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Hydrology and Earth System Sciences An interactive open-access journal of the European Geosciences Union
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Preprints
https://doi.org/10.5194/hess-2020-339
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/hess-2020-339
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  10 Jul 2020

10 Jul 2020

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This preprint is currently under review for the journal HESS.

Evaluating a land surface model at a water-limited site: implications for land surface contributions to droughts and heatwaves

Mengyuan Mu1, Martin G. De Kauwe1, Anna M. Ukkola2, Andy J. Pitman1, Teresa E. Gimeno3,4, Belinda E. Medlyn5, Dani Or6, Jinyan Yang5, and David S. Ellsworth5 Mengyuan Mu et al.
  • 1ARC Centre of Excellence for Climate Extremes and Climate Change Research Centre, University of New South Wales, Sydney 2052, Australia
  • 2ARC Centre of Excellence for Climate Extremes and Research School of Earth Sciences, Australian National University, Canberra 0200, Australia
  • 3Basque Centre for Climate Change, Leioa 48940, Spain
  • 4IKERBASQUE, Basque Foundation for Science, 48008, Bilbao, Spain
  • 5Hawkesbury Institute for the Environment, Western Sydney University, Sydney 2751, Australia
  • 6Department of Environmental Sciences, ETH Zurich, Zurich 8092, Switzerland

Abstract. Land surface models underpin coupled climate model projections of droughts and heatwaves. However, the lack of simultaneous observations of individual components of evapotranspiration, concurrent with root-zone soil moisture, has limited previous model evaluations. Here, we use a comprehensive set of observations from a water-limited site in southeastern Australia including both evapotranspiration and soil moisture to 4.5 m depth to evaluate the Community Atmosphere-Biosphere Land Exchange (CABLE) land surface model. We demonstrated that alternative process representations within CABLE had the capacity to improve simulated evapotranspiration, but not necessarily soil moisture dynamics – highlighting problems of model evaluations against water fluxes alone. Our best simulation was achieved by resolving a soil evaporation bias; a more realistic initialisation of the groundwater aquifer state; higher vertical soil resolution informed by observed soil properties; and further calibrating soil hydraulic conductivity. Despite these improvements, the role of the empirical soil moisture stress function in simulated water fluxes remained important: using a site calibrated function reduced the median level of water stress by 36 % during drought and 23 % at other times. These changes in CABLE not only improve the seasonal cycle of evapotranspiration, but also affect the latent and sensible heat fluxes during droughts and heatwaves. Alternative parameterisations led to differences of ~ 150 W m−2 in the simulated latent heat flux during a heatwave, implying a strong impact of parameterisations on the capacity for evaporative cooling and feedbacks to the boundary layer (when coupled). Overall, our results highlight the opportunity to advance the capability of land surface models to capture water cycle processes, particularly during meteorological extremes, when sufficient observations of both evapotranspiration fluxes and soil moisture profiles are available.

Mengyuan Mu et al.

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Short summary
Land surface model (LSM) is a critical tool to study land responses to droughts and heatwaves, but lacking comprehensive observations limited past model evaluations. Here, we use a novel dataset at a water-limited site, evaluate a typical LSM with a range of competing model hypotheses widely used in LSMs and identify marked uncertainty due to the differing process assumptions. We show the extensive observations constrain model processes and allow better simulate land responses to these extremes.
Land surface model (LSM) is a critical tool to study land responses to droughts and heatwaves,...
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