No abstract

The response of wheat crops to elevated CO 2 (eCO 2) was measured and modelled with the Australian Grains Free‐Air CO 2 Enrichment experiment, located at Horsham, Australia. Treatments included CO 2 by water, N and temperature. The location represents a semi‐arid environment with a seasonal VPD of around 0.5 kPa. Over 3 years, the observed mean biomass at anthesis and grain yield ranged from 4200 to 10 200 kg ha−1 and 1600 to 3900 kg ha−1, respectively, over various sowing times and irrigation regimes. The mean observed response to daytime eCO 2 (from 365 to 550 μmol mol−1 CO 2) was relatively consistent for biomass at stem elongation and at anthesis and LAI at anthesis and grain yield with 21%, 23%, 21% and 26%, respectively. Seasonal water use was decreased from 320 to 301 mm (P = 0.10) by eCO 2, increasing water use efficiency for biomass and yield, 36% and 31%, respectively. The performance of six models (APSIM‐Wheat, APSIM‐Nwheat, CAT‐Wheat, CROPSYST, OLEARY‐CONNOR and SALUS) in simulating crop responses to eCO 2 was similar and within or close to the experimental error for accumulated biomass, yield and water use response, despite some variations in early growth and LAI. The primary mechanism of biomass accumulation via radiation use efficiency (RUE) or transpiration efficiency (TE) was not critical to define the overall response to eCO 2. However, under irrigation, the effect of late sowing on response to eCO 2 to biomass accumulation at DC65 was substantial in the observed data (~40%), but the simulated response was smaller, ranging from 17% to 28%. Simulated response from all six models under no water or nitrogen stress showed similar response to eCO 2 under irrigation, but the differences compared to the dryland treatment were small. Further experimental work on the interactive effects of eCO 2, water and temperature is required to resolve these model discrepancies.

Heat waves have a significant impact on crop production and quality of many staple grains including wheat. Under climate change, changing weather patterns including greater temperature volatility are also likely to further affect yield stability. Simulation modeling provides a powerful tool to investigate the interactive effects of abiotic factors and develop adaptive strategies; however, many of these models do not adequately account for the step change response to high temperature occurring during the crop reproductive phase. Empirical response data will support the development of robust algorithms for crop models. We present results from three experiments on the response of wheat to a range of acute high-temperature treatments. For 35, 37, and 42°C and 1, 3, and 5 d of exposure (~6 h d -1 ), expressed as heat sum, °C×h (above 32°C), high temperature applied 5 d prior to anthesis reduced grain number and yield by 0.16 and 0.15% per °C×h respectively, whereas individual grain weight and grain nitrogen concentration increased by 0.03 and 0.06% per °C×h, respectively. For high temperature applied after anthesis, individual grain weight decreased by 0.05% per °C×h, grain nitrogen concentration increased by 0.03% per °C×h, and yield was reduced by 0.07% per °C×h. The often asymptotic response meant exponential functions provided a superior fit. Water availability prior to anthesis attenuated wheat response to high temperature. Such algorithms could contribute to improving our prediction of the step change response of wheat to high temperature within crop models and more broadly contribute to developing strategies for crop adaptation to climate change using a modeling approach.

Higher transpiration efficiency (TE) has been proposed as a mechanism to increase crop yields in dry environments where water availability usually limits yield. The application of a coupled radiation and TE simulation model shows wheat yield advantage of a high-TE cultivar (cv. Drysdale) over its almost identical low-TE parent line (Hartog), from about À7 to 558 kg/ha (mean 187 kg/ha) over the rainfed cropping region in Australia (221-1,351 mm annual rainfall), under the present-day climate. The smallest absolute yield response occurred in the more extreme drier and wetter areas of the wheat belt.However, under elevated CO 2 conditions, the response of Drysdale was much greater overall, ranging from 51 to 886 kg/ha (mean 284 kg/ha) with the greatest response in the higher rainfall areas. Changes in simulated TE under elevated CO 2 conditions are seen across Australia with notable increased areas of higher TE under a drier climate in Western Australia, Queensland and parts of New South Wales and Victoria. This improved efficiency is subtly deceptive, with highest yields not necessarily directly correlated with highest TE. Nevertheless, the advantage of Drysdale over Hartog is clear with the benefit of the trait advantage attributed to TE ranging from 102% to 118% (mean 109%). The potential annual cost-benefits of this increased genetic TE trait across the wheat growing areas of Australia (5 year average of area planted to wheat) totaled AUD 631 MIL (5-year average wheat price of AUD/260 t) with an average of 187 kg/ha under the present climate. The benefit to an individual farmer will depend on location but elevated CO 2 raises this nation-wide benefit to AUD 796 MIL in a 2°C warmer climate, slightly lower (AUD 715 MIL) if rainfall is also reduced by 20%.

This paper illustrates the hydrological limitations and underlying assumptions of 4 catchment modelling approaches representing different generic classes of predictive models. These models are commonly used to estimate the impacts of land use and management change on stream flow and salinity regimes within a target region. Three approaches are based on a simple conceptual framework that assumes a single layer groundwater aquifer and requires minimal information and calibration (Zhang-BC2C, CAT1D-BC2C and LUCICAT), whereas the fourth approach (CAT3D) adopts a fully distributed highly parameterised catchment model capable of simulating complex multi-layered groundwater aquifer systems. All models were applied to the Gardiner subcatchment within the Goulburn–Broken region of Victoria, identified as a National Action Plan for Salinity priority subcatchment. Current condition simulation results were compared with observed stream flow and groundwater hydrograph data. Results show that the simple frameworks predicted whole-of-catchment mean annual salt and water yield with minimum parameterisation. The fully distributed framework produced similar catchment-scale responses to the simple approaches, but required more intensive input data and solution times. However, the fully distributed framework provides finer temporal and spatial scale information within the catchment. The more detailed models (such as CAT3D) also have the predictive capacity to assess the within-catchment dynamics at a range of scales and account for landscape position and complex surface/groundwater interactions. This paper concludes that the simple frameworks are useful for judging the whole-of-catchment impacts of broad-scale land use change on catchment water yields and salinity and therefore provide valuable tools for community engagement. However, the within-catchment dynamics are not well represented and particular care must be taken when applying such models in those catchments where the interaction between groundwater and surface features result in saturated areas that are disconnected from streams. Adoption of a distributed groundwater modelling environment similar to that of CAT3D provides higher spatial resolution relative to the lumped broad scale groundwater glow system (GFS) based parameterisation adopted by the BC2C rapid assessment approaches. The developers of the BC2C model acknowledge that such models are currently limited to upland local and intermediate groundwater flow systems. Given that the majority of land salinisation is located in regions dominated by intermediate and regional groundwater systems, this tool is not well suited to adequately model regional processes. In contrast, the CAT3D distributed groundwater models are likely to be applicable across a range of scales and provide the capacity to assess the trade offs between salinity recharge and discharge intervention strategies. We conclude that more complex models (e.g. CAT3D) are needed to identify at the land management scale (paddock/farm) cost effective land use and land management changes within the catchment to improve catchment health.