Measuring and modeling the water balance of low rainfall cropping systems

Abstract

In low rainfall cropping systems, understanding the water balance and in particular the storage of soil water in the rooting zone for use by crops, is considered as critical information for devising risk management strategies for grain-based farming. Crop-soil modelling approaches remain a cost-effective option to understand the interactions between rainfall, soil and crop growth from which management options can be derived. The objective of this study was to assess error in the prediction of soil water content at key decision points in the season against continuous, multi-layer soil water measurements made with frequency domain reflectometry (FDR) probes in long-term experiments in the Mallee region of South Australia and New South Wales. Using field estimates of crop lower limit or drained upper limits were found to be more reliable than lab-based estimates despite the fact that plant available water capacity (PAWC) between the methods did not substantially differ. Using the Agricultural Production Systems sIMulator (APSIM) to simulate plant available water over three year rotations, predicted soil water was within 7 mm (PAWC 64 – 99 mm) of the measured data across all sowing events and rotations. Simulated (n=46) wheat grain production resulted in a root mean square error (RMSE) of 492 kg/ha which is only marginally smaller than other field studies deriving soil water limits from less detailed methods. The paper shows that using field derived data of soil water limits and soil specific settings for parameterisation of other properties determining soil evaporation and water redistribution enables the APSIM model to be widely applied to managing climate risk in low rainfall environments