Effects of tillage and traffic on crop production in dryland farming systems: I. Evaluation of PERFECT soil-crop simulation model

Agricultural production systems are complex involving variability in climate, soil, crop, tillage management and interactions between these components. The traditional experimental approach has played an important role in studying crop production systems, but isolation of these factors in experimental studies is difficult and time consuming. Computer simulation models are useful in exploring these interactions and provide a valuable tool to test and further our understanding of the behavior of soil-crop systems without repeating experimentation. Productivity erosion and runoff functions to evaluate conservation techniques (PERFECT) is one of the soil-crop models that integrate the dynamics of soil, tillage and crop processes at a daily resolution. This study had two major objectives. The first was to calibrate the use of the PERFECT soil-crop simulation model to simulate soil and crop responses to changes of traffic and tillage management. The second was to explore the interactions between traffic, tillage, soil and crop, and provide insight to the long-term effects of improved soil management and crop rotation options. This contribution covers only the first objective, and the second will be covered in a subsequent contribution. Data were obtained from field experiments on a vertisol in Southeast Queensland, Australia which had controlled traffic and tillage treatments for the previous 5 years. Input data for the simulation model included daily weather, runoff, plant available water capacity, and soil hydraulic properties, cropping systems, and traffic and tillage management. After model calibration, predicted and measured total runoffs for the 5-year period were similar. Values of root mean square error (RMSE) for daily runoff ranged from 5.7 to 9.2mm, which were similar to those reported in literature. The model explained 75-95% of variations of daily, monthly and annual runoff, 70-84% of the variation in total available soil water, and 85% of the variation in yield. The results showed that the PERFECT daily soil-crop simulation model could be used to generate meaningful predictions of the interactions between crop, soil and water under different tillage and traffic systems. Ranking of management systems in order of decreasing merit for runoff, available soil water and crop yield was (1) controlled traffic zero tillage, (2) controlled traffic stubble mulch, (3) wheeled zero tillage, and (4) wheeled stubble mulch.