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The Murray-Darling Basin is Australia's food bowl and home to many iconic water bodies that are culturally and ecologically highly valued. The recent Millennium Drought (from mid-1990s to 2009) was the most severe hydrological drought since records started in the late 19th century. It severely impacted on the basin and for many acted as a wake-up call. To address the ongoing declines in water resources and environmental conditions and to prepare the region for climate change, Australia's Governments are currently attempting to introduce a new comprehensive, and integrated approach to the management of the basin's water resources. In this paper, long-term time series of climate, hydrological and environmental data are used to analyze how compounding stresses have gradually affected the hydrological system and its services. Major hydroclimatic stresses considered in this paper include salinity, water use, droughts, and climate change. Other, more localized or minor stresses exist (groundwater extraction, farm dams, afforestation, bush fires, cyanobacterial blooms and pollutants) and are reviewed more briefly. The history of water policy and planning shows that Government actions have been strongly influential on the basin. A shift in the strategic goals from water development to the protection and restoration of environmental assets is noticeable since the mid 1990s. Median climate change projections by 2030 indicate smaller reductions in rainfall and runoff than those observed during the recent Millennium Drought, but have a relatively high uncertainty attached to them. The use of regional approaches to reduce that uncertainty, such as statistical downscaling, points to a sizeable decline in rainfall by the end of the century. Most climate projections used for planning consider greenhouse emission scenarios that have smaller global emission trends than the one observed over the last decade. Other, 'less optimistic' scenarios have to be considered for long-term water planning and food security. Compounding all these stresses, is the naturally high hydroclimatic variability of this semi-arid region, that may have been insufficiently considered during previous water development and planning efforts. Successful water planning will need to balance cultural and ecological values with food production, account for high natural variability and uncertainty in climate change projections, learn from past mistakes and be cognizant of future hydrological changes.

The Murray-Darling Basin is Australia's food bowl and home to many iconic water bodies that are culturally and ecologically highly valued. The recent Millennium Drought (from mid-1990s to 2009) was the most severe hydrological drought since records started in the late 19th century. It severely impacted on the basin and for many acted as a wake-up call. To address the ongoing declines in water resources and environmental conditions and to prepare the region for climate change, Australia's Governments are currently attempting to introduce a new comprehensive, and integrated approach to the management of the basin's water resources. In this paper, long-term time series of climate, hydrological and environmental data are used to analyze how compounding stresses have gradually affected the hydrological system and its services. Major hydroclimatic stresses considered in this paper include salinity, water use, droughts, and climate change. Other, more localized or minor stresses exist (groundwater extraction, farm dams, afforestation, bush fires, cyanobacterial blooms and pollutants) and are reviewed more briefly. The history of water policy and planning shows that Government actions have been strongly influential on the basin. A shift in the strategic goals from water development to the protection and restoration of environmental assets is noticeable since the mid 1990s. Median climate change projections by 2030 indicate smaller reductions in rainfall and runoff than those observed during the recent Millennium Drought, but have a relatively high uncertainty attached to them. The use of regional approaches to reduce that uncertainty, such as statistical downscaling, points to a sizeable decline in rainfall by the end of the century. Most climate projections used for planning consider greenhouse emission scenarios that have smaller global emission trends than the one observed over the last decade. Other, 'less optimistic' scenarios have to be considered for long-term water planning and food security. Compounding all these stresses, is the naturally high hydroclimatic variability of this semi-arid region, that may have been insufficiently considered during previous water development and planning efforts. Successful water planning will need to balance cultural and ecological values with food production, account for high natural variability and uncertainty in climate change projections, learn from past mistakes and be cognizant of future hydrological changes.

Despite strong demand for information to support the sustainable use of Australia's natural resources and conserve environmental values and despite considerable effort and investment, nation-wide environmental data collection and analysis remains a substantially unmet challenge. We review progress in producing national environmental reports and accounts, identify challenges and opportunities, and analyse the potential role of research in addressing these. Australia's low and concentrated population density and the short history since European settlement contribute to the lack of environmental data. There are additional factors: highly diverse data requirements and standards, disagreement on information priorities, poorly measurable management objectives, lack of coordination, over-reliance on researchers and businesses for data collection, lack of business engagement, and short-term, project-based activities. New opportunities have arisen to overcome some of these challenges: enhanced monitoring networks, standardisation, data management and modelling, greater commitment to share and integrate data, community monitoring, increasing acceptance of environmental and sustainability indicators, and progress in environmental accounting practices. Successes in generating climate, water and greenhouse gas information appear to be attributable to an unambiguous data requirement, considerable investment, and legislative instruments that enhance data sharing and create a clearly defined role for operational agencies. Based on the analysis presented, we suggest six priorities for research: (1) common definitions and standards for information that address management objectives, (2) ecological measures that are scalable from local to national level, (3) promotion of long-term data collection and reporting by researchers, (4) efficient satellite and sensor network technologies and data analysis methods, (5) environmental modelling approaches that can reconcile multiple data sources, and (6) experimental accounting to pursue consistent, credible and relevant information structures and to identify new data requirements. Opportunities exist to make progress in each of these areas and help secure a more sustainable future. ; This research was supported under the Australian Research Council's Linkage Projects funding scheme (project number LP130100789).

Despite strong demand for information to support the sustainable use of Australia's natural resources and conserve environmental values and despite considerable effort and investment, nation-wide environmental data collection and analysis remains a substantially unmet challenge. We review progress in producing national environmental reports and accounts, identify challenges and opportunities, and analyse the potential role of research in addressing these. Australia's low and concentrated population density and the short history since European settlement contribute to the lack of environmental data. There are additional factors: highly diverse data requirements and standards, disagreement on information priorities, poorly measurable management objectives, lack of coordination, over-reliance on researchers and businesses for data collection, lack of business engagement, and short-term, project-based activities. New opportunities have arisen to overcome some of these challenges: enhanced monitoring networks, standardisation, data management and modelling, greater commitment to share and integrate data, community monitoring, increasing acceptance of environmental and sustainability indicators, and progress in environmental accounting practices. Successes in generating climate, water and greenhouse gas information appear to be attributable to an unambiguous data requirement, considerable investment, and legislative instruments that enhance data sharing and create a clearly defined role for operational agencies. Based on the analysis presented, we suggest six priorities for research: (1) common definitions and standards for information that address management objectives, (2) ecological measures that are scalable from local to national level, (3) promotion of long-term data collection and reporting by researchers, (4) efficient satellite and sensor network technologies and data analysis methods, (5) environmental modelling approaches that can reconcile multiple data sources, and (6) experimental accounting to pursue consistent, credible and relevant information structures and to identify new data requirements. Opportunities exist to make progress in each of these areas and help secure a more sustainable future. ; This research was supported under the Australian Research Council's Linkage Projects funding scheme (project number LP130100789).

Water accounting is becoming a common term in water resources management, especially in Australia after the Council of Australian Governments (COAG) reached an intergovernmental agreement on the National Water Initiative (NWI) in 2004. Water accounting aims to provide a complete picture of water resources for a region over a reporting period. It provides quantitative information about all water resource components such as inflows, outflows, rainfall, evaporation, water use, surface water availability and change, groundwater availability and change etc on a consistent basis. It has already been occasionally used in water resources management and planning processes. For example, the Commonwealth Water Act 2007 assigns the Bureau of Meteorology (the Bureau hereinafter) the role of "compiling and maintaining water accounts for Australia, including a set of water accounts to be known as the National Water Account." Among other obligations under the Act, the Bureau is required to publish this National Water Account (NWA) annually in a form readily accessible by the public. The NWA will provide reliable information such as water rights, water availability and water use that has previously been difficult to access or unavailable to general users in a standardised form. The Bureau, together with water agencies and other government agencies, has successfully prepared and published Pilot NWA for 2007-2008, and will publish the first NWA in 2011. Uncertainty of a measurand is "a parameter associated with the result of a measurement or estimation that characterises the dispersion of the values that could reasonably be attributed to the measurand." Most, if not all, line items in water accounting are subject to uncertainties. The sources of uncertainty vary, depending on the origin of data and processing procedures. Possible uncertainty sources include direct measurement uncertainty, indirect measurement uncertainty (processing errors from recording, estimation, interpolation, aggregation, sampling, etc), and hydrological model uncertainty (model structure and model parameters). Quantitative uncertainty information in water accounting can encourage users to query the precision of the underlying data and information, thereby improving their understanding of the nature of water stocks and flows. It can help identify the main contributors to the uncertainty thus enabling resources to be allocated appropriately in order to improve the accounting in future. It can also improve risk control in informed water management and planning. Thus, it is crucial to include uncertainty in water accounting, such as the NWA. As an activity under the Water Resources Information Research and Development Alliance (WIRADA) between the Bureau and CSIRO, we develop a consistent uncertainty framework for assessing, quantifying and reporting of quantitative uncertainty for water accounting. The proposed hierarchical decision-tree-based uncertainty framework is intended to help systematically choose and apply consistent and reliable approaches for quantifying and reporting uncertainty information about water accounting items. For each line item, the decision tree considers its nature, source and logical foundation. It will provide a pathway to some end point through a couple of simple decisions. For each end point (leaf node) in the decision-tree, we recommend a practical uncertainty analysis technique, ranging from classical uncertainty analysis techniques, uncertainty propagation law, Monte Carlo, literature to expert elicitation. Hierarchical structures are ubiquitous in the water accounting. They may originate from geographical hierarchy of reporting entities like subcatchment, catchment, and a nation, or from reporting detail hierarchy like detailed, intermediate, and simple water accounts, or from temporal hierarchy like monthly and yearly water accounts. We also suggest techniques to aggregate uncertainty from a lower level of accounting items. We simply list software implementation possibilities, including add-ins for spreadsheet like Microsoft Excel, standalone software, and general statistical software. For the work in progress, we also discuss a couple of issues in this preliminary uncertainty framework for water accounting.

• Drought has many faces, monitoring them requires a vast range of observations • Key questions: who is affected, how bad is it, who's fault is it, and will it get better? • Recent legislation has made the Bureau of Meteorology the natural home for

Water accounting is becoming a common term in water resources management, especially in Australia after the Council of Australian Governments (COAG) reached an intergovernmental agreement on the National Water Initiative (NWI) in 2004. Water accounting aims to provide a complete picture of water resources for a region over a reporting period. It provides quantitative information about all water resource components such as inflows, outflows, rainfall, evaporation, water use, surface water availability and change, groundwater availability and change etc on a consistent basis. It has already been occasionally used in water resources management and planning processes. For example, the Commonwealth Water Act 2007 assigns the Bureau of Meteorology (the Bureau hereinafter) the role of "compiling and maintaining water accounts for Australia, including a set of water accounts to be known as the National Water Account." Among other obligations under the Act, the Bureau is required to publish this National Water Account (NWA) annually in a form readily accessible by the public. The NWA will provide reliable information such as water rights, water availability and water use that has previously been difficult to access or unavailable to general users in a standardised form. The Bureau, together with water agencies and other government agencies, has successfully prepared and published Pilot NWA for 2007-2008, and will publish the first NWA in 2011. Uncertainty of a measurand is "a parameter associated with the result of a measurement or estimation that characterises the dispersion of the values that could reasonably be attributed to the measurand." Most, if not all, line items in water accounting are subject to uncertainties. The sources of uncertainty vary, depending on the origin of data and processing procedures. Possible uncertainty sources include direct measurement uncertainty, indirect measurement uncertainty (processing errors from recording, estimation, interpolation, aggregation, sampling, etc), and hydrological model uncertainty (model structure and model parameters). Quantitative uncertainty information in water accounting can encourage users to query the precision of the underlying data and information, thereby improving their understanding of the nature of water stocks and flows. It can help identify the main contributors to the uncertainty thus enabling resources to be allocated appropriately in order to improve the accounting in future. It can also improve risk control in informed water management and planning. Thus, it is crucial to include uncertainty in water accounting, such as the NWA. As an activity under the Water Resources Information Research and Development Alliance (WIRADA) between the Bureau and CSIRO, we develop a consistent uncertainty framework for assessing, quantifying and reporting of quantitative uncertainty for water accounting. The proposed hierarchical decision-tree-based uncertainty framework is intended to help systematically choose and apply consistent and reliable approaches for quantifying and reporting uncertainty information about water accounting items. For each line item, the decision tree considers its nature, source and logical foundation. It will provide a pathway to some end point through a couple of simple decisions. For each end point (leaf node) in the decision-tree, we recommend a practical uncertainty analysis technique, ranging from classical uncertainty analysis techniques, uncertainty propagation law, Monte Carlo, literature to expert elicitation. Hierarchical structures are ubiquitous in the water accounting. They may originate from geographical hierarchy of reporting entities like subcatchment, catchment, and a nation, or from reporting detail hierarchy like detailed, intermediate, and simple water accounts, or from temporal hierarchy like monthly and yearly water accounts. We also suggest techniques to aggregate uncertainty from a lower level of accounting items. We simply list software implementation possibilities, including add-ins for spreadsheet like Microsoft Excel, standalone software, and general statistical software. For the work in progress, we also discuss a couple of issues in this preliminary uncertainty framework for water accounting.

• Drought has many faces, monitoring them requires a vast range of observations • Key questions: who is affected, how bad is it, who's fault is it, and will it get better? • Recent legislation has made the Bureau of Meteorology the natural home for

On average, Australia is a dry continent with many competing uses for water. Hence, there is an urgent need to know actual evapotranspiration (ET a) patterns across wide areas of agricultural and natural ecosystems, as opposed to just point measurements of ET a. The Australian Government has tasked the science agencies with operationally developing monthly and annual estimates of ET a and other hydrological variables, and with forecasting water availability over periods of days to decades, as part of its national water assessment programme. To meet these needs, Australian researchers have become leaders in developing large-area methods for estimating ET a at regional and continental scales. Ground methods include meteorological models, eddy covariance towers, sap flow sensors and catchment water balance models. Remote sensing methods use thermal infrared, mid infrared and/or vegetation indices usually combined with meteorological data to estimate ET a. Ground and remote sensing ET a estimates are assimilated into the Australian Water Resource Assessment, which issues annual estimates of the state of the continental water balance for policy and planning purposes. The best ET a models are estimated to have an error or uncertainty of 10% to 20% in Australia. Developments in Australian ET a research over the past 20years are reviewed, and sources of error and uncertainty in current methods and models are discussed.

On average, Australia is a dry continent with many competing uses for water. Hence, there is an urgent need to know actual evapotranspiration (ET a) patterns across wide areas of agricultural and natural ecosystems, as opposed to just point measurements of ET a. The Australian Government has tasked the science agencies with operationally developing monthly and annual estimates of ET a and other hydrological variables, and with forecasting water availability over periods of days to decades, as part of its national water assessment programme. To meet these needs, Australian researchers have become leaders in developing large-area methods for estimating ET a at regional and continental scales. Ground methods include meteorological models, eddy covariance towers, sap flow sensors and catchment water balance models. Remote sensing methods use thermal infrared, mid infrared and/or vegetation indices usually combined with meteorological data to estimate ET a. Ground and remote sensing ET a estimates are assimilated into the Australian Water Resource Assessment, which issues annual estimates of the state of the continental water balance for policy and planning purposes. The best ET a models are estimated to have an error or uncertainty of 10% to 20% in Australia. Developments in Australian ET a research over the past 20years are reviewed, and sources of error and uncertainty in current methods and models are discussed.