An assessment of the severity of recent reductions in rainfall and runoff in the Murray-Darling Basin

The Murray-Darling Basin (MDB) in south-eastern Australia has been in the grip of a serious drought for over a decade. Recent reductions in runoff have been unprecedented in the historical record. We test annual time series of rainfall and runoff from 1895 to 2006 for the presence of step changes and trends in order to place recent years in the context of the historical record, and characterise the severity of the recent low rainfall and runoff using average recurrence intervals. The largest signal identified in historical rainfall and runoff is a step-change increase in rainfall and runoff occurring in the mid-1940s. However, other dry periods comparable to the recent dry sequence are evident in the historical record, particularly at the beginning of the 20th century and around 1936-1945. The recent reduction in rainfall and runoff is most severe in the southernmost parts of the MDB, with the runoff reduction being significantly more severe than the rainfall reduction. The average recurrence interval of the reduction in mean annual rainfall during 1997-2006 in the southernmost parts of the MDB is up to 100 years, whereas the average recurrence interval of the reduction in mean annual runoff during 1997-2006 is more than 300 years. We explore possible reasons for the larger than expected runoff decrease including proportionally larger decreases in autumn and winter rainfall in the winter-dominant rainfall regime of the southern MDB, less interannual variability of rainfall and increased temperatures. Crown Copyright (C) 2009 Plublished by Elsevier B.V. All rights reserved.