Modelling monthly streamflows in two Australian dryland rivers: Matching model complexity to spatial scale and data availability

SummaryThere is new pressure to develop some of Australia’s dryland river basins for irrigation as development on highly regulated rivers has halted with increasing ecological degradation. Sparse populations, limited infrastructure, and minimal water resource development in Australian dryland river basins mean streamflow records are few and typically short in duration. The Paroo and Warrego basins (74,003 and 62,908 km2, respectively; largely undeveloped) in the northwest of the Murray-Darling Basin of Australia, each have four streamflow gauges with more than 15 years of daily record, and four and eight years, respectively, of coincident record. These short records do not fully characterise the highly variable flow regimes. Improved characterisation of the flow regime and determination of the potential impacts of water resource development can be based on streamflow simulation using climate data. Numerous floodplain lakes and wetlands are important ecological assets in these basins. For these environments impacts will be a result of changes in the higher (above mean) flows, which can be reasonably assessed by changes in monthly streamflows. However, high spatial and temporal variability of runoff generation, and the delicate hydrologic balance (runoff is a small fraction of rainfall) in dryland regions means the relationship between rainfall and runoff is complex, and a daily time step model was therefore required to adequately simulate monthly streamflow. It was neither necessary, nor (because of limited data) possible to employ a complex spatially distributed model. Use of a highly parameterised, lumped-parameter conceptual model was also deemed unnecessarily complex, however, it was recognised that a simple black box model was unlikely to be successful. As a compromise, we implemented a simple conceptualisation of the daily water balance in PC spreadsheet software. The model was calibrated and validated using monthly streamflow volumes. The model accounts for 76–88% and 60–81% of the variance in monthly streamflows in the Paroo (four stations) and Warrego (three stations) basins, respectively. We use the model to explore the relative importance of local and catchment rainfall in flood generation, and to predict the timing and magnitude of large floods in these basins over the last 100 years.