Identifying threshold responses of Australian dryland rivers to future hydroclimatic change

Z T Larkin,S Tooth,A J R Carthey,T J Ralph,K A Fryirs

doi:10.1038/s41598-020-63622-3

Abstract

Rivers provide crucial ecosystem services in water-stressed drylands. Australian dryland rivers are geomorphologically diverse, ranging from through-going, single channels to discontinuous, multi-channelled 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 km2 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 bedrock-alluvial) 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.

Highlights

Rivers provide crucial ecosystem services in water-stressed drylands
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
Catchment aridity, and key hydrological variables have been established in previous research, including how the magnitude and variability of runoff and streamflow in eastern Australian rivers is strongly modulated by climatic modes (e.g. El Niño-Southern Oscillation (ENSO), Interdecadal Pacific Oscillation (IPO) and the Southern Annular Mode (SAM))[25,26,27,28,29]. While these inter- and multi-decadal hydroclimatic relationships are relatively well understood, the distinct trends and clusters that emerge from our analysis demonstrate, for the first time, the significant overarching control that dominant hydroclimate exerts on the www.nature.com/scientificreports

Summary

Introduction

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. Analyses of future changes have tended to focus solely on dryland hydrology, such as surface water availability or river flow regimes [e.g.5,13–15], rather than on the implications of these changes for river response and physical structure (e.g. number of channels, sinuosity, lateral stability, landform assemblages) This is a critical knowledge gap, as dryland river geomorphology provides the physical template atop which complex ecosystems and anthropogenic land uses operate and intersect[16], thereby defining the range and quality of ecosystem service delivery. Previous research has suggested that regional continua of dryland river types may be related to hydroclimatic gradients, with greater aridity leading to a greater propensity for channel breakdown[21,22] To refine these concepts and identify thresholds of change relevant for Australian dryland rivers, we characterise the hydrology and geomorphology of 29 rivers draining >1,800,000 km[2] of continental eastern and central Australia, and establish the hydroclimatic conditions under which different river types persist. Dominant river pattern meandering or anabranching/ anastomosing meandering/straight, anabranching/ anastomosing, or distributary straight, anabranching/ anastomosing, or distributary, with trunk channel terminating in a floodout (unchannelled plain)

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Conclusion