Water, sediment and nutrient budgets for Somerset and Wivenhoe reservoirs, Australia.

Abstract

Annual inputs and outputs of water, total suspended solids (TSS), total nitrogen (TN) and total phosphorus (TP) were estimated for two reservoirs over 14 from July 1997 to June 2011. Somerset and Wivenhoe are major drinking and flood mitigation reservoirs in southeast Queensland, Australia (27° 7' S, 152° 33' E and 27° 24'S, 152° 36' E, respectively), and they are linked by the Stanley River. Inflows are highly episodic and concentrated in the austral summer (wet season) therefore data is provided for water years defined from July to June. Annual inputs of water, TSS, TN and TP from the catchments were estimated from daily flow and loads predicted by the Source Catchments (SC) model. Loads of TSS, TN and TP exported from the reservoirs were calculated by multiplying concentrations measured at the dam walls by the volumes of released, and integrating over annual timescales. Concentrations of TSS, TN and TP were primarily from monthly quality sampling, with missing data during December 2010 and January 2011 estimated from hourly turbidity profiles. Annual retention in each reservoir is the sum of catchment inputs (SC model predications) and loads from the upstream reservoir (in the case of Wivenhoe), minus reservoir outputs. There were four notable hydrological events during the study period: a major drought from 2001 to 2009, high flow in February 1999 and February 2008, and a major flood in January 2011. Uncertainty was quantified for all terms, but confidence in the uncertainty estimated for the period July 2010-June 2011 was low due to the large scale of the January 2011 flood. A full description of methods for determining loads and uncertainty is available in O'Brien et al. 2016.