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Fine sediment generated from catchments of the Great Barrier Reef (GBR) is considered to be one of the main pollutants of serious concern affecting the quality of water entering the reef. Estimation of the amount of sediment that is generated from these catchments and the proportion that enters the reef lagoon for a given combination of land use and land management is being carried out under the Paddock to Reef (P2R) Program using the eWater Source modelling platform and the Dynamic SedNet plugin. Currently, water quality monitoring data from selected sites, including end of system sites, are being used to manually calibrate and validate the water quality model. However, the manual calibration attempted so far has mainly been based on trial-and-error. The main objectives of the simple one-at-a-time (OAT) local sensitivity analysis reported here are to: (1) identify parameters that the model reacts most sensitively to in order to simplify and accelerate the calibration of the model; (2) examine if the results of the sensitivity analysis depend on the spatial and temporal scales of investigation; and (3) compare the results to a more computationally intensive global sensitivity analysis reported in an associated global sensitivity analysis paper to be presented in this conference (Bennett and Fentie, 2017). The choice of parameters included in the sensitivity analysis was determined based on a preliminary investigation on a relatively small test sub-catchment in the Burnett catchment by the first author. The preliminary investigation showed that fine sediment load is most sensitive to changes in three parameters (i.e., streambank sediment bulk density (ρs), hillslope sediment delivery ratio (HSDR) and gully sediment delivery ratio (GSDR)), which represent the three sources of sediment (streambank, hillslope and gully erosion, respectively) in the Source/Dynamic SedNet water quality model. The other three parameters included in the analysis – floodplain deposition settling velocity (Vp), channel average terminal fall velocity for fine sediment remobilization (ωdep), and channel average terminal fall velocity for fine sediment deposition (ωmob) – represent the effect of settling velocity of particles on sediment deposition on the floodplain and in the stream, and channel remobilization, respectively. In order to explore the effect of catchment size and other characteristics on the sensitivity analysis, three locations with differing catchment area were chosen in this study. The location with the smallest catchment area drains the upper parts of the Burnett catchment while the location with the largest catchment area represents the Burnett catchment end of system (EoS). The third location is at the outlet of the Mary catchment. Temporal variability in results of the sensitivity analysis was investigated by conducting the analysis at an annual time step summarised from the daily time-step model outputs of the 28 years modelling period of the Great Barrier Reef catchment modelling for Report Card 2016. Results show that: (1) the modelled fine sediment loads were relatively highly sensitive to changes in streambank sediment bulk density (ρs), hillslope sediment delivery ratio (HSDR) and gully sediment delivery ratio (GSDR), but were only marginally sensitive to changes in the other three parameters; (2) the sensitivity of fine sediment load to changes in parameters varied across the three locations; and (3) the sensitivity of fine sediment load to changes in parameters varied annually. Sensitivities from this study have similar rankings of parameters to those from Bennet and Fentie (2017). Both approaches determined that modelled fine sediment load is relatively insensitive to changes in particle settling velocity parameters for both stream deposition and channel remobilization. Based on these findings it is concluded that (1) the simple one-at-a-time sensitivity analysis conducted in this study is adequate in determining the sensitivity of modelled sediment to changes in model parameters; and (2) since results of the local sensitivity analysis vary spatially from catchment to catchment and temporally from year to year, the analysis needs to be carried out for each specific catchment and period of interest.

Simple sensitivity analysis of a catchment scale water quality model at a range of spatio-temporal scales