (Table 1). Despite using similar scenarios to Chiew et al. (1995), these results were substantially drier. The differences between these results may be related to the different representation in their respective models of the effect of temperature and Ep changes on runoff. Table 1. Climate scenario impact on precipitation and streamflow in snow-free and snow-affected catchments in Victoria in 2030. From Schreider et al. (1996, 1997), based on CSIRO (1992). Scenario Precipitation Streamflow (% change) (% change) Most dry Snow free -7 - 36 Snow affected -6 - 30 Most wet Snow free + 13 0 Snow affected + 13 +9 Wang et al. (1999) investigated the Campaspe system using a scenario derived from the CSIRO regional model nested in a mixed-layer global climate model (GCM), which produced rainfall decreases in the first half of the year and increases in the second half, resulting in a net annual rainfall decrease. This was used to investigate the impact of climate change on security of water right. Irrigation water is allocated on the basis of 100% water right and a further 120% of sales water, giving a total of 220% of the water right in a year when supply is not limited. Security of the actual water right is measured as the percentage of years that 100% of the water right can be supplied. This percentage was reduced by only 1% in 2030 (0.8°C global warming), 4% in 2070 (1.8°C global warming) and 16% for a 4.1°C global warming, but the relatively effective maintenance of security was at the expense of downstream environmental flows. Hassall & Associates (1998) reported on an extensive study into the effects of climate change on the economy and ecology of the Lower Macquarie Valley. Precipitation and (Ep) scenarios from CSIRO’s regional climate model nested in a mixed-layer GCM were used to provide estimates for 2030 that were towards the dry end of the CSIRO (1996) scenarios. The changes in river flow simulated by the Integrated Quantity and Quality Model (IQQM) Macquarie Model (Department of Land and Water Conservation, 1995ab) are shown in Table 2. Average sub-catchment streamflow decreases for the most wet and most dry scenarios were 12% and 32% respectively. Accumulated economic losses for the livestock, cotton and wheat industries were $38 million and $152 million respectively. Most losses were in the livestock industry and all sectors took productivity increases due to higher concentrations of CO2 into account (Hassall & Associates, 1998). These losses can be considered as relevant within the scope of the latest research described in Part Two of this document.

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Table 2. Total Annual Yields of the Macquarie River (GL) Catchment Most wet Rainfall Change (% change) (%) Upstream Burrendong Dam -4 -11 Downstream Burrendong Dam -8 -14 Overall -6 -12

Most dry Rainfall Change (% change) (%) -7 -30 -11 -37 -9 -32

Future impacts on the Murray-Darling Basin Planning for climate change in the MDB has been hampered by the large uncertainties associated with how the greenhouse effect may affect the regional climate, particularly for the direction and magnitude of rainfall change. However, we believe the latest research described in this paper and in the papers it draws upon, is sufficient to warrant the development and implementation of adaptation strategies to cope with climate change consistent with the policies described earlier. Under current climate, the Murray-Darling Basin (MDB) is affected by large climate variability, high water-use as a proportion of total streamflow and competing demands for water. Plans to secure environmental flows in the MDB would be threatened by possible reductions in flow in coming decades. This possibility is investigated through three avenues: • Climate change projections for Australia, recently released by CSIRO, have been re-investigated for the MDB. • Conclusions from recent impact modelling and risk assessment on flows and water use in the Macquarie River catchment have been used to infer possible basin-wide changes. • The impacts of climate variability and land-use change. Projected regional climate The patterns of seasonal P and Ep changes investigated in detail for the Macquarie catchment are similar to the broader changes simulated over the MDB as shown in Figure 1. The pattern of rainfall change is one of increases or decreases in summer and autumn, with increases dominating over the northern half of the Basin, and predominantly decrease in winter and spring. Winter-spring decreases are more strongly evident in the southern half of the basin. Cool season rainfall (and runoff) is proportionally more important relative to the annual total in southern areas of the basin (Table B2). This pattern of rainfall change may be at least partially explained in terms of simulated changes in atmospheric circulation. Pressure patterns show an increase in the mid southern latitudes that would be consistent with a southward shift in the westerly rain belt, an important source of rainfall in southern Australia, particularly in the cooler months (not shown). Whether this is a robust aspect of simulated regional rainfall change within coupled ocean-atmosphere models needs to be further examined. Increases in Ep as observed in the Macquarie study are also generally applicable across the MDB (CSIRO, 2001). Increases in potential evaporation in winter and spring are typically stronger in southern parts of the basin, as would be expected given the stronger tendency for rainfall decreases in these regions. Along with the rainfall changes discussed above, this would contribute to greater flow decreases in southern catchments compared to those simulated in the Macquarie study. Open water evaporation over lakes, reservoirs and wetlands would also be expected to increase. Such changes have not yet been quantified. Ranges of temperature increases are lower in the southern half of the MDB and tend to be less in winter. Higher temperatures will affect water temperature, possible leading to increased algal blooms. Higher temperatures in the uplands of the MDB will change the ratio of solid precipitation to rainfall. Schreider et al. (1997) indicated that snow-affected catchments were a little less sensitive to evaporation increases than neighbouring snow-free catchments. Changes in seasonality of flow (with the peak occurring earlier in the year) may also be expected in snow-affected catchments (although in the Schreider et al. study, this effect was quite small). Macquarie River risk assessment implications for the MDB Many of the results from Macquarie River risk assessment detailed in Part Two can be applied more broadly to the MDB. Similar patterns of regional P and Ep change across the MDB show that similar outcomes in terms of streamflow are likely. However, quantified outcomes for the MDB cannot be provided until each significant

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catchment in terms of water supply, has been modelled directly. Furthermore, several important uncertainties were not addressed in the Macquarie risk assessment, and these also need to be explored within a modelling system to determine whether they would significantly alter the conclusions. Risk analysis indicated the most likely 90% of the total range of change in streamflow and water supply is 0% to 15% in 2030 and 0% to 35% in 2070. Changing the input assumptions alters the extremes of the total range of outcomes but has relatively little effect on the central 90%. Taking into account the climate projections for the MDB described above, this range may be slightly more favourable to the north, where summer and autumn rainfall increases are possible. The range is likely to become more negative (larger decreases) to the south because of the dependence of streamflow on winter-spring rainfall which is expected to decrease, based on the current evidence. Two areas of uncertainty, that have not bee simulated, concern daily rainfall changes and hydrological uncertainties. Daily rainfall is expected to become more intense in most areas where mean rainfall increases or remains the same, and may possibly become more intense even with slight decreases in mean rainfall. How these trends may combine with the seasonal pattern of mean rainfall change is uncertain. It may be expected that summer-autumn increases in intensity could be larger than winter-spring changes, which may vary between increase and decrease. However, this needs to be investigated using daily model data output from a number of climate models, a data- and time-intensive procedure. This effect is likely to most significant in small catchments and those with a relatively high proportion of runoff compared to rainfall. How changes in extreme rainfall may affect flood distributions (an important part of environmental flows) and flood risks remains unknown. Hydrological uncertainties are many, and are probably less significant than climatic uncertainties (Arnell and Liu, 2001) but need to be accounted for in planning. The type of rainfall-runoff used in this study falls into a group of conceptual rainfall runoff-models that approximate hydrological processes within a series of optimised parameters, ranging from several to about two dozen. Whether these parameters adequately represent processes under climate change is poorly known, and few studies have been carried out where several such models have been compared within a single catchment (e.g. Boorman and Sefton, 1997). This would be a very valuable exercise to carry out within the MDB. Topographic variations, especially in the eastern highlands, are generally poorly represented in climate models because of their large grid size. Since, the interaction between mountains and various weather systems influences the distribution of rainfall, poor topographic representation remains an uncertainty. However, the 60km and 125km CSIRO regional models used in this study contain a better representation of topography and did not have vastly different results to the GCMs. Also, poor soil development in a number of rainshadow areas in southeastern Australia suggests that these types of topographic controls have persisted throughout a series of past climate changes. The Bayesian and uncertainty analysis carried out for the Macquarie catchment also indicate that, under the present modelling structure, the probability distribution functions (PDFs) of streamflow are little affected by changes in the input range or distribution of uncertainties. Most of the uncertainty is due to rainfall and magnitude and change, and changes in the ranges and distribution of such changes produced similar PDFs to those produced using altered input assumptions. In summary, it is very likely that the general tendency for reduced flows under enhanced greenhouse conditions simulated for the Macquarie catchment will apply throughout the wider MDB. With regard to the effect of rainfall change, reductions in flow in catchments in southern areas of the Basin may be larger than those from the Macquarie study, whereas flow reductions in northern catchments may be more moderate. More reliable estimates of changes in water resources and allocation implications for other catchments in the MDB would require explicit modelling for these catchments (see draft program of research below).

Climate variability and land-use change Decadal scale variability in rainfall in this project is a period of rainfall where an average higher, lower or close to the long-term mean is sustained for several or more decades. These periods can switch between droughtdominated and flood-dominated modes in a relatively short time. Little is known about their dynamics, but they affect many parts of the world, especially in the tropics and mid-latitudes. Drought-dominated and flooddominated regimes observed in the Macquarie River catchment persisted across the MDB switching from drought- to flood-dominated mode in 1948. The shift between regimes was abrupt, having a significant affect on the simulation of flows, shifting from about 25% less than the long-term mean to about 25% greater after 1948.

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This is supported by observed flow changes elsewhere in the MDB. These modes of rainfall variability are likely to continue in the future but they are unpredictable, their dynamics are poorly understood and it is difficult at any time to be certain which mode is current. However, the research in the Macquarie showed that the combination of climate change and decadal rainfall variability is much more important than either mechanism on its own. Tree-planting and other forms of revegetation across the MDB are being planned for the following purposes: Commercial plantations: Australian governments have committed themselves to a tripling of plantation forests by 2020. Carbon sequestration: Increases in vegetation biomass as an allowed mechanism to meet CO2 emission targets (1990 levels plus 8%), have been set by the Kyoto Protocol. Salinity remediation: Late in 2000, the federal government announced a plan to spend $1.4 billion over 10 years on salinity remediation. A principal remediation activity is the revegetation of recharge areas. Biodiversity management and ecosystem services: Re-establishment of indigenous species and communities is aiming to halt the decline in biodiversity and ecosystem services. Three scenarios of reforestation covering from 2% to 10% of the upper Macquarie catchment reduced flows by 4% to 17%. Under climate change, these reductions were mainly additive, suggesting that the joint effect of reforestation and climate change further increases the risk to streamflow. The revegetation programs listed above, all valuable in their own right, could potentially lead to similar scale reductions over the MDB. This highlights the need for tree-planting programs to be carefully targeted to maximise the above benefits while minimising streamflow losses.

Critical thresholds Critical thresholds marking an unacceptable level of harm for a system or aspect of a system can be used to assess the risk associated with a driver, or combination of drivers such as climate change and variability. Two thresholds were constructed for the Macquarie catchment: the minimum amount of flow required to ensure the continued breeding of water birds in the Macquarie Marshes, and irrigation allocations falling below a level of 50% for five consecutive years. The risk of exceeding both thresholds under a drought-dominated regime increases from 1% in a normal rainfall regime to about 30%. In 2070, these probabilities are 60–70% in a drought-dominated regime, 30–40% in a normal climate and 10–20% in a flood-dominated regime. Critical thresholds and other forms of thresholds can, and are set up for monitoring environmental flows. For instance, River Red Gum forests need a particular level of flood frequency to remain healthy. Other well known thresholds in the MDB are the 500 Ec limit for total dissolved salts in the Murray River at Morgan and the proportion of current flow compared to natural flow. We recommend measuring the impacts on important functions in the MDB in two ways: 1. Using risk and uncertainty analysis to determine the most likely outcomes for important impacts, e.g. 0% to –15% changes in streamflow for the Macquarie by 2030, to use in general planning. A monitoring and attribution program can determine observational rules that signal if climate is changing in such a way that these limits may be exceeded (e.g. rainfall trends having a likelihood of being above or below a certain limit). 2. Determine important thresholds within the system that are associated with legislated targets, plans and policies, sustainable performance or an unacceptable level of harm. Use risk assessment to assess how likely such thresholds are to be met or avoided within a certain range of conditions. Again, a monitoring program can determine whether thresholds are more or less likely to be exceeded over time. Bayesian analysis can also look how risk profiles may change when new information becomes available.

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Part Two The Macquarie Study Based on the earlier Macquarie Study (Hassall and Associates (1998), and with funding from the Rural Industries Research and Development Corporation, CSIRO and research partners, the NSW Department of Land and Water Conservation (DLWC) and Hassall and Associates, carried out a risk assessment of climate change on the water resources of the Macquarie River catchment (Jones et al., 2001a and b; Jones and Page, 2001). This work is the most relevant available regarding the issue of climate change impacts on water resources and their management in the MDB. Relevant methods and outcomes are described in Part Two of this report. The Region The Macquarie River Catchment is situated in the central eastern part of the Murray-Darling Basin and covers about 75,000 km2 (Fig. 2). It rises on the western slopes of the Great Dividing Range, 100 km west of Sydney, and flows WNW into the Darling River. Rainfall ranges from about 1,200 mm pa in the upper catchment to 20% from the long-term historical mean. The flood-dominated regime contains both the wettest and driest years for the entire series. Table 3. Average rainfall anomalies and mean annual storage volume in the Burrendong Dam for drought-dominated and flood-dominated periods under current climate simulated using the 1996 flow management rules. Drought-dominated period Flood-dominated period 1890–1948 1949–1996 Rainfall anomaly –20% +23% Flow anomaly –23% +27% Figure 4 shows Burrendong Dam and Macquarie Marshes inflows along with the percentage of irrigation allocations met within a water year (July 1 to June 30) for the period 1890/91–1995/96. The effect of decadal variability is obvious. Flows and irrigation allocations are much lower for the first half of the 20th century than they are for the second half. During the period 1947/48 to 1978/79, irrigation allocations only fall below half on one occasion, and 100% allocations were supplied almost 80% of the time. Between 1947 and 1996, three extreme shortfalls (