An understanding of the interaction between groundwater and surface water systems is fundamental to the effective management of water resources. Importantly, groundwater discharge to streams often accounts for a significant proportion of total streamflow, particularly during low-rainfall periods. This study has applied a consistent approach to classifying streams and assessing groundwater discharge to streams in order to characterise streamflow and baseflow patterns in different geographic regions across Australia.
Historical streamflow data for 233 Australian stream gauges were analysed. Long-term and seasonal (high-flow and low-flow) baseflow indices (BFI) were determined using the Lyne and Hollick recursive digital filter to quantify the relative baseflow component of streamflow. Classifying streams into perenniality and seasonality groups highlights distinct characteristics of baseflow regarding timing, magnitude and variability.
Five distinct perenniality groups were identified by hierarchical clustering of percentage flow days. Non-perennial streams (flow < 90% of the time) have a long-term mean BFI of 0.25 while perennial streams (flow > 90% of the time) have a long-term mean BFI of 0.46.
Two overarching seasonal flow regimes (Summer or Winter-dominant) or eight sub-groups were distinguished by hierarchical clustering of mean monthly streamflow data. Streams within each of the seasonal flow regimes have a characteristic streamflow and baseflow pattern. All streams assessed in Northern Australia are Summer-dominant; a number of these streams have sustained baseflow throughout the dry season. Winter-dominant streams predominate in the southern half of Australia. The timing and relative discharge of groundwater to streams vary between seasonal flow regimes due to differences in the dominant sources of water to the stream at different times of the year.
Knowledge of stream perenniality, seasonal streamflow regimes and the respective streamflow and baseflow relationships is important from a water management perspective, particularly for conjunctive use of groundwater and surface water resources.