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Many Victorian catchments experienced a rainfall‐runoff relationship (RRR) shift during the Millennium Drought (1997–2009). Less annual streamflow was generated from annual rainfall during the drought than would have been expected for the same rainfall before the drought. Changes in weather systems, such as cyclones, fronts, and thunderstorms, were also observed during this period. In this study, we assess the role that changes in weather systems played in the RRR shift. We find that catchments that experienced a RRR shift tended to receive less of their rainfall from frontal weather systems, and more of their rainfall from thunderstorm‐enhanced cyclone and cyclone‐front hybrid systems during the drought than catchments that showed little change in RRR. Overall, changes in the proportion of rainfall from weather systems alone explained up to 43% of the variance in the RRR shift between catchments. Including parameters that quantify catchment processes and physiographic features indicated that weather system changes are not the primary indicator of a RRR shift. Catchment mean slope, aridity and valley confinement, together with proportional rainfall change of cyclone‐thunderstorms and cyclone‐front‐thunderstorms, explained 60% of the variance in the RRR shift between catchments. Focusing on streamflow availability, we find the importance of different weather systems and catchment characteristics varies depending on warm or cool season, eastern or western Victoria, and before or during the drought. Following the drought, some catchments largely maintained the RRR shift that occurred during the drought, and also experienced less of their rainfall from fronts and cyclones compared to the other catchments. Key Points: Weather system changes in Victoria, Australia are partly related to a change in rainfall‐runoff relationship (RRR) during a decade‐long droughtLess rainfall from fronts and more rainfall from thunderstorm enhanced systems were an indicator of RRR changeInternal catchment characteristics were a greater indicator of RRR change than changes in weather systems [ABSTRACT FROM AUTHOR]

Environmental policy is often implemented using market instruments. In some cases, including carbon taxing, the links between financial products and the environmental objectives, are transparent. In other cases, including water markets, the links are less transparent. In Australia's Murray–Darling Basin (MDB), financial water products are known as 'entitlements', and are similar to traditional financial products, such as shares. The Australian water market includes 'Low Security' entitlements, which are similar to 'sub-prime' mortgage bonds because they are unlikely to yield an amount equal to their financial worth. Nearly half the water purchased under the Murray–Darling Basin Plan for environmental purposes is 'Low Security'. We suggest that the current portfolio of water held by the Australian Government for environmental purposes reflects the mortgage market in the lead-up to the global financial crisis. Banks assumed that the future value of the mortgage market would reflect past trends. Similarly, it is assumed that the future value of water products will reflect past trends, without considering climate change. Historic records of allocations to 'Low Security' entitlements in the MDB suggest that, in the context of climate change, the Basin Plan water portfolio may fall short of the target annual average yield of 2075 GL by 511 GL. We recommend adopting finance sector methods including 'hedging' 'Low Security' entitlements by purchasing an additional 322–2755 GL of 'Low Security', or 160–511 GL of 'High Security' entitlements. Securing reliable environmental water is a global problem. Finance economics present opportunities for increasing the reliability of environmental flows. [ABSTRACT FROM AUTHOR]

Meteorological time‐series data are a fundamental input to hydrological investigations. But sourcing data is often laborious and plagued with difficulties. In an effort to improve efficiency and rigor we present an R‐package, named AWAPer (https://github.com/peterson-tim-j/AWAPer), for the efficient estimation of daily area weighted catchment average and spatial variance of meteorological variables, including evapotranspiration. The package allows creation and updating of a data‐cube of gridded daily data from 1900 onwards. Once created, point and area weighted estimates can be extracted at user‐defined locations and time periods for anywhere within Australia. Examples of point and catchment average extraction are presented. [ABSTRACT FROM AUTHOR]

Uncertainty analysis is an integral part of any scientific modeling, particularly within the domain of hydrological sciences given the various types and sources of uncertainty. At the center of uncertainty rests the concept of equifinality, that is, reaching a given endpoint (finality) through different pathways. The operational definition of equifinality in hydrological modeling is that various model structures and/or parameter sets (i.e., equal pathways) are equally capable of reproducing a similar (not necessarily identical) hydrological outcome (i.e., finality). Here we argue that there is more to model equifinality than model structures/parameters, that is, other model components can give rise to model equifinality and/or could be used to explore equifinality within model space. We identified six facets of model equifinality, namely, model structure, parameters, performance metrics, initial and boundary conditions, inputs, and internal fluxes. Focusing on model internal fluxes, we developed a methodology called flux mapping that has fundamental implications in understanding and evaluating model process representation within the paradigm of multiple working hypotheses. To illustrate this, we examine the equifinality of runoff fluxes of a conceptual rainfall‐runoff model for a number of different Australian catchments. We demonstrate how flux maps can give new insights into the model behavior that cannot be captured by conventional model evaluation methods. We discuss the advantages of flux space, as a subspace of the model space not usually examined, over parameter space. We further discuss the utility of flux mapping in hypothesis generation and testing, extendable to any field of scientific modeling of open complex systems under uncertainty. Key Points: We characterized different facets of model equifinality and discussed them within the context of conceptual hydrological modelingWe introduced the new model evaluation method of Flux Mapping to explore model behavior, particularly process representationEven within a very narrow margin of model error/performance, different modes of model response (i.e., internal flux dynamics) can be equally active [ABSTRACT FROM AUTHOR]

• We studied changes in recession behaviour of drought-affected catchments. • We detected steeper post-drought recessions in catchments impacted by the drought. • We attributed changes to disconnection of storage and increased transmission losses. Recession analysis is commonly adopted to characterise catchment behaviour, particularly in relation to the role of catchment storage in streamflow production. Recession techniques are data hungry and characterise average behaviour over long periods of recorded data, making them generally unsuitable in transient conditions. Nevertheless, in the context of multi-annual drought, sufficient data may be available to characterise temporal change by examining different subperiods. In this work, we assessed changes in recession behaviour of catchments affected by multi-annual drought in south-eastern Australia, focusing on how changes in recession patterns over time correlate to drought-induced shifts in annual rainfall-runoff relationships. We applied two methods, drawn from the vast methodological literature on recession analysis. They consider different information and entail different assumptions; one is based on average daily recession rates and the other on hourly recession slope plots. Despite the differences, their results are consistent: catchments that exhibited significant changes in rainfall-runoff relationship during the drought also display significantly steeper recessions during and after the drought. Subsequent investigation suggests that only a portion of the increases in recession speed are attributable to increased evaporative demand. Rather, they are likely caused by decreased connectivity between subsurface storage and catchment surface water and increased transmission losses through the streambed. With this work, we demonstrate the usefulness of recession analysis to investigate sources and effects of non-stationary behaviour, providing insights into the impacts of long-term drought on streamflow production and catchment storage dynamics. [ABSTRACT FROM AUTHOR]

Summary Future projections of water supply under climate change scenarios are fundamental for efficient water resource planning. However, runoff projections are affected by uncertainties in the modelling process that limit their utility to decision makers. The main source of uncertainty in runoff projections are the Global Climate Models (GCMs) used to produce future climate projections. The impact on projected runoff of this uncertainty has mainly been assessed through comparison of multi-model runs of future climate with little exploration of uncertainties inside the models due to different parameterisations. Here we investigate the uncertainty response of projected runoff due to perturbed physics parameter variations within a GCM using a novel 2500 member ensemble from the HadCM3L model. Our research evaluates the uncertainties in runoff modelling for southwest Western Australia, a Mediterranean climate region which has experienced reductions in precipitation during the last decades. Results for future projections in southwest Western Australian catchments indicate reductions in modelled precipitation between 0% and 40% and increases in temperature that fluctuate between 0.5 °C and 3 °C by 2050–2080 compared to 1970–2000, which lead to reductions in projected runoff of between 10% and 80%. This range of uncertainty for projected runoff is larger than that calculated for previous estimates of within-model uncertainties of runoff. The perturbed physics approach indicates that current water management assessments underestimate uncertainties in runoff projections. [ABSTRACT FROM AUTHOR]