Abstract
Abstract. Information about the carbon cycle potentially constrains the water cycle, and vice versa. This paper explores the utility of multiple observation sets to constrain a land surface model of Australian terrestrial carbon and water cycles, and the resulting mean carbon pools and fluxes, as well as their temporal and spatial variability. Observations include streamflow from 416 gauged catchments, measurements of evapotranspiration (ET) and net ecosystem production (NEP) from 12 eddy-flux sites, litterfall data, and data on carbon pools. By projecting residuals between observations and corresponding predictions onto uncertainty in model predictions at the continental scale, we find that eddy flux measurements provide a significantly tighter constraint on continental net primary production (NPP) than the other data types. Nonetheless, simultaneous constraint by multiple data types is important for mitigating bias from any single type. Four significant results emerging from the multiply-constrained model are that, for the 1990–2011 period: (i) on the Australian continent, a predominantly semi-arid region, over half the water loss through ET (0.64 ± 0.05) occurs through soil evaporation and bypasses plants entirely; (ii) mean Australian NPP is quantified at 2.2 ± 0.4 (1σ) Pg C yr−1; (iii) annually cyclic ("grassy") vegetation and persistent ("woody") vegetation account for 0.67 ± 0.14 and 0.33 ± 0.14, respectively, of NPP across Australia; (iv) the average interannual variability of Australia's NEP (±0.18 Pg C yr−1, 1σ) is larger than Australia's total anthropogenic greenhouse gas emissions in 2011 (0.149 Pg C equivalent yr–1), and is dominated by variability in desert and savanna regions.
Highlights
We expect evapotranspiration (ET) and long-term streamflgroowss(pprreimciapriytatpiorondu–cTEtihTon)eo(GbCsPerPryv)ao(taisonnpdshhteoenrbceee constraints net primary production (NPP)), and on information about gross primary productivity (GPP) and NPP to provide significant constraints on the partitioning of ET into transpiration and soil evaporation
Coupled carbon and water cycles were simulated using a modified version of the CABLE land surface scheme in the BIOS2 modelling environment, a fine spatial resolution (0.05◦) offline environment built on capability developed for the Australian Water Availability Project (King et al, 2009; Raupach et al, 2009)
CABLE consists of five components (Wang et al, 2011): (1) the radiation module describes radiation transfer and absorption by sunlit and shaded leaves; (2) the canopy micrometeorology module describes the surface roughness length, zero-plane displacement height, and aerodynamic conductance from the reference height to the air within canopy or to the soil surface; (3) the canopy module includes the coupled energy balance, transpiration, stomatal conductance and photosynthesis of sunlit and shaded leaves; (4) the soil module describes heat and water fluxes within soil and snow at their respective surfaces; and (5) the ecosystem carbon module accounts for the respiration of stem, root and soil organic carbon decomposition
Summary
Coupled carbon and water cycles were simulated using a modified version of the CABLE land surface scheme in the BIOS2 modelling environment, a fine spatial resolution (0.05◦) offline environment built on capability developed for the Australian Water Availability Project (King et al, 2009; Raupach et al, 2009). BIOS2 includes: (1) a modification of the CABLE land surface scheme (Kowalczyk et al, 2006; Wang et al, 2011) as described below; (2) infrastructure for the treatment of inputs (gridded vegetation cover, meteorological data and parameters) and outputs for optimum efficiency; (3) a weather generator for downscaling of meteorological data; and (4) model-data fusion capability. Modifications to CABLE, SLI and CASA-CNP for use in BIOS2 are detailed in the Appendices. Additional CASA-CNP modifications, made to improve model performance against observations in this application, included using static allocation coefficients (rather than allocation coefficients dependent upon phenology, temperature, and soil moisture), and holding the ratio of NPP to GPP constant in time, instead of using the default growth respiration/maintenance respiration paradigm which is known to be problematic. CASA-CNP carbon pools were initialised by spinning the model 200 times for the 1970–1989 period using CABLE output for this period. The simulation period was 1990–2011, for which monthly outputs at 0.05◦ (∼ 5 km) spatial resolution were produced
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