On the Treatment of Soil Water Stress in GCM Simulations of Vegetation Physiology

P L Vidale,M Todt,B Balan-Sarojini,A Verhoef,G Egea,P C Mcguire,O Müller,W Peters

doi:10.3389/fenvs.2021.689301

Abstract

Current land surface schemes in weather and climate models make use of the so-called coupled photosynthesis–stomatal conductance (A–gs) models of plant function to determine the surface fluxes that govern the terrestrial energy, water and carbon budgets. Plant physiology is controlled by many environmental factors, and a number of complex feedbacks are involved, but soil moisture control on root water uptake is primary, particularly in sub-tropical to temperate ecosystems. Land surface models represent plant water stress in different ways, but most implement a water stress factor, β, which ranges linearly (more recently also curvilinearly) between β = 1 for unstressed vegetation and β = 0 at the wilting point, expressed in terms of volumetric water content (θ). β is most commonly used to either limit A or gs, and hence carbon and water fluxes, and a pertinent research question is whether these treatments are in fact interchangeable. Following Egea et al. (Agricultural and Forest Meteorology, 2011, 151 (10), 1,370–1,384) and Verhoef et al. (Agricultural and Forest Meteorology, 2014, 191, 22–32), we have implemented new β treatments, reflecting higher levels of biophysical complexity in a state-of-the-art LSM, Joint UK Land Environment Simulator, by allowing root zone soil moisture to limit plant function non-linearly and via individual routes (carbon assimilation, stomatal conductance, or mesophyll conductance) as well as any (non-linear) combinations thereof. The treatment of β does matter to the prediction of water and carbon fluxes: this study demonstrates that it represents a key structural uncertainty in contemporary LSMs, in terms of predictions of gross primary productivity, energy fluxes and soil moisture evolution, both in terms of climate means and response to a number of European droughts, including the 2003 heat wave. Treatments allowing ß to act on vegetation fluxes via stomatal and mesophyll routes are able to simulate the spatiotemporal variability in water use efficiency with higher fidelity during the growing season; they also support a broader range of ecosystem responses, e.g., those observed in regions that are radiation limited or water limited. We conclude that current practice in weather and climate modelling is inconsistent, as well as too simplistic, failing to credibly simulate vegetation response to soil water stress across the typical range of variability that is encountered for current European weather and climate conditions, including extremes of land surface temperature and soil moisture drought. A generalized approach performs better in current climate conditions and promises to be, based on responses to recently observed extremes, more trustworthy for predicting the impacts of climate change.

Highlights

Water availability exerts a major control on vegetation gross primary productivity (GPP), as well as on the land surface energy balance
In terms of presentation, in order to reduce the number of plots and subplots, CTL is only shown in terms of long-term soil moisture memory, as QB is virtually identical to CTL in terms of landatmosphere fluxes
First the climatological biases of some variables of interest will be shown, the 2003 heat wave response will be used as a surrogate for future climate conditions, as suggested for instance in Schär et al (2004), where it was shown how the 2003 event represents an outlier for current climate, but is, instead, compatible with a temperature probability distribution function (PDF) extracted from climate projections forced by a strong climate change scenario

Summary

Introduction

Water availability exerts a major control on vegetation gross primary productivity (GPP), as well as on the land surface energy balance. Land-atmosphere feedbacks involving anomalously high near-surface vapor pressure deficit and leaf boundary temperature (see e.g., Ball et al, 1987), further exacerbate concurrent soil drought and atmospheric aridity (Zhou et al, 2019) Such conditions are more likely when meteorological drought occurs, often as a result of stagnant atmospheric conditions (e.g., summertime blocking). Such events see a reduction in carbon uptake, with possible consequences for plant growth and below-ground carbon allocation, and increase the Bowen ratio, raise surface temperature and can lead to further dessication of the soils, at a Clausius-Clapeyron rate (see for instance the review in Seneviratne et al, 2010 and Vargas Zeppetello et al, 2019)

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