Modelling current and future carbon and water balances for Australia's forests, woodlands and rangelands

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Donohue, Randall; McVicar, Tim ORCID ID icon; Li, Lingtao




Landscape carbon and water cycles are continuously changing in response to a plethora of dynamic process including climatic conditions, natural disturbances and anthropogenic activities. In order to be able to quantify – and perhaps even predict – such changes across the vast Australian continent, the effects of these processes on the carbon and water cycles need to be modelled. This report describes the modelling of these two cycles across Australia’s forests, woodlands and rangelands under current climatic conditions and under two hypothetical future climatic conditions. Modelling of the carbon cycle was performed using the steady state carbon balance model ‘VAST’ (Barrett, 2011). VAST represents the landscape using ten carbon stores – two above-ground biomass stores, two above-ground litter stores and three stores each of below-ground biomass and soil C. In addition, the fluxes between each store are estimated. The water cycle is modelled using the BCP model (Donohue et al., 2012) that estimates the steady state water fluxes (that is, evaporation and runoff). The ‘current’ climatic conditions were represented using the long-term average conditions for the period 1982-2006. The two future scenarios are for 2100 and are based on the IPCC AR5 Representative Concentration Pathways (RCP) 4.5 and 8.5. Under these two scenarios, global-average 2100 air temperature is predicted to increase by 2.4°C and 4.6°C, respectively, compared to 1980-1999 averages. Specific parameterisations of these future climates for the Australian region were adapted from AR4 parameterisations, which yielded predictions that Australian average temperatures may rise by 11% for RCP 4.5 and by 22% RCP 8.5, respectively. The corresponding values for the predicted change in Australian precipitation are -3.1% and -5.9%, relative to 1980-1990 averages. Across the whole study area, the average carbon store for current conditions is estimated at about 28 kg m-2 and the annual Gross Primary Productivity (GPP) is 0.44 kg m-2 a-1. Approximately one third of stored carbon is held above ground and two-thirds is below-ground, and 64% of all storage is soil carbon. Only around 26% of carbon is stored as above-ground living biomass. Currently, of the average 470 mm a-1 of precipitation received across Australia’s forests, woodlands and rangelands, around 80% is evaporated and 20% becomes runoff. Under the RCP4.5 and RCP8.5 future scenarios, total carbon stores are predicted to fall by around 7 and 12%, and GPP by about 3% and 6%, all respectively. Changes in the living biomass stores (leaves, stems and roots) are in proportion to the change in GPP (which itself changes in direct proportion to that in precipitation) whereas the litter and soil stores are predicted t change due to the combined effect of changes in precipitation and temperature. Evaporation is predicted to fall by between 3% and 6% and runoff by between 7% and 14%.


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Technical Report (Author)


Donohue, Randall; McVicar, Tim; Li, Lingtao. Modelling current and future carbon and water balances for Australia's forests, woodlands and rangelands. CSIRO; 2012.

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