Research Article 2014 WRR Burdekin sediment budget

Water Resources Research 10.1002/2013WR014386 2005/2006, when drought had reduced reservoir water levels to 60% of capacity (Figure 3a). Otherwise the dam was almost full prior to each wet season; despite its considerable volume (1.86 ML), full capacity is <6% of the average annual inﬂow. BFD overﬂow waters were the primary source (i.e., 65–95%) to end-of- river discharge over the study period, with the remainder contributed from the Lower Burdekin subcatch- ment, including the Bowen River (Figure 3). An important ﬁnding of this study is that the Upper Burdekin is the dominant sediment source to the BFD under all streamﬂow conditions, contributing 76–95% of the total sediment inﬂux in each of the ﬁve water years studied. The Cape, Belyando, and Suttor subcatchments each contributed only 1–11% of the total sediment load into the dam in any water year during the study period (Figure 3). The contrast between the Upper Burdekin and these other subcatchments was greatest in the 2007/2008 water year when the Belyando and Suttor subcatchments combined contributed 54% of total inﬂow into the dam due to above average events across their catchment areas, but contributed only 15% of the total sediment load (0.92 mil- lion tonnes) into the dam (Figure 3b). In comparison, the Upper Burdekin contributed 4.66 million tonnes, or 77% of the sediment load into the BFD, while contributing only 33% of total inﬂow (Figure 3b). The BFD reservoir trapped an average of 66% (80% CI 5 60–72) of annual suspended sediment inﬂux over the ﬁve study years (Figure 3; Lewis et al., 2013]. The dominance of the Upper Burdekin subcatchment as a major sediment source to end-of-river export has been diminished by the construction of this reservoir and its sediment trapping efﬁciency. Assuming equal trapping of sediment within the reservoir contributed from all upstream subcatchments, the Upper Burdekin contributed 14–43% to annual end-of-river sedi- ment export during this study period. The Lower Burdekin subcatchment, including the Bowen River, is now the major sediment source, despite representing only 12% of the entire Burdekin catchment area (Fig- ure 3). The Bowen River contribution to end-of-river sediment export ranged from 31–50% over the study period, representing 48–81% of the Lower Burdekin subcatchment contribution (Figure 3). However, these Bowen River contributions do not include the 2009/2010 water year due to a high uncertainty in the load estimate; high uncertainties for the Bowen River were also calculated for the 2007/2008 and 2008/2009 load estimates (see load estimates in red, Figure 3b). The high uncertainties result from a lack of TSS con- centration data available during these above average discharge years and the difﬁculties developing discharge-TSS concentration relationships using TSS data collected only in below average and average water years. In particular, TSS data were not available for the largest streamﬂow event in the above average (RI513) 2007/2008 water year (see supporting information FS01f). Uncertainties in the Bowen load outputs in this study highlight the importance of (1) prioritising critical water quality monitoring sites to inform management decision-making and (2) prioritizing the capture of larger discharge events in sampling regimes for more precise load calculations. Our results demonstrate the importance of measured stream TSS concentration and ﬂow data to accurately estimate loads and source areas of suspended sediment in comparison to catchment modeling-only stud- ies. The study of McKergow et al. [2005] [see also Brodie et al., 2003] based on the SedNet model showed delivery of a large proportion of the suspended sediment from a small proportion of the catchment, although their ﬁndings were limited to an assessment of the entire GBR catchment area (i.e., no speciﬁc numerical data for the Burdekin catchment were presented). Furthermore inaccurate and unrealistic assumptions in the modeling approach at the time, such as overestimates of dam trapping [see Lewis et al., 2013] and underestimates of gully and stream bank erosion [see Wilkinson et al., 2013] are now known to have produced poor estimates of actual subcatchment spatial sources of suspended sediment. In contrast, our study using measured ﬁeld data for suspended sediment and particle size provides far more accurate estimates which can be compared and used to calibrate recently improved modeling estimates using the Source Catchments model and updated versions of SedNet [Wilkinson et al., 2014]. 5.2. Subcatchment Annual Sediment Yields: A Comparison With Other Tropical River Studies End-of-river sediment yields for the Burdekin are low (<115 t km 22 yr 21 ) when compared to published yields from other tropical catchments around the world (Table 3). Most catchment studies across the tropi- cal belt have been conducted in wet tropical rainforest (Af), monsoon (Am), and savannah (Aw) climates [Peel et al., 2007] within South America and South-East Asia, where annual rainfall typically exceeds 2000 mm y 21 and yields exceed 500 t km 2 yr 21 (Table 3). Although the Burdekin catchment is deﬁned largely as semiarid (Bsh), it is the higher rainfall and steeper terrain of the coastal areas (Aw and Cwa) that are the primary hydrological drivers of this catchment; climatic conditions that position it somewhere BAINBRIDGE ET AL. C 2014. American Geophysical Union. All Rights Reserved. V 9080

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