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University of Newcastle (2020)

Changes to catchment rainfall-runoff response during and following the Millennium drought

Hughes Donna

Titre : Changes to catchment rainfall-runoff response during and following the Millennium drought

Auteur : Hughes Donna

Université de soutenance : University of Newcastle

Grade : Masters Research - Master of Philosophy (MPhil) 2020

Résumé
Droughts have occurred throughout Australia’s history, the Millennium drought (around 1997 to 2009) was particularly severe in the southeast of Australia. The magnitude of the rainfall reduction compared to the reduction in runoff was unprecedented from the historic record. A change in the annual rainfall-runoff relationship was evident for some catchments located in southeast Australia, change was more commonly associated with flatter and less forested catchments with drier climate. The reduction in runoff was not solely attributed to changes in rainfall and temperature experienced in the Millennium drought, catchment processes such connectivity of groundwater and surface water, were also proposed to contribute to the reduction. The end of the meteorological drought was associated with two La Niña events in 2010–11 and 2011–12 with high rainfall in many parts of southeast Australia. Studies have shown that for catchments in southeast Australia the duration of the hydrologic drought was longer than meteorological drought. The lag between the ends of these types of droughts (rainfall and runoff) was, on average over 130 catchments assessed, 7 months for streamflow. Lags were longer for catchments in the southern parts of southeast Australia with some catchments having a lag of several years. This investigation was motivated to understand the runoff response in the post drought period and association with changes in runoff response evident in the drought. Of interest is the persistence of the anomalous low flows after the end of the hydrologic drought that may exacerbate impacts from droughts if reduced runoff response continues after a prolonged drought. Detailed hydrologic analysis was undertaken for five catchments in southeast Australia. Catchments selected were expected to have a change in runoff response in the Millennium drought period so that recovery or persistence of runoff response following the end of the drought could be assessed. A multiple assessment approach was undertaken that considered annual, seasonal and daily data. Analysis used observed rainfall and recorded streamflow up to 2017 allowing assessment past the high rainfall and streamflow years of 2010 to 2012. Analysis indicated that four catchments (all with moderate rainfall between 600-750 mm/yr. and low forest cover) had a different runoff response in the Millennium drought period compared to the pre-drought period. In the post drought period runoff response for all four catchments was more aligned to runoff response in the drought than runoff response in the pre-drought period, except for some periods with high rainfall. In contrast the fifth catchment (with higher rainfall and predominately forested) did not have a different runoff response in the Millennium drought compared to the pre-drought period and runoff response in the post drought period was similar to the runoff response in both the pre-drought and drought periods. Results present a prima facie case that for certain catchments, runoff response may not have returned to pre-drought conditions over the 8-year period assessed.

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