Identifying threshold responses of Australian dryland rivers to future hydroclimatic change

Abstract

Rivers provide crucial ecosystem services in water-stressed drylands. Australian dryland rivers are geomorphologically diverse, ranging from through-going, single channels to discontinuous, multichannelled systems, yet we have limited understanding of their sensitivity to future hydroclimatic changes. Here, we characterise for the first time the geomorphology of 29 dryland rivers with catchments across a humid to arid gradient covering >1,800,000 km 2 of continental eastern and central Australia. Statistical separation of five specific dominantly alluvial river types and quantification of their present-day catchment hydroclimates enables identification of potential thresholds of change. Projected aridity increases across eastern Australia by 2070 (RCP4.5) will result in ~80% of the dryland rivers crossing a threshold from one type to another, manifesting in major geomorphological changes. Dramatic cases will see currently through-going rivers (e.g. Murrumbidgee, Macintyre) experience step changes towards greater discontinuity, characterised by pronounced downstream declines in channel size and local termination. Expanding our approach to include other river styles (e.g. mixed bedrockalluvial) would allow similar analyses of dryland rivers globally where hydroclimate is an important driver of change. Early identification of dryland river responses to future hydroclimatic change has far-reaching implications for the ~2 billion people that live in drylands and rely on riverine ecosystem services. Rivers are lifelines in climatically variable and water-stressed drylands, the dry subhumid through hyperarid environments that cover 40-50% of the Earth's land surface and host ~28% of the world's population 1,2. Dryland rivers are fundamentally important for human populations, providing a plethora of provisioning, regulating, supporting and cultural ecosystem services 1,3. Yet dryland rivers exist in marginal environments and are threatened by declines in water availability due to the impacts of climate change (e.g. decreased rainfall, increased temperature and evapotranspiration, and greater climatic variability) and other human activities (e.g. river regulation, flow diversion and abstraction, and land use change) 4-6. Rivers are not static conduits of water, sediment and nutrients, but adjust dynamically to a suite of internal and external drivers. Among various external drivers (e.g. tectonic activity, sea level fluctuations, climate), research has shown that late Quaternary hydroclimatic changes have driven substantial geomorphological changes to many dryland rivers globally, including during the mid to late Holocene [e.g. 7-10 ]. Indeed, in tectonically stable settings such as continental Australia, and in reaches where rivers are free from significant bedrock influence, hydroclimatic changes are the principal driver of river response and resulting channel-floodplain geomorphology. To date, however, assessment of the potential likelihood and pathways of hydrological and geomorphological changes in dryland rivers due...

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