Challenges of classifying and mapping perennial freshwater systems within highly variable climate zones: A case study in the Murray Darling Basin, Australia.

Perennial freshwater systems are valuable natural resources that provide important ecological services globally. However, in highly variable climates, such as Australia, water availability in rivers and streams can vary greatly from year to year and from decade to decade. Further, across Australia and many other regions, perennial river systems are projected to decrease because of anthropogenic climate change, placing the ecosystems they support under additional pressure. Quantifying the potential impacts of climate change on perennial freshwater systems requires robust databases of existing water features with accurate classifications. This is a challenge for rivers that display a high degree of interannual variability since the river classification can be dependent on the period of available data. In this study, we carry out a regional scale comparison of three different spatial databases commonly used in environmental and ecological assessments of perennial systems of Australia, namely Geodata, Geofabric and Water Observations from Space (WOfS). Focusing on the southern Murray Darling Basin (MDB), due to its national and international significance and its highly variable flow regimes, we show that no single spatial database is reliable by itself in terms of perennial water classification, with notable differences likely arising from variations in the periods analysed and methods used to classify the systems. Further, an analysis of high-quality gauged streamflow data (with approximately 40-year daily records) for four sub-catchments, and long-term simulation data (>100 years) for two sub-catchments in the lower MDB, confirm that flow persistence can be non-stationary through time, with some 'perennial' systems exhibiting sustained periods of cease to flow (i.e. becoming non-perennial) during prolonged droughts. This study demonstrates that due consideration is required in developing baseline classification of perennial freshwater systems for assessing future changes and measuring adaptive capacity.

Compound climate extremes driving recent sub-continental tree mortality in northern Australia have no precedent in recent centuries.

Compound climate extremes (CCEs) can have significant and persistent environmental impacts on ecosystems. However, knowledge of the occurrence of CCEs beyond the past ~ 50 years, and hence their ecological impacts, is limited. Here, we place the widespread 2015-16 mangrove dieback and the more recent 2020 inland native forest dieback events in northern Australia into a longer historical context using locally relevant palaeoclimate records. Over recent centuries, multiple occurrences of analogous antecedent and coincident climate conditions associated with the mangrove dieback event were identified in this compilation. However, rising sea level-a key antecedent condition-over the three decades prior to the mangrove dieback is unprecedented in the past 220 years. Similarly, dieback in inland forests and savannas was associated with a multi-decadal wetting trend followed by the longest and most intense drought conditions of the past 250 years, coupled with rising temperatures. While many ecological communities may have experienced CCEs in past centuries, the addition of new environmental stressors associated with varying aspects of global change may exceed their thresholds of resilience. Palaeoclimate compilations provide the much-needed longer term context to better assess frequency and changes in some types of CCEs and their environmental impacts.