Preprints
https://doi.org/10.5194/gmd-2021-151
https://doi.org/10.5194/gmd-2021-151

Submitted as: development and technical paper 18 Jun 2021

Submitted as: development and technical paper | 18 Jun 2021

Review status: this preprint is currently under review for the journal GMD.

Step-wise modifications of the Vegetation Optimality Model

Remko Christiaan Nijzink1, Jason Beringer2, Lindsay Beaumont Hutley3, and Stanislaus Josef Schymanski1 Remko Christiaan Nijzink et al.
  • 1Luxembourg Institute of Science and Technology, Environmental Research and Innovation, Catchment and Eco-hydrology Research Group, Belvaux, Luxembourg
  • 2School of Agriculture and Environment, The University of Western Australia, Crawley, WA, Australia, 6909
  • 3Research Institute for the Environment and Livelihoods, Charles Darwin University, Darwin, NT, Australia, 0909

Abstract. The Vegetation Optimality Model (VOM, Schymanski et al., 2009, 2015) is an optimality-based, coupled water-vegetation model that predicts vegetation properties and behaviour based on optimality theory, rather than calibrating vegetation properties or prescribing them based on observations, as most conventional models do. In order to determine wheter optimality theory can alleviate common shortcomings of conventional models, as identified in a previous model inter-comparison study along the North Australian Tropical Transect (NATT) (Whitley et al., 2016), a range of updates to previous applications of the VOM have been made for increased generality and improved comparability with conventional models. To assess in how far the updates to the model and input data would have affected the original results, we implemented them one-by-one while reproducing the analysis of Schymanski et al. (2015).

The model updates included extended input data, the use of variable atmospheric CO2-levels, modified soil properties, implementation of free drainage conditions, and the addition of grass rooting depths to the optimized vegetation properties. A systematic assessment of these changes was carried out by adding each individual modification to the original version of the VOM at the flux tower site of Howard Springs, Australia.

The analysis revealed that the implemented changes affected the simulation of mean annual evapo-transpiration (ET) and gross primary productivity (GPP) by no more than 20 %, with the largest effects caused by the newly imposed free drainage conditions and modified soil texture. Free drainage conditions led to an underestimation of ET and GPP, whereas more fine-grained soil textures increased the water storage in the soil and resulted in increased GPP. Although part of the effect of free drainage was compensated for by the updated soil texture, when combining all changes, the resulting effect on the simulated fluxes was still dominated by the effect of implementing free drainage conditions. Eventually, the relative error for the mean annual ET, in comparison with flux tower observations, changed from an 8.4 % overestimation to an 10.2 % underestimation, whereas the relative errors for the mean annual GPP stayed similar with a change from 17.8 % to 14.7 %. The sensitivity to free drainage conditions suggests that a realistic representation of groundwater dynamics is very important for predicting ET and GPP at a tropical open-forest savanna site as investigated here. The modest changes in model outputs highlighted the robustness of the optimization approach that is central to the VOM architecture.

Remko Christiaan Nijzink et al.

Status: open (until 13 Aug 2021)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • CEC1: 'Comment on gmd-2021-151', Astrid Kerkweg, 09 Jul 2021 reply
    • AC1: 'Reply on CEC1', Remko C. Nijzink, 12 Jul 2021 reply
  • AC2: 'Comment on gmd-2021-151', Remko C. Nijzink, 12 Jul 2021 reply
  • RC1: 'Comment on gmd-2021-151', Anonymous Referee #1, 17 Jul 2021 reply
    • AC3: 'Reply on RC1', Remko C. Nijzink, 27 Jul 2021 reply

Remko Christiaan Nijzink et al.

Model code and software

VOM Stanislaus Josef Schymanski http://doi.org/10.5281/zenodo.3630081

VOMcases Remko Christiaan Nijzink, Stanislaus Josef Schymanski https://renkulab.io/gitlab/remko.nijzink/vomcases

Remko Christiaan Nijzink et al.

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Short summary
The Vegetation Optimality Model (VOM) is a coupled water-vegetation model that predicts vegetation properties, rather than determining them based on observations. A range of updates to previous applications of the VOM have been made for increased generality and improved comparability with conventional models. This showed that there is a large effect in the simulated water and carbon fluxes caused by the assumption of deep groundwater tables and updated soil profiles in the model.