Modelling water and carbon canopy fluxes

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Modelling the water and carbon fluxes from forest canopies provides useful insight into the dynamics of the exchange of water vapour for atmospheric CO₂ and the processes that govern this exchange. The work presented in this thesis aimed to answer four questions related to modelling of canopy gas-exchange. The first two questions involved the development of a simple empirical model of canopy water-use to see whether i) water fluxes from a canopy could be estimated without the need for canopy conductance and ii) could such a model be applied across multiple sites without the need for site-specific calibration? The remaining two questions involved the modification and improvement of a highly mechanistic and complex soil-plant-atmosphere (SPA) continuum model, which was done in order to iii) replicate canopy gas-exchange for a Australian tropical savanna and iv) to improve the simulated leaf gas-exchange process of a SPA model. A simple empirical model of canopy water-use (Ec), a modified Jarvis-Stewart (MJS) model, was developed in order to circumvent the problem of requiring surface conductance as an input in order to calculate transpiration. This was accomplished by modelling an empirical relationship of the multivariate response of Ec to solar radiation (Rs), vapour pressure deficit (Dv) and soil moisture content (θs). The MJS model was shown to provide favourable short- and mid-term (annual) estimates of Ec that only required three more readily available abiotic inputs (Rs, Dv and θs) and a small set of site-calibrated model parameters. Predictions of Ec determined from the MJS model were able to replicate the observed data and compared favourably with the established Penman-Monteith (PM) equation and a statistical benchmark created using an artificial neural network (ANN). In addition to this, the applicability of the MJS model was tested for five disparate Australian woodland sites, where model parameters were calibrated for each individual site and simultaneously for all sites. The result was that while MJS model was able to give a good representation of the measured data using site-specific parameters, using a parameter set that describes an average response of Ec to the environment performed equally well. This was despite each site being comprised of different tree species and occurring over different soil profiles. This showed that the MJS model is partially insensitive to variation in the values of the model parameters and that the number of inputs into the MJS can be further reduced. The conclusion was that this model is broadly applicable for many sites in temperate Australia and one that can be used as a tool in the management of water resources. While the MJS model provided a useful management tool, in order to investigate the dynamics of water and carbon gas-exchange from forest canopies, the more complex SPA model of Williams et al. (1996a) was used. While the SPA model has been applied in ecosystems globally with much success, the lack of C₄ photosynthesis has limited its application to savanna ecosystems. Modification of the SPA model was therefore undertaken in order to improve its applicability to savannas through incorporation of C₄ photosynthesis. This was an important improvement as savannas are dominated by C₄ grasses, which contribute significantly to ecosystem water and carbon fluxes. This modification allowed the SPA model to be parameterised to a savanna site in northern Australia, which was simulated over 5 years to replicate measurements of carbon and water fluxes derived from eddy-covariance. The SPA model allowed C₃ and C₄ water and carbon fluxes to be separated and this showed that the C₄ grasses contribute significantly to total savannah productivity (48%), but a much smaller amount to total water-use (23%). Additionally, it was determined the seasonal variation in leaf area index was driving the seasonality in productivity and water-use and the savanna site was determined to be energy-limited (limited by its light interception). The modification and application of the SPA model to a savanna site highlighted important issues in the way leaf gas-exchange is represented in the model. An investigation into the leaf gas-exchange process handled by SPA showed that there was an imbalance between assimilation and transpiration, as a result of simulated stomatal conductance being increased to unreasonably high levels in order to maximise carbon gain. In order to correct this problem, the modelled gas-exchange was modified to follow the optimality hypothesis of Cowan and Farquhar (1977), such that carbon gain is maximised while water lost from the leaf is simultaneously minimised. This improvement was tested in a purely theoretical exercise, where leaf gas-exchange (default and improved schemes) was simulated over a drought. The result of this simulation was that the improved scheme produced a reduction in canopy water-use, while carbon gain remained high and comparable with that of the default scheme.
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