Evaluating of soil water balance (SWB-Sci) model for water and nitrogen interactions in pasture: Example using annual ryegrass

Poor management of irrigation water and nitrogen (N) fertiliser in ryegrass cropping systems reduces forage yield, and quality, N-use efficiency, and potentially leads to atmospheric and water pollution. The objective of this study was to calibrate and validate the soil water balance (SWB-Sci) model for annual ryegrass (Lolium multiflorum) pasture and assess the impact of different irrigation strategies on forage yield, irrigation water use and N use efficiency. Field trials were conducted using annual ryegrass under a range of irrigation and N application strategies to calibrate and test the SWB-Sci model. Once calibrated and validated, the model was used to investigate and compare the impact of different irrigation strategies on forage yield, unproductive water loss due to drainage and runoff and N leaching. Using the moderate deficit (irrigating to 60–80% of the field capacity) irrigation strategy, reduced water loss due to drainage and runoff by 54–66%, irrigation water by 37–63%, and N leaching by 69–72% compared to the common farmers’ practice of 25mm irrigation per week, without significant forage yield reduction. The SWB-Sci model showed ways of improving water and N fertilisation use efficiency using moderate deficit irrigation approaches in high rainfall areas.