Flood water quality and marine sediment and nutrient loads from the Tully and Murray catchments in north Queensland, Australia

Abstract

Abstract. Current estimates of sediment and nutrient loads from the Tully-Murray floodplain to the Great Barrier Reef lagoon are updated by taking explicit account of flood events. New estimates of flood discharge that include over-bank flows are combined with direct measurements of sediment and nutrient concentrations in flood waters to calculate the loads of sediment and nutrient delivered to the ocean during 13 floods that occurred between 2006 and 2008. Although absolute concentrations of sediment and nutrients were quite low, the large volume of water discharged during floods means that they make a large contribution (30-50%) to the marine load. By not accounting for flood flows correctly, previous estimates of the annual average discharge are 15% too low and annual loads of nitrogen and phosphorus are 47% and 32% too low respectively. However, because sediments may be source-limited, accounting for flood flows simply dilutes their concentration and the resulting annual average load is similar to that previously estimated. Flood waters also carry more dissolved organic nitrogen than dissolved inorganic nitrogen and this is the opposite of their concentrations in river water. Consequently, dissolved organic nitrogen loads to the ocean may be around twice those previously estimated from riverine data.

Relationships between land use and nutrient concentrations in streams draining a 'wet-tropics' catchment in northern Australia

“…During flood events in the Tully River, nitrate concentrations rise with increasing discharge to a broad peak between 10 000 and 40 000 ML day −1 , beyond which higher discharge leads to falling nitrate concentrations (Mitchell et al 2007). Wallace et al (2009) showed that over-bank floods in the Tully River occurred at discharges greater than 40 000 ML day −1 and that these floodwaters had low concentrations of nitrate, but relatively high concentrations of DON. Hence, it appears that dilution of nitrate commences at a discharge of ∼10 000 ML day −1 , whereas beyond 40 000 ML day −1 further reduction occurs when the waters flood over onto the broader coastal plain.…”

“…Further reductions occur over the coastal plain during bank overflows (Wallace et al 2009). These factors raise the issue of scaling of the subcatchment areas sampled and the extrapolation of measurements from small areas to the whole catchment.…”

Catchment modelling of sediment, nitrogen and phosphorus nutrient loads with SedNet/ANNEX in the Tully - Murray basin

“…Recent research has identified that over-bank flows in the Tully-Murray that are not measured by the Euramo gauging station can contribute extra volumes of water, sediment and nutrient to the GBR (Wallace et al 2009). These additional loads may result in an underestimate by gauging of the annual average N load of ∼40-75% and the P load of ∼30-50%.…”

Target setting for pollutant discharge management of rivers in the Great Barrier Reef catchment area

“…For instance, in the Tully catchment, Wallace et al (2009) showed that a large proportion of the total load of suspended sediment and nitrogen was present in waters in overbank flow on the floodplain and this was not included in the load calculation made at the gauging station (Euramo) in the river channel. Similarly, it is clear that much of the nitrate lost from sugarcane fertiliser in the lower Burdekin reaches the GBR via small stream discharge and possibly groundwater discharge and is thus not included in loads measured at Home Hill in the Burdekin River (Brodie and Bainbridge 2008).…”