A model for simulating soil water regime in alternating fallow-crop systems

Abstract

In developing a methodology for quantitative agroclimatic assessment within a sub-humid to semi-arid region in central Queensland with sparse meteorological and agronomic data, a set of simple working functions depicting evaporative losses of an alternating fallow-crop system was derived. Given weekly rainfall and estimates of potential evaporation as basic inputs, these functions are used to generate a continuous simulation of week-to-week changes in soil water. Despite data inadequacies, a close simulation of temporal changes in soil water regime was obtained using distinctive functions for fallowed and cropped intervals, and by making evapotranspiration rates dependent upon stage of crop development and status of available soil water obtained as a feedback from the model. The levels of stepped functions used in the model to express evaporative loss during fallowed and cropped intervals were fitted in part by applying previous functions presented by Slatyer (1960b), and in part by iterative procedures, using soil water extraction as determined by gravimetric sampling and phenological data from an experimental site within the region. Sufficient data were available to enable the formulation of functions for cotton, grain sorghum, and wheat. The model takes account of prevailing agronomic practices and dynamic aspects of crop growth and development, and makes provision for future refinements in available data.