IMPROVING WHEAT SIMULATION CAPABILITIES IN AUSTRALIA FROM A CROPPING SYSTEMS PERSPECTIVE - III - THE INTEGRATED WHEAT MODEL (I-WHEAT)

Previous work has identified several short-comings in the ability of f our spring wheat and one barley model to simulate crop processes and r esource utilization. This can have important implications when such mo dels are used within systems models where final soil water and nitroge n conditions of one crop define the starting conditions of the followi ng crop. In an attempt to overcome these limitations and to reconcile a range of modelling approaches, existing model components that worked demonstrably well were combined with new components for aspects where existing capabilities were inadequate. This resulted in the Integrate d Wheat Model (I_WHEAT), which was developed as a module of the croppi ng systems model APSIM. To increase predictive capability of the model , process detail was reduced, where possible, by replacing groups of p rocesses with conservative, biologically meaningful parameters. I_WHEA T does not contain a soil water or soil nitrogen balance. These are pr esent as other modules of APSIM. In I_WHEAT, yield is simulated using a linear increase in harvest index whereby nitrogen or water limitatio ns can lead to early termination of grainfilling and hence cessation o f harvest index increase. Dry matter increase is calculated either fro m the amount of intercepted radiation and radiation conversion efficie ncy or from the amount of water transpired and transpiration efficienc y, depending on the most limiting resource. Leaf area and tiller forma tion are calculated from thermal time and a cultivar specific phylloch ron interval. Nitrogen limitation first reduces leaf area and then aff ects radiation conversion efficiency as it becomes more severe. Water or nitrogen limitations result in reduced leaf expansion, accelerated leaf senescence or tiller death. This reduces the radiation load on th e crop canopy (i.e. demand for water) and can make nitrogen available for translocation to other organs. Sensitive feedbacks between light i nterception and dry matter accumulation are avoided by having environm ental effects acting directly on leaf area development, rather than vi a biomass production. This makes the model more stable across environm ents without losing the interactions between the different external in fluences. When comparing model output with models tested previously us ing data from a wide range of agro-climatic conditions, yield and biom ass predictions were equal to the best of those models, but improvemen ts could be demonstrated for simulating leaf area dynamics in response to water and nitrogen supply, kernel nitrogen content, and total wate r and nitrogen use. I_WHEAT does not require calibration for any of th e environments tested. Further model improvement should concentrate on improving phenology simulations, a more thorough derivation of coeffi cients to describe leaf area development and a better quantification o f some processes related to nitrogen dynamics. (C) 1998 Elsevier Scien ce B.V.