Modelling the flow regime of arid zone, floodplain rivers

The requirements of ecological studies and water resource management plans are driving demand for hydrological models of the rivers of the arid zone. Knowledge of the hydrology of Australia’s arid zone is poor, yet is critical in understanding the ecology of the region. The research presented in this thesis seeks to address some shortcomings in our understanding of the hydrology of the Australian arid zone. In particular, the research examines the requirements for modelling the flow regime of arid zone rivers, concentrating on the rivers of the Lake Eyre Basin (LEB). The LEB has exceptionally low annual runoff of 3.5 mm, its major rivers develop over extremely low gradients and are characterised by very wide floodplains and complex anastomosing flow paths in their mid to lower reaches. This research was driven by both a practical and theoretical impetus. Practically, hydrological data were required at the waterbody scale for a large number of sites across three river systems of the LEB, for use in a study, known as ARIDFLO, of the ecological responses to hydrological conditions. Because of the remoteness of these sites and the paucity of gauging stations on these rivers, modelling of the rivers was the only method for delivering the required discharge data. Theoretically, the challenge was set for creating hydrological models for some extraordinarily complex river systems, in terms of their size, catchment characteristics and flow regime variability. A grid-based, semi-distributed, daily time-step conceptual model was developed and applied to six river reaches covering areas between 10,000 to 60,000 km and encompassing three river systems ranging in catchment area up to 330,000 km. The model simulated the complex flow paths and large transmission losses of these rivers. Analyses of observed data guided the model development and provided insight into hydrological processes operating in the LEB, including: • Spatial variability of rainfall, • Runoff processes, • Streamflow transmission losses, • Surface water – groundwater interactions, • Salinity variations of surface waterbodies. The grid-based structure of the model required innovative algorithms to control cell connectivity during flow routing in the wide middle and lower reaches of the LEB rivers but allowed the representation of moderate amounts of variation in spatial characteristics of the catchment. Threshold cell volumes, determined from a combination of flow information and remote sensing, were used to determine when changes in connectivity occurred for each grid cell on a daily time-step. Importantly, the model structure was flexible and allowed further spatial characterisation of catchment attributes that could increase model performance and accuracy, through the definition of new land-types, as well permitting the generation of output discharge data for any number of internal cells. The definition of flow paths was found to be critical for modelling the timing and transmission losses of flood events in the mid and lower reaches of the large rivers of the LEB. Satellite images provided an a priori means to further develop, constrain and calibrate hydrological models of these reaches, in addition to the traditional use of gauging station data.