Contemporary and palaeo-drought episodes in the Snowy Mountains and Murray-Darling Basin, Australia: A multi-scaled approach to understanding drought

Samuel K Marx; H McGowan; B S Kamber; J Denholm

Water availability is a dominant issue with in the Murray-Darling Basin (MDB), Australia's most important agricultural region. The climate of much of the Basin is semi-arid and as a result the MDB is subject to extended droughts such as has occurred over the last 6 years. These drought episodes have significant social, environmental and economic effects, for example, drought was estimated to have wiped 6.6 million from the Australian economy in 2002. While the recent drought is the most severe on record, analysis of historical inflow records to the Snowy Mountains Hydro Scheme in the headwaters of the Murray River imply that extended drought episodes are an intrinsic feature of the regions climate. Importantly, they suggest drought episodes correspond to the interaction of the El Niño Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO). Decadal patterns in water availability are thus evident in the instrument record of the Scheme's catchments, and these correlate with changes in Pacific Ocean SSTs as described by the PDO. Positive periods of the PDO index are inversely related to inflows, with periods of relatively low inflows occurring from the 1920s to the late 1940s and again from the 1980s onwards, while particularly severe droughts are often associated with ENSO events during these periods. To confirm whether this association between ENSO, PDO and the hydrometeorology of the Scheme's catchments has existed for longer than the historical record we present a 7000 year drought record for the MDB using rates of mineral dust deposition as a proxy for drought conditions. This is possible within Australia because dust activity on both historical (last 100 years) and Quaternary timescales increases during periods of greater aridity. Chronologies of dust deposition were constructed from cores extracted from ombrotrophic peat bogs located in the Scheme's headwaters in the Snowy Mountains. Results show multi-decadal variability in dust deposition rates super-imposed on longer global scale shifts in climate. The source areas of dust deposited in the core through time were established using a trace-element provenance model. Prior to 5000 years B.P. rates of dust deposition were relatively low, while modeling of dust provenance confirmed dust source areas in eastern Australia, including in the MDB, were less active due to more humid climatic conditions. After 5000 years B.P, dust flux increased markedly in response to drying of the Australian continent which has previously been attributed to the onset of contemporary ENSO conditions. The MBD basin became more active as a dust source in response to this regional scale drying. Spectral analysis undertaken on the fine scale variability of dust deposition in the top of the core indicates multi- decadal patterns in aridity, coincident with PDO type variability. These patterns which are evident for the last 3000- 4000 years suggest multi-decadal dry phases are an intrinsic feature the MDB, during which the chances of severe and prolonged drought are likely to be high. These findings have major implications for water resource management. Importantly, they confirm the need for water resource mangers to include the effects of decadal climate variability on water resource availability in the MDB.

Contemporary and palaeo-drought episodes in the Snowy Mountains and Murray-Darling Basin, Australia: A multi-scaled approach to understanding drought

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