Abstract
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. Several updates to previous applications of the VOM have been made for the study in the accompanying paper of Nijzink et al. (2022), where we assess whether 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). Therefore, we assess in this technical paper how the updates to the model and input data would have affected the original results of Schymanski et al. (2015), and we implemented these changes one at a time. 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 evapotranspiration (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 in comparison with the results of Schymanski et al. (2015), 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 remained similar, with an overestimation that slightly reduced 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.
Highlights
Novel modelling approaches that are able to explicitly model vegetation dynamics, such as vegetation cover or root surfaces, may lead to an overall improved understanding of carbon and water flux exchanges with the atmosphere
Recent model inter-comparison studies confirm that models with explicit vegetation dynamics are needed, as Whitley et al (2016) showed that prescribing rooting depths and the lack of dynamic representations of leaf area index (LAI) led to highly variable performances for a selection of terrestrial biosphere models (TBMs)
To compare previous simulations using the Vegetation Optimality Model (VOM)-AoB2015 with the VOM-v0.5 set-up that includes the modifications as outlined in Sect. 2.2.9, each modification was applied to the previous set-up in a one-step-at-a-time approach to quantify the influence of each change in isolation
Summary
Novel modelling approaches that are able to explicitly model vegetation dynamics, such as vegetation cover or root surfaces, may lead to an overall improved understanding of carbon and water flux exchanges with the atmosphere. Nijzink et al.: Influence of modifications (from AoB2015 to v0.5) in the Vegetation Optimality Model scenarios This makes the models rely on the quality of the data and does not enhance our understanding of the vegetation dynamics, which is highly important regarding the feedbacks between the land and the atmosphere in a changing climate. The costs and benefits at different sites are determined in a consistent way, leading to vegetation properties that solely depend on physical conditions, such as meteorological forcing, soils and hydrology. As a result, this leads to a systematic and consistent explanation of vegetation behaviour under different external conditions at different sites
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