title: Effects of climate change on flooding and water quality in a subtropical Australian catchment
Creators: name: Eccles, Rohan
Publisher: name: Griffith University
Year: 2021
Subjects: subject: Climate change
subject: Flooding
subject: Subtropical regions
subject: Water quality issues
subject: Logan-Albert catchment
subject: South East Queensland

description: Anthropogenic changes over many decades, such as urbanisation, industrialisation, the introduction of livestock, loss of riparian vegetation, and adoption of fertilisers, have altered the natural characteristics of many rivers, exacerbating flooding and increasing nutrient and sediment loads. Anthropogenic climate change is expected to alter hydrological regimes throughout the world, causing increases in extreme precipitation, as well as altered temporal, seasonal and spatial patterns of rainfall. The hydrological extremes (e.g. floods and droughts) will impact water resources and water quality. In recent decades research to address climate change impacts on flooding and water quality has increased. However, relatively little research on this topic has been published on Australian catchments and even less for subtropical regions compared to other regions of the world. The majority of the research has also been focused on large river systems, while comparatively little has considered small and mid-sized catchments (e.g., &lt; 1000 km2). Water quality problems and flooding are major issues for the coastal regions of southeast Queensland (SEQ). The International Panel for Climate Change (IPCC) has identified SEQ as one of the ‘hotspots’ for climate change in Australia and flooding and water quality issues are likely to be exacerbated in the future. Water quality in the Logan and Albert rivers is already particularly poor, with marked ongoing increases in sediment and nutrient loads since European settlement. In addition, the catchment is anticipated to be subject to substantial population growth, which may further exacerbate water quality issues, while increasing the number of residents and properties at risk of flooding. The issues of flooding are compounded by significant urban development located along the low-lying coastal floodplain. Sea level rise linked to climate change may act synergistically with inland hydrological changes to intensify flooding. In this context, the primary aims of this research are to: (i) summarise understanding of the impacts of climate change on flooding in subtropical regions, (ii) identify the principal water quality issues in the subtropical Logan-Albert catchment of South East Queensland, (iii) and determine the impacts of climate change on high flows, flooding, and water quality in the Logan-Albert catchment. An improved understanding of the effects of climate change in subtropical Australia will benefit regional and local managers by allowing them to be better prepared for the range of future possible eventualities of hydrological extremes, flooding, and water quality. Effects of climate change on flooding in subtropical and tropical regions was assessed by a systematic evaluation of the literature which revealed adoption of a wide variety of methodologies for change impact assessments. The evaluation showed there was strong regional bias towards studies conducted in Asian catchments, while there was a paucity of research on catchments in Africa, Latin America, and particularly Australia, for which only one study was found. There was also an appreciable bias in the research towards studies conducted on large river systems, with little research conducted on smaller to midsized catchments similar in size to the Logan-Albert catchment. The impacts of climate change were shown to vary significantly across the various regions of the tropics and subtropics, with the most consistent increases in flooding predicted for South Asia, South East Asia, and the western Amazon. Results were more varied for Latin America and Africa, though there was a notable paucity of studies. Mitigating model uncertainty and improving data accessibility in subtropical regions were recognised as the principal issues faced by researchers. An examination of long-term water quality monitoring data was conducted for the Logan and Albert estuaries and tributaries, to improve understanding of the drivers of water quality change. A seasonal Mann-Kendall test was applied at 15 sites to reveal spatial patterns of water quality change, while a Before-After Control-Impact (BACI) test was employed to assess the impacts of a recently constructed (Wyaralong) upstream dam on estuarine water quality. Significant decreases in nutrient concentrations and turbidity were observed at sites along the upper and middle Logan estuary, which was principally attributed to the construction of Wyaralong dam. Significant decreases in total phosphorus (TP) and nitrate concentrations along the lower Albert estuary were principally attributed to wetter conditions over the second half of the study period, which acted to dilute loads from a nearby wastewater treatment plant. The impacts of climate change in the lower Logan-Albert catchment were examined through a novel coupled 1D-2D hydrodynamic model, which simulated flooding and inundation. The hydrological Nedbør-Afstrømnings Model (NAM) was employed to simulate the rainfall-runoff response under current and future climate conditions. An ensemble of 11 high-resolution dynamically downscaled climate models was applied with high (Representative Concentration Pathway 8.5 - RCP8.5) and intermediate (RCP4.5) emission scenarios. The climate models predicted an increase in the seasonal intensity of precipitation in the 2020s, 2050s, and 2080s, relative to the baseline (1980-2010). Decreases were predicted for the winter and spring dry season and increases for the summer wet season. Predicted changes to the magnitude of mean and high flows in the three future periods largely followed the pattern predicted for precipitation, though decreases during the winter dry season were amplified, while increases in summer were diminished due to increased evaporation rates. There was considerable variation in the magnitude of predicted future flooding events, ranging from 5 to 100-year year average recurrence intervals (ARI). The largest events (100-year ARI) tended to increase by the 2080s, while smaller events (5-year ARI) tended to decrease. Floodplain inundation resulting from a 100-year ARI event increased in all future periods, and the inclusion of sea level rise near doubling floodplain inundation by the 2080s. The same ensemble of climate models was applied with the Soil Water Assessment Tool (SWAT) catchment model to assess climate change impacts on total nitrogen (TN), TP, and total suspended sediment (TSS) loads in the 2020s, 2050s, and 2080s in the Logan and Albert rivers. Predicted decreases of streamflow from SWAT during the winter dry season were not as significant as those predicted from NAM, but similar changes were predicted for the remainder of the year by the two models. Changes to TN, TP, and TSS loads in the three future periods mostly reflected the pattern of change predicted for streamflow, with the largest decreases coinciding with the dry season. Compared with the baseline, decreases in TSS loads were 34.3 and 54.2% by the 2080s at the Logan and Albert rivers, respectively and were larger than decreases predicted for streamflow, TN and TP. Decreases to streamflow, however, may have the effect of increasing the realtive importance of point load sources, which were found to be important drivers of estuarine water quality. Increased point loads in the future may therefore have the effect of increased in-stream pollutant concentrations. This is particularly important as point source contributions are typically dominated by dissolved inorganic nutrients that are more readily bioavailable than the organic and particulate nutrients associated with diffuse sources. The outcomes of this Ph.D. study provide new insights into the potential impacts of climate change in a relatively unstudied region that has been identified as a hotspot for climate change. Flooding and changes in water quality from climate change are likely to be have substantial impacts throughout subtropical South East Queensland, especially when combined with pressure from rapidly growing human populations. Given these issues, mitigations measures may need to be employed to mitigate future flooding and water quality issues.