Applications of digital baseflow separation techniques for model validation, Wairarapa valley, New Zealand

The representation of groundwater processes in hydrological models is crucial, as the connectivity between groundwater and surface water is significant. It is particularly important for regions such as the Wairarapa that experience high water stresses. Intensified agriculture has increased demand for irrigation, which can lead to depletion and degradation of reservoirs. This study compared observed streamflow records to TopNet-0 and TopNet-GW model outputs at points along the Mangatarere stream, a sub-catchment in the Wairarapa valley, New Zealand. Model performance was assessed using a suite of quantitative and qualitative comparisons. This analysis aimed to assess the similarities and differences between observed flow and the model outputs with respect to their model structures. Baseflow estimates from recursive digital filters were also compared at these sites to assess the groundwater representation of the models. The investigation can be considered representative of the wider Ruamahanga catchment, as the geology and hydrology in the region is relatively analogous. Flow infilling and baseflow separation was undertaken at 13 Wairarapa flow gauges to provide considerations to the model outputs. Options investigated for flow infilling included a straight infill or calculation of the flow difference at each point. Potential multipliers included a long-term or a monthly option. The difference infill, coupled with the long-term multiplier, was found to be the optimum method. Independent baseflow estimates included a Q90/Q50 flow duration curve index and indices generated from the Eckhardt and Bump & Rise recursive digital filters. The two digital filters produced similar statistics but were found to employ uncertain parameters that significantly affect outputs. TopNet-GW benefitted from up-to-date calibrations and as such produced generally excellent simulations in comparison to observed streamflow. With the addition of the deep groundwater conceptual reservoir in the structure of the model, simulated flow baseflow index estimates and graphical assessment of flow recession curves indicate TopNet-GW reproduces groundwater processes well despite potential over-representation of baseflow at the expense of high flow periods during peak flows. These findings highlight the importance of combining subsurface and surface flow dynamics to resolve water management issues and improve model performance at the catchment scale