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

Mengyuan Mu,David Ellsworth,Martin De Kauwe,Dani Or,Jinyan Yang,Anna Ukkola,Teresa Gimeno,Belinda Medlyn,Andy Pitman

doi:10.5194/egusphere-egu21-4083

Abstract

&lt;p&gt;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 a depth of 4.5 m to evaluate the Community Atmosphere-Biosphere Land Exchange (CABLE) land surface model. We demonstrate 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 influencing the simulated water fluxes remained important: using a site calibrated function reduced the soil water stress on plants 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. The range of parameterisations tested led to differences of ~150 W m&lt;sup&gt;-2&lt;/sup&gt; 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.&lt;/p&gt;

Highlights

Droughts and heatwaves can have severe and long-lasting impacts on terrestrial ecosystems (Allen et al, 2015; Reichstein et al, 2013) and humans (Matthews et al, 2017; Pal and Eltahir, 2016)
It is worth noting that during the summer of 2014 there was an outbreak of psyllids leading to canopy defoliation (Gherlenda et al, 2016), which may explain part of the model–data mismatch (CABLE only accounts for canopy defoliation via a decline in leaf area index (LAI) but not other damage, e.g. to the phloem)
Land surface schemes used in climate models range in complexity, and different approaches translate into contrasting predictions of the exchange of carbon, energy and water (Fisher and Koven, 2020)

Summary

Introduction

Droughts and heatwaves can have severe and long-lasting impacts on terrestrial ecosystems (Allen et al, 2015; Reichstein et al, 2013) and humans (Matthews et al, 2017; Pal and Eltahir, 2016). Global climate models are commonly used to project how anthropogenic climate change will affect the magnitude, frequency and intensity of droughts and heatwaves. Heatwaves are projected to increase in the future in response to climate change (Dosio et al, 2018; Zhao and Dai, 2017). The future of droughts is less clear: projections of an increase in future droughts are common in the literature (Ault, 2020), yet regional precipitation projections re-. M. Mu et al.: Evaluating a land surface model at a water-limited site main uncertain (Collins et al, 2013) and land surface processes relevant to drought are poorly represented in climate models (Ukkola et al, 2018a)

Methods

Results

Conclusion