Catchment salt balances in the Queensland Murray–Darling Basin, Australia

• Catchment salt balances were calculated for the Queensland Murray–Darling Basin.
• The Basin is accumulating salt at the regional scale.
• Small eastern catchments are exporting salt.
• Variation in method of calculation affects the result and interpretation.

SummaryCatchment salt mass balances and export/import ratios were calculated for 55 gauging stations in nine major catchments across the Queensland Murray–Darling Basin (QMDB), Australia. Salt inputs were comprised of atmospheric, groundwater and inter-basin transfer contributions, while exports were derived from model runs calibrated to streamflow data and flow-salt relationships. Catchment atmospheric salt inputs were larger than groundwater inputs in the major catchments, with the exception of the Condamine catchment. Across the whole QMDB, the magnitude of atmospheric and groundwater inputs is potentially equal. Average annual streamflow salt export is generally much less than salt input, even when atmospheric inputs alone are considered, and is strongly influenced by episodic, large events. The exceptions to this are some smaller salt-affected upland catchments in the eastern QMDB where flow is more continual (i.e. baseflow occurs) and stream salinity is higher – a result of long-term land use change impacts. Variability in catchment salt export/import ratio (E/I) as a result of different calculation methods for both inputs and outputs creates a wide range in possible E/I for some sites, but trends remain the same. Losses of stream water to floodplains, seepage and extractions in lower portions of catchments leads to significant reductions in E/I with distance downstream. It appears that in general, the natural status of the QMDB is one of salt accumulation and significant hydrologic changes – as represented through salt mass balance calculations – are largely confined to the eastern half of the Basin, although further change may yet express in the landscape.