Great Barrier Reef Source Catchment’s modelling: Enhanced simulation and water quality targeting through event based assessment

A decline in marine water quality entering the Great Barrier Reef (GBR) lagoon is associated with terrestrial runoff discharged from adjacent GBR catchments. Reef Plan 2013 outlines water quality targets to address this decline, with the eWater CRC Source Catchment modelling framework used to report progress towards meeting these targets. Catchment modelling is an ideal tool to investigate constituent budgets and the potential impact of management strategies and it follows that the better a catchment model performs spatially and temporally the greater the confidence in targeted management actions. However model assessment data can often be sporadically collected, over different time periods, different locations and from disparate groups of projects. An advantage of the Source Catchment’s framework is the ability to generate daily outputs for discrete periods and locations, thus facilitating aggregation of the disparate monitoring data for use in model calibration and validation. A Source Catchment model has been built for the second largest GBR catchment the Burdekin. The model identifies constituent sources, supply and losses; and explores the impact of a change in land management on loads discharged to the reef lagoon. The Burdekin, like many rivers has had the majority of its water quality sampling located at the end of system gauging station and to better utilise this data in model assessment, events were defined and catalogued by identifying the source of runoff to this site. Speculating; that the process may add to the limited information on the spatial and temporal sources of pollutants within the catchment. An important feature of the Burdekin catchment is the large Burdekin Falls Dam (BFD), and the consensus is that the area below the dam is a major sediment source and an appropriate area for the targeting of ameliorative management action. The classification system identified 35 flow events between 1986 and 2009 at Burdekin End of valley, of these events, 16 were classed as originating from “Above Dam”, 8 “Below Dam”, and 11 were classed as “Mixed” in origin. Of the 16 “Above Dam” events, 11 were classified as discrete flow events from the Upper Burdekin catchment. In contrast the classification system did not identify a discrete sub-catchment source for the “Below Dam” events. “Below Dam” classified events had the highest event mean sediment concentrations (EMC) 0.62 g/l, since they only contributed 6% of the total discharge, these events contributed just 9% of the total sediment load. In contrast, the “Above Dam” events contributed the largest proportion of discharge (41%) and sediment load (42%), with a lower EMC (0.40 g/l). “Mixed” events contributed 34% of the total discharge and 39% of the load, with a similar EMC (0.45 g/l) to the “Above Dam” events. The classification of “Other” had the lowest EMC (0.19 g/l) with 19% of the total discharge and contributing just 9% of the total sediment load. All events irrespective of classification have a proportion of flow and load sourced from above and below the BFD. Nonetheless, it was possible to estimate the above dam and below dam contribution to the total TSS load for the study period. The analysis suggests that 60% of the total load is sourced from “above the dam” and 40 % from “below the dam” for the assessment period (1986 and 2009) The work suggests that above the dam sources may generate greater loads then previously considered and should therefore be further investigated. Additionally the source catchment’s model assessed in this paper was found to under predict “Above Dam” classified events and it appears that a lack of generation from the Upper Burdekin catchment is a likely source of error. Importantly we have outlined a method for utilising water quality data, through event cataloguing of spatial source and daily timestep modelling. A similar approach may prove beneficial for other model users.