Validation and calibration of Source water quality models in the Great Barrier Reef catchments

Abstract

The Paddock to Reef (P2R) Integrated Monitoring, Modelling and Reporting Program uses modelled catchment loads to estimate annual load reductions as a result of improved management practices in the Great Barrier Reef (GBR) catchments. Calibrating and validating catchment models across a large spatial scale is challenging, and requires a range of approaches to continue to refine and improve modelled load estimates. Catchment models have been built for the six Natural Resource Management (NRM) regions draining to the GBR in the Source framework. The NRM regions range in area from 9,000 km 2 to 155,000 km 2 , with the total modelled area 423,000 km 2. Load estimates are modelled for 10 water quality constituents. Many improvements have been made over the past seven years to improve confidence in modelled loads. An example includes hydrology recalibration and baseflow optimisation to align with estimates derived from gauged flow. Typically, the larger, drier regions required more gauges to be recalibrated than for the wetter regions. In Cape York (CY) following recalibration of the model, the baseflow proportion was improved from an average of 43% to 33%, which better aligns with the baseflow estimate from gauged flow of 31%. A second notable model update is the improved representation of Dissolved Inorganic Nitrogen (DIN) in sugarcane areas. The DIN runoff model was updated to incorporate a new algorithm which correlates fertiliser input to nitrogen in runoff, derived from the pool of P2R field monitoring data. This daily model of DIN concentration in runoff was based on >200 sugarcane field monitored DIN runoff events. These changes have resulted in modelled DIN loads now being within ± 30% of measured loads in the Wet Tropics (WT), compared to ± 50% in previous model iterations.