Integration of paddock scale modelling and Source

The final P2R Program report.

Executive summary

The linkage between the Agricultural Production Systems sIMulator (APSIM) model used for paddock scale sugarcane land use constituent load estimation and the Source basin scale model is currently being undertaken by redistributing monthly loads generated from APSIM across the Source model hydrology.

This approach can result in unrealistic constituent concentrations in the basin scale model output if the paddock scale sugarcane is under irrigation. In these cases, paddock scale models tend to estimate higher runoff rates than the basin scale model. The paddock scale loads are then distributed across a smaller than expected runoff volume,resulting in unrealistic modeled constituent concentrations. This is a particular problem for pesticides, where the frequency of high in-stream concentrations, as well as long-term loads,are an important indicator of ecosystem health.

The current model linkage also limits the ability to directly translate some on-farm management practices simulated in APSIM to hydrology in the catchment outlet,such as irrigation management and recycling pits. These practices have the potential to influence both hydrology and constituent loads.

To address these problems, this study has investigated an alternative model linkage by constructing a Source plugin to directly link the sugarcane paddock scale model hydrology and loadstoSource by:

Application of the new model linkage to the Pioneer (rain-fed), Barratta (Burdekin—irrigated sugarcane) and Tully–Johnstone (Wet Tropics) models for the water quality constituents dissolved inorganic nitrogen (DIN), atrazine and Diuron demonstrated:

The key parameters of the new plugin are the delivery ratio for drainage contribution to the stream,and the surface and drainage constituent delivery ratios. Adjustment of these ratios is available to the modeller as calibration parameters to achieve model fit. Following from this study, further work has been recommended to:

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