A review of load tools available for calculating pollutant exports to the Great Barrier Reef lagoon: a case study of varying catchment areas

Several load methods and software programs have been developed to calculate sediment, nutrient and other pollutant exports from waterways of the Great Barrier Reef (GBR) catchments. These different methods can produce large discrepancies in the calculation of catchment loads. Such discrepancies reduce the confidence of these methods for application within the Reef Water Quality Protection Plan process, such as the setting of end-of-river load targets as well as the comparison to modelled outputs, namely the SedNet and ANNEX models (e.g. Brodie et al. 2003). We present a case study based on intensive monitoring data collected over a range of spatial (paddock, a Dry Tropics sub-catchment and a large Dry Tropics end-of-river catchment) and temporal (hourly - yearly) scales. We simulate changes in load calculations of total suspended solids based on the selected removal of monitoring data from intervals throughout the flow hydrograph. We attempt to quantify errors in load calculations based on the different load tools, as well as investigate optimal load methods and ideal sampling frequencies over these different catchment areas. Three software programs were used to calculate loads including WQ Loads Tool, Brolga and GUMLEAF. We found that all three software programs provided suitable methods to calculate loads for the catchments of the GBR where continuous flow and concentration data were available. The linear interpolation (and associated inter sample mean) methods were optimal in the Brolga and Loads Tool program while the flow regime estimators provided ideal load estimates in the GUMLEAF program. Our findings suggest that six samples evenly-spaced over the flow hydrograph (e.g. 2-3 samples on rise, 1 on peak and 2-3 on the falling limb) will provide reliable load estimations (within 10% of our best estimate) at the paddock scale provided that the optimal methods are used. We recommend that at least daily (but up to 4-5 samples per day in catchments with very high TSS concentrations on the rising limb such as the Bowen sub-catchment) sampling is suitable for load estimations at the sub-catchment scale. One sample collected every two days is an adequate sampling frequency for load calculations of larger catchments of the GBR such as the Burdekin River. We note that researchers need to account for the uncertainty in all load estimates before the significance of longterm trends can be analysed