The co-occurrence of droughts and heatwaves can have significant impacts on many socioeconomic and environmental systems. Groundwater has the potential to moderate the impact of droughts and heatwaves by moistening the soil and enabling vegetation to maintain higher evaporation, thereby cooling the canopy. We use the Community Atmosphere Biosphere Land Exchange (CABLE) land surface model, coupled to a groundwater scheme, to examine how groundwater influences ecosystems under conditions of co-occurring droughts and heatwaves. We focus specifically on south-east Australia for the period 2000-2019, when two significant droughts and multiple extreme heatwave events occurred. We found groundwater plays an important role in helping vegetation maintain transpiration, particularly in the first 1-2 years of a multi-year drought. Groundwater impedes gravity-driven drainage and moistens the root zone via capillary rise. These mechanisms reduced forest canopy temperatures by up to 5 °C during individual heatwaves, particularly where the water table depth is shallow. The role of groundwater diminishes as the drought lengthens beyond 2 years and soil water reserves are depleted. Further, the lack of deep roots or stomatal closure caused by high vapour pressure deficit or high temperatures can reduce the additional transpiration induced by groundwater. The capacity of groundwater to moderate both water and heat stress on ecosystems during simultaneous droughts and heatwaves is not represented in most global climate models, suggesting that model projections may overestimate the risk of these events in the future.
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Acknowledgements. Mengyuan Mu, Martin G. De Kauwe, Andy J. Pitman, Anna M. Ukkola, and Sanaa Hobeichi acknowledge support from the Australian Research Council (ARC) Centre of Excellence for Climate Extremes (CE170100023). Mengyuan Mu acknowledges support from the UNSW University International Postgraduate Award (UIPA) scheme. Martin G. De Kauwe and Andy J. Pitman acknowledge support from the ARC Discovery Grant (DP190101823) and Anna M. Ukkola support from the ARC Discovery Early Career Researcher Award (DE200100086). Martin G. De Kauwe acknowledges support from the NSW Research Attraction and Acceleration Program (RAAP). We thank the National Computational Infrastructure at the Australian National University, an initiative of the Australian Government, for access to supercomputer resources. Mengyuan Mu thanks the University of Nanjing for hosting her research through 2020.
© 2021 Mengyuan Mu et al.