Regional significance of historical trends and step changes in Australian streamflow

Abstract. The Hydrologic Reference Stations is a network of 467 high quality streamflow gauging stations across Australia, developed and maintained by Bureau of Meteorology, as part of ongoing responsibility under the Water Act 2007. Main objectives of the service are to observe and detect climate-driven changes in observed streamflow and to provide a quality controlled dataset for research. We investigate linear and step changes in streamflow across Australia in data from all 467 streamflow gauging stations. Data from 30 to 69 years duration ending in February 2019 was examined. We analysed data in terms of water year totals and for the four seasons. The commencement of water year varies across the country – mainly from February–March in the south to September–October in the north. We summarised our findings for each of the 12 Drainage Divisions defined by Australian Geospatial Fabric (Geofabric), and continental Australia as a whole. We used statistical tests to detect and analyse linear and step changes in seasonal and annual streamflow. Linear trends were detected by Mann-Kendall – Variance Correction Approach (MK3), Block Bootstrap Approach (MK3bs) and Long Term Persistence (Mk4) tests. The Nonparametric Pettitt test was used for step change detection and identification. Regional significance of these changes at the drainage division scale was analysed and synthesised using the Walker test. The Murray Darling Basin, with Australia’s largest river system, showed statistically significant decreasing trends for the region in annual total and all four seasons. Drainage Divisions in New South Wales, Victoria and Tasmania showed significant annual and seasonal decreasing trends. Similar results were found in south-west Western Australia, South Australia and north-east Queensland. There was no significant spatial pattern observed in Central and mid-west Australia, one possibility being the sparse density of streamflow stations and or length of data. Only the Timor Sea drainage division in northern Australia showed increasing trends and step changes in annual and seasonal streamflow and were regionally significant. Most of the step changes occurred during 1970–99. In the south-eastern part of Australia, majority of the step changes occurred in the 1990s, before the onset of the millennium drought. Long term linear trends in observed streamflow and its regional significance are consistent with observed changes in climate experienced across Australia. Findings from this study will assist water managers for long term infrastructure planning and management of water resources under climate variability and change across Australia.