Final LDC WQ Report | Page 38

Program ( GBRCLMP ) 2016-17 sediment load estimation . The cyclone Debbie event (~ 1 in 10 year ) is of considerable importance in determining the long-term sediment budget .

• The hydrology component of the model is currently being evaluated using new calibration techniques and rainfall dataset plus statistical performance criteria developed for hydrological and water quality models . The Source Catchment model discharge is being compared to active and historical discharge and rainfall gauge datasets which will help inform catchment behaviour at spatial and temporal scales , and thereby , through multiple lines of evidence , allow relevant changes to be applied to model parameters for better model fit . Essentially when looking at Myuna events , this should increase confidence in the likely relative contribution of the Broken , Little Bowen and potentially Rosella Creek . At present it is often difficult to tell with confidence the source of the water which contributes to uncertainty in results at a sub-catchment scale .

Using this information with improved spatial datasets collated during the project ( e . g ., the “ Soil and land conservation technical support for priority hotspots in Reef catchments project ” undertaken by DoR , 2020- 21 ), a major model rebuild will commence later in 2021 / 2022 ( Phase 4 ). This will take into account additional water quality data from the GBRCLMP , the tributary results from the LDC Community Water Quality Monitoring program , particle size data and evaluation of the model hydrology .

Simple sensitivity runs have been performed and improved methods / protocol have been developed to assimilate less statistically robust datasets in the production of average annual loads for model comparison and calibration . Once the Report Card data is processed , an update can be reported at a basin scale ( likely to be in early 2022 ). This will provide an estimate of the outcome of the total effort of the LDC Project in its final year as a contribution to the Burdekin basin end of catchment load reduction .

Previously BBB monitored sub-catchment loads used in Phases 1 and 2 model validations ( Table 5 ) were constrained to the limited GBRCLMP dataset and prior linear regression of efficiency ( LRE ) modelling . Although a useful first version , these were limited in samples and years of coverage , therefore , adding considerable uncertainty associated with the range of discharge conditions the catchment encounters . In addition , as other downstream and tributary water quality data were identified these were discarded , as new techniques would be required to extract useful information about BBB loads .

Importantly , the period after the Phase 3 model re-build coincided with the initiation of the LDC Project and provided an opportune moment to analyse the increasing GBRCLMP dataset and assimilate the LDC Community Water Quality Monitoring Project water quality and tracing data . This will thereby improve the catchment modelling and the conceptual understanding of water quality loads and process in the BBB .

Importantly after a major re-build the P2R Program protocol is to apply only minor model changes between 5 yearly reviews .

Table 5 . Summary of the Source Catchments catchment modelling cycle in the BBB .

Phase 1 Modelling 2009 – 2013

Management Practice data baseline year ; climate period

Report Card 2011 Report Card 2012 Report Card 2013

Minor model changes Minor model changes Minor model changes

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