title: Spatio-temporal dynamics and hydro-ecology of intermittent streams in eastern Australia
Creators: name: Yu, Songyan
Publisher: name: Griffith University
Year: 2019
Subjects: subject: Intermittent streams
subject: River channel drying
subject: Hydrological variability
subject: Freshwater ecosystems

description: Intermittent streams that cease to flow for some period of most years are prevalent across global river networks. Their spatial extent is projected to increase in regions experiencing drying trends related to climate change and water extraction for human uses. Intermittent streams sustain biodiversity by hosting a unique combination of aquatic, amphibious, and terrestrial assemblages as a result of their wet and dry phases. Compared to perennial streams, the ecological values of intermittent streams are not well-appreciated or understood, and thus intermittent streams are less commonly incorporated into policy, management, and regulatory decisions. As research on intermittent streams is increasing, there have been strident calls for better recognition and protection of intermittent streams. This thesis aims to develop new methods to address some key issues related to spatio-temporal dynamics and hydro-ecology of intermittent streams, with a focus on eastern Australia. River channel drying caused by intermittent stream flow is a widely-recognised factor shaping stream ecosystems. There is a strong need to quantify spatio-temporal variations in the hydrology of intermittent streams over broad spatial scales to inform ecological understanding and management. This is challenging because observational stream gauges are sparsely distributed and provide only point estimates of discharge. In this study, I developed models to simulate monthly discharge across river catchments. Due to the common issue of over-estimating low flows in discharge simulations, I also identified appropriate zero flow thresholds to mitigate this uncertainty. I quantified spatial and temporal patterns of flow intermittency for every stream segment within river networks of five major catchments in south-eastern Queensland (SEQ), eastern Australia. Results showed that the temporal dynamics of flow intermittency varied dramatically inter-annually over the period of 1900-2016, with the proportion of intermittent streams ranging in length from 3 % to nearly 100% of river networks, but there was no evidence of an increasing trend towards flow intermittency over this period. This approach to generating spatially explicit and catchment-wide estimates of streamflow intermittency can facilitate improved ecological understanding and management of intermittent streams. Compared with monthly discharge simulations, daily discharge simulations can provide more detailed representation of the dynamic aspects of hydrological processes and potentially enables more ecologically relevant characterisation of hydrology. However, models of daily stream flow are more complex and often need to take river routing processes into account. I developed models to simulate daily stream flows for contiguous sub-catchments across entire river networks in two hydro-climatically distinctive regions (SEQ vs. the Tamar River II catchment). I evaluated the models in terms of their ability to represent different ecologically important components of flow regime and quantified environmental correlates of differences in model accuracy within and between regions. The models showed generally good performance in both regions. However, average- and high flows were better predicted than low flows in SEQ because it is difficult to represent climate and hydrogeological processes influencing the low-flow part of the hydrograph. Spatial variation in flow characteristics revealed the highly dynamic nature of flow permanence in space and time, with intermittent flows affecting between 29% and 80% of the river network over the period of 1911-2017. I discuss the pros and cons of the applications of modelled monthly and daily flows, and conclude that the appropriate choice of modelling time step depends on the primary objectives of the research. The monthly time-step is suitable for quantifying ecologically relevant spatial and temporal variations in streamflow intermittency, but may be insufficient for studies aimed at quantifying ecological responses to short term flow events. The hydrological variability of intermittent streams poses challenges for resident aquatic biota which require access to permanent surface water-bodies to persist during dry spells and to recolonise suitable habitats when flows resume. However, research to quantify the dynamics and environmental determinants of variation in surface water extent is usually conducted over limited spatial and/or temporal extents. One of the biggest barriers to this kind of research is the difficulty in obtaining observed data of surface water extent across river networks. In this study, I demonstrated a newly-developed field method for rapid surface water assessment, and then developed predictive models relating observed water extent to environmental attributes at 241 surveyed stream segments in SEQ. I used the models to predict daily variations in surface water dynamics throughout entire river networks over the past century, based on available long-term environmental attributes. Descriptors of surface water extent could be accurately modelled, with good internal and external validation performance. Long-term variations in surface water extent were highly dynamic through space and time, although the overall length of river networks with surface water remained relatively stable from year to year. This study provides valuable insights into the potential priority conservation areas for aquatic biota across the study region. Systematic conservation prioritisation methods are increasingly being applied to freshwater ecosystems to identify candidate areas for ecosystem management and biodiversity protection. However, applications with emphasis on intermittent streams are scarce. The hydrological variability of intermittent streams means that the spatial distribution of dry season aquatic refuges within river networks and the temporal dynamics of hydrological connectivity between them are critical for the persistence of aquatic biodiversity. I developed a new approach to incorporating both surface water persistence and hydrological connectivity into systematic conservation prioritisation in intermittent streams. I also included multiple freshwater fish species distributions as explicit targets for habitat prioritisation, and incorporated estimates of their relative mobility to maximise potentially re-colonisable stream length from refuges. Compared with the situation without mobility, the inclusion of species mobility could significantly reduce the number of aquatic refuges required to meet the set conservation targets. High priority aquatic refuges were widely distributed across the study river networks, encompassing streams in various orders from main stems to headwaters. The research can help enhance both the resistance and resilience of freshwater biodiversity in intermittent stream ecosystems. The thesis concludes with practical learnings from these modelling studies for intermittent stream research and management, namely, 1) that discharge simulations (monthly or daily) throughout river networks confirmed the prevalence of intermittent streams and revealed the highly dynamic nature of flow intermittency over space and time; 2) that spatial and temporal dynamics of surface water availability within stream reaches can be modelled through the combination of observed surface water extent with long-term environmental attributes; and 3) that systematic prioritisation of aquatic refuges by incorporating both surface water persistence and hydrological connectivity enables to efficiently meet conservation targets for species representation and cost-effective conservation management. The thesis also concludes with future challenges and directions for intermittent stream research.