Towards baseflow index characterisation at national scale in New Zealand

Abstract Streamflow is typically divided into two components for hydrograph separation, quickflow and baseflow. Baseflow is the portion of streamflow that contains groundwater flow and flow from other delayed sources and is of key importance for river basin ecology and water resources planning and management. The BaseFlow Index (BFI) is defined as the ratio of long-term mean baseflow to total streamflow. Knowledge of the BFI is not directly available for ungauged catchments and hence for most of the terrestrial land surface. In this study, the BFI was determined for all river reaches in New Zealand. First a recursive digital filtering technique was applied to separate baseflow from total streamflow for 482 gauged sites across New Zealand, whereby an individual filter parameter was determined for each catchment. Based on the baseflow and total streamflow data the long-term BFI for each gauged site was determined, as well as seasonal values of BFI. BFI varies between 0.20 and 0.96 with an average of 0.53, which indicates that 53% of long-term streamflow in New Zealand is likely to originate from groundwater discharge and other delayed sources. Long-term BFI values for all river reaches that comprise the New Zealand river network were predicted using the random forest technique. Furthermore, the winter to summer BFI for all river reaches in New Zealand were also determined. Distinct spatial patterns of the BFI were identified. While the spatial distribution and the magnitude of the BFI was determined by a combination of factors, certain patterns can be attributed to geological formations in New Zealand, namely the volcanic plateau region and the Southern Alps. While the dataset determined in this work can support work specifically pertaining to water resources planning and management in New Zealand, in particular water supply, stream ecology and pollution risk, the methodology devised to calculate the BFI for gauged sites and to predict the BFI for ungauged sites is applicable to any region around the world.