Watersheds may not recover from drought

The Millennium Drought (southeastern Australia) provided a natural experiment to challenge the assumption that watershed streamflow always recovers from drought. Seven years after the drought, the runoff (as a fraction of precipitation) had not recovered in 37% of watersheds, and the number of recovered watersheds was not increasing. When recovery did occur, it was not explained by watershed wetness. For those watersheds not recovered, ~80% showed no evidence of recovering soon, suggesting persistence within a low-runoff state. The post-drought precipitation not going to runoff was found to be likely going to increased evapotranspiration per unit of precipitation. These findings show that watersheds can have a finite resilience to disturbances and suggest that hydrological droughts can persist indefinitely after meteorological droughts.

Unravelling socio-hydrological processes behind cascading drought-to-flood disasters

<p>Future climate projections show a strengthening of the hydrological cycle with more droughts and floods expected in many regions of the world. This means a higher likelihood of cascading drought-to-flood disasters such as the Millennium Drought – Brisbane flooding in Australia or the California drought – Oroville spillway collapse in the US. Droughts allow ample time for impacts and adaptation, which influence hazard, exposure, and vulnerability of a subsequent flood. When we treat the flood risk as independent from the drought this might lead to large underestimations of future risk.</p><p>Here, we present the PerfectSTORM project (‘STOrylines of futuRe extreMes’). In this project we will study drought-to-flood events to provide the understanding needed to prevent major disasters in the future. We will use a mixed-methods approach based on a combination of qualitative and quantitative storylines of past and future drought-to-flood risk in case studies and extrapolation of this rich case study information to the global scale. Qualitative storylines will be collected with narrative interviews and mental simulation workshops and will be analysed to develop timelines and causal loop diagrams. Quantitative storylines will be developed from timeseries of hydrological and social data that will be analysed to distinguish interrelated drivers and modelled with system dynamics modelling. These storylines will then be combined in an iterative way using innovative data visualisation as a basis for co-creating management solutions.</p><p>To generalise our case study understanding, a range of global datasets will be analysed to find global types and hotspots of drought-to-flood events. This information will be combined with the system dynamics model developed in the case studies and a global multi-dimensional possibility space will be developed. This will allow us to explore positive pathways for future management of drought-to-flood events in different parts of the world. The PerfectSTORM project will provide in-depth understanding of the hydrosocial feedbacks and dynamic vulnerability of cascading hazards.</p>

Conceptual Model Modification and the Millennium Drought of Southeastern Australia

Long-term droughts observed in southern Australia have changed relationships between annual rainfall and runoff and tested some of the assumptions implicit in rainfall–runoff models used in these areas. Predictive confidence across these periods is when low using the more commonly used rainfall–runoff models. Here we modified the GR4J model to better represent surface water–groundwater connection and its role in runoff generation. The modified model (GR7J) was tested in 137 catchments in south-east Australia. Models were calibrated during “wetter” periods and simulation across drought periods was assessed against observations. GR7J performed better than GR4J in evaluation during drought periods where bias was significantly lower and showed improved fit across the flow duration curve especially at low flows. The largest improvements in predictive performance were for catchments where there were larger changes in the annual rainfall–runoff relationship. The predictive performance of the GR7J model was more sensitive to objective function used than GR4J. The use of an objective function that combined daily and annual error produced a better goodness of fit when measured against 80, 50 and 20 percent excedance flow quantiles and reduced evaluation bias, especially for the GR7J model.

The Impact of Anthropogenic Forcing and Natural Processes on Past, Present, and Future Rainfall over Victoria, Australia

Cool-season (April to October) rainfall dominates the annual average rainfall over Victoria, Australia, and is important for agriculture and replenishing reservoirs. Rainfall during the cool season has been unusually low since the beginning of the Millennium Drought in 1997 (~12% below the twentieth-century average). In this study, 24 CMIP5 climate models are used to estimate 1) the extent to which this drying is driven by external forcing and 2) future rainfall, taking both external forcing and internal natural climate variability into account. All models have preindustrial, historical, and twenty-first-century (RCP2.6, RCP4.5, and RCP8.5) simulations. It is found that rainfall in the past two decades is below the preindustrial average in two-thirds or more of model simulations. However, the magnitude of the multimodel median externally forced drying is equivalent to only 20% of the observed drying (interquartile range of 40% to −4%), suggesting that the drying is dominated by internally generated rainfall variability. Underestimation of internal variability of rainfall by the models, however, increases the uncertainties in these estimates. According to models the anthropogenically forced drying becomes dominant from 2010 to 2029, when drying is evident in over 90% of the model simulations. For the 2018–37 period, it is found that there is only a ~12% chance that internal rainfall variability could completely offset the anthropogenically forced drying. By the late twenty-first century, the anthropogenically forced drying under RCP8.5 is so large that internal variability appears too small to be able to offset it. Confidence in the projections is lowered because models have difficulty in simulating the magnitude of the observed decline in rainfall.

Characterising transition towards more ephemeral streams in Australian catchments

<p>Many streams experienced a prominent increase in proportion of cease to flow conditions during and after the multiyear drought in Australia (Millennium drought, circa 1997 – 2009). Change in zero flow occurrence frequency reflects the general transition of stream reaches from gaining to losing conditions, from losing to losing more, and ultimately to the disconnected state. We track and characterise these changes in groundwater-surface water connection using zero flow conditions as a proxy and explore the spatial and temporal patterns in flow regime transformation. The implications for upstream / downstream water availability and management of environmental flows and ecosystems are discussed in view of projected drier future climate.</p>

To what degree does land-atmosphere feedback exacerbate drought in South East Australia?

<p>South East Australia is characterised by a diverse climate ranging from lush, temperate mountain ranges to hot and arid grasslands. The region is home to Australia's largest river system, the Murray-Darling. The Murray-Darling Basin is an important agricultural region, generating almost 50% of Australia's total irrigated agricultural production in 2018. Rainfall in this region is typically highly variable and subject to severe drought. The Millennium Drought (2001-2009), widely known as the worst drought on record and one of the most severe in the world, has now been superseded by a worse drought (2017-present), setting a new extreme in the drought record. During the current drought, rainfall, root zone soil moisture and water storages have reached record-breaking low levels. High temperatures have also broken historical records on multiple occasions since the drought began. Drought conditions and exceptionally high temperatures have dried the landscape, which has led to intense bushfires that have so far ravaged over 5 million hectares.</p><p>Yet the degree to which the land surface exacerbates drought in the Murray-Darling Basin remains unknown. In other words, the relative importance of local versus remote processes affecting rainfall, particularly during drought, is uncertain. Where does the moisture come from, and how strongly do local land surface processes attenuate or amplify this atmospheric moisture to affect local rainfall? Establishing the evaporative source regions that supply moisture for rainfall can help reveal the mechanisms driving anomalously low rainfall. In the case of drought, it can help reveal whether anomalous rainfall was due to a reduction in source evaporation, anomalous atmospheric circulation (i.e., the moisture was generated but transported somewhere else), land surface control on the atmosphere through feedbacks, or a combination of factors.</p><p>We used a Lagrangian back-trajectory approach to determine the long-term average evaporative source regions that supply Australia's rainfall, and the level of recycling that rainfall undergoes. The back-trajectory model tracked water vapour from the location of rainfall events backward in time and space and identified the evaporative origin. From this, we calculated the proportion of rainfall falling across the Murray-Darling Basin that originated as evapotranspiration from the Basin itself; that is, the rainfall recycling ratio.</p><p>By combining this long-term baseline of source region and rainfall recycling with anomalies of source region evaporation and local atmospheric boundary layer properties, we found that the drivers of low rainfall changed through time during the Millennium Drought. At the peak of the Drought the anomalously low rainfall was driven by a lack of atmospheric moisture advected from the identified typical source region; at other times the low rainfall was due to local conditions unfavorable for the precipitation of available moisture. Overall we found that land surface control on the atmosphere exacerbated the Millennium Drought by approximately 10%.</p>

Relative Contribution of Anthropogenic Forcing and Natural Processes to Rainfall Variability over Victoria, Australia

<p>Victoria is the second-most populated and most densely populated state in Australia with a population of over 6.5 million. Over two thirds of the population live in greater Melbourne. It is also a major area for agriculture and tourism and is the second largest economy in Australia, accounting for a quarter of Australia's Gross Domestic Product. Any changes in Victoria's climate has huge impacts in these sectors. Rainfall over Victoria during the cool season (e.g. April to October) has been unusually low since the beginning of the Millennium Drought in 1997 (~12% below the 20<sup>th</sup> century average). Cool season rainfall contributes two-third to annual rainfall and is very important for many crops and for replenishing reservoirs across the state. Here we examine the extent to which this reduction in cool season rainfall is driven by external forcing, and the prospects for future multi-decadal rainfall, taking both external forcing and internal natural climate variability into account.</p><p>We analyse simulations from 40 global climate models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) under preindustrial and historical forcing, as well as three scenarios for the 21<sup>st</sup> century: Representative Concentration Pathway (RCP)2.6, RCP4.5 and RCP8.5, which vary markedly in the amount of greenhouse gas emitted over the coming century. While the 1997-2018 average rainfall for cool season is below the preindustrial average in more than two-thirds of models under the three scenarios, the magnitude of the externally-forced drying is very small (median decline is around -2.5% in all three scenarios with an interquartile range around -5% to +1%). The model ensemble results suggest that external forcing contributed only 20% (interquartile range -41% to 4%) to the drying observed in 1997-2018, relative to 1900-1959. These results suggest that the observed drying was dominated by natural, internal rainfall variability. While the multi-model median is below average from 1997-2018 onwards, the externally-forced drying only becomes clear from 2010-2029, when the proportion of models exhibiting drying increases to over 90% under all three scenarios. This agreement reflects the increase in the magnitude of the externally-forced drying. We estimate that there is a 12% chance that internal rainfall variability will completely offset the externally-forced drying averaged over 2018-2037, regardless of scenario. By the late 21<sup>st</sup> century the externally forced change under RCP8.5 is so large that drying – even after taking internally variability into account - appears inevitable. </p><p>Confidence in the modelled projections is lowered because models have difficulty in simulating the magnitude of the observed decline in rainfall. Some of this difficulty appears to arise because most models seem to underestimate multidecadal rainfall variability. Other candidates are: the observed drying may have been primarily due to the occurrence of an extreme, internally-driven event; the models underestimate the magnitude of the externally-forced drying in recent decades; or some combination of the two. If externally-forced drying is underestimated because the response to greenhouse gases is underestimated then the magnitude of projected changes might also be underestimated.</p>

Monitoring climate-driven and anthropogenic impacts on hydrology and agriculture in South-Eastern Australia in the 21st century

<p>With the climate change, drought events likely become more frequent and severe in Australia, where the worst droughts were recorded during the 21st century. Particularly, in the South-East of the country, the so called "Millennium Drought" showed below average annual precipitation for an entire decade. The precipitation record was then increased by extreme precipitation events generated from the La Niña events in 2010 and 2011. Afterwards, dry conditions began again to develop. The climate-driven events and anthropogenic adaptions to the circumstances resulted in strong impacts on the hydrological resources and agricultural production. In fact, simulating hydrological processes within the (semi-)arid region of South-East Australia is very challenging especially during extreme events. In previous studies, we found a strong underestimation of the decline of total terrestrial water storage (TWS) and of groundwater in comparison to remote sensing data and in-situ station networks. Thus, we successfully calibrated the W3RA water balance model and simultaneously assimilated TWS anomalies obtained from the Gravity Recovery And Climate Experiment (GRACE) satellite mission to improve the model's skill during extreme meteorological conditions. In this presentation, we focus on the comparison of remote sensing observations and W3RA simulations after implementing the calibration and data assimilation with existing data records on anthropogenic intervention into the water cycle, as well as on agricultural production. Our results indicate high correlations between meteorological, hydrological and agricultural variables, and we observe strong similarities in the long-term trends and break points.</p>