APSIMS WATER AND NITROGEN MODULES AND SIMULATION OF THE DYNAMICS OF WATER AND NITROGEN IN FALLOW SYSTEMS

APSIM (Agricultural Production Systems Simulator) is a software system which provides a flexible structure for the simulation of climatic an d soil management effects on growth of crops in farming systems and ch anges in the soil resource. The focus of this paper is the predictive performance of APSIM for simulation of soil water and nitrate nitrogen in contrasting soils (vertisols and alfisols) and environments. The t hree APSIM modules that determine the dynamics of water, carbon, and n itrogen in the soil system (viz. SOILWAT, SOILN and RESIDUE v.1) are d escribed in terms of the processes represented with particular emphasi s on aspects of their coding that differ from their precursors in CERE S and PERFECT. The most fundamental change is in SOILN, which now prov ides a formal balance of both carbon and nitrogen in the soil and incl udes a labile soil organic matter pool that decomposes more rapidly th an the bulk of the soil organic matter. Model performance, in terms of prediction of soil water and nitrate, is evaluated during fallows, th ereby avoiding complications arising from water use and nitrogen uptak e by a crop. One data set is from a long-term experiment on a vertisol in southeast Queensland which studied two tillage treatments (convent ional and zero tillage) in combination with fertiliser nitrogen inputs for the growth of wheat; soil water and nitrate were measured twice e ach year (pre-planting and post-harvest). The second comes from experi ments at Katherine, Northern Territory, where legume leys growing on a lfisols were chemically killed and ensuing changes in soil water and n itrate were measured during a single season. For both datasets, the pr edictive ability of the model was satisfactory for water and nitrate, in terms of both the total amounts in the whole profile and their dist ribution with depth. Since neither of these datasets included measurem ents of the runoff component of the water balance, this aspect of mode l performance was evaluated, and shown to be generally goad, using dat a from a third source where runoff had been measured from contour bay catchments. (C) 1997 Elsevier Science Ltd.