IMPROVING WHEAT SIMULATION CAPABILITIES IN AUSTRALIA FROM A CROPPING SYSTEMS PERSPECTIVE - II - TESTING SIMULATION CAPABILITIES OF WHEAT GROWTH

To simulate cropping systems, crop models must not only give reliable predictions of yield across a wide range of environmental conditions, they must also quantify water and nutrient use well, so that the statu s of the soil at maturity is a good representation of the starting con ditions for the next cropping sequence. To assess the suitability for this task a range of crop models, currently used in Australia, were te sted. The models differed in their design objectives, complexity and s tructure and were (i) tested on diverse, independent data sets from a wide range of environments and (ii) model components were further eval uated with one detailed data set from a semi-arid environment. All mod els were coded into the cropping systems shell APSIM, which provides a common soil water and nitrogen balance. Crop development was input, t hus differences between simulations were caused entirely by difference in simulating crop growth. Under nitrogen non-limiting conditions bet ween 73 and 85% of the observed kernel yield variation across environm ents was explained by the models. This ranged from 51 to 77% under var ying nitrogen supply. Water and nitrogen effects on leaf area index we re predicted poorly by all models resulting in erroneous predictions o f dry matter accumulation and water use. When measured light intercept ion was used as input, most models improved in their prediction of dry matter and yield. This test highlighted a range of compensating error s in all modelling approaches. Time course and final amount of water e xtraction was simulated well by two models, while others left up to 25 % of potentially available soil water in the profile. Kernel nitrogen percentage was predicted poorly by all models due to its sensitivity t o small dry matter changes. Yield and dry matter could be estimated ad equately for a range of environmental conditions using the general con cepts of radiation use efficiency and transpiration efficiency. Howeve r, leaf area and kernel nitrogen dynamics need to be improved to achie ve better estimates of water and nitrogen use if such models are to be use to evaluate cropping systems. (C) 1998 Elsevier Science B.V.