Simulating rice in farming systems – modelling transitions between aerobic and ponded soil environments in APSIM

Abstract: Adapting farming systems to reduced availability of irrigation water is an emerging research issue in irrigated districts throughout the globe. Water shortages in parts of the rice-growing world have prompted research into a range of alternate agricultural practices, including expansion of rice as a component in diverse farming systems, in rotation with dryland and aerobically irrigated crops and pastures, utilising a range of modified tillage and residue management practices. Evaluation of potential future adaptation strategies can be assisted by well-tested farming systems models that capture interactions between soil water and nutrient dynamics, crop growth, climate and management inputs/practices. APSIM represents such a model, however due to its ‘dryland heritage ’ APSIM has previously been unequipped to describe the soil water, carbon and nitrogen dynamics as soil environments progress from aerobic to anaerobic and back again, such as occurs in crop rotations involving ponded rice and other non-ponded crops (wheat, maize, legumes, pastures etc.). Various relevant chemical and biological processes that occur in long-term ponded water were also unaccounted for in APSIM. In this paper we describe how we incorporated this new functionality into APSIM. This includes evaluation of previous efforts to model anaerobic soil environments; conceptualization of the required new functionality within the existing APSIM framework; and design of code structure to facilitate seamless transitions betwee