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. Many studies underline the importance of groundwater assessment at thelarger, i.e. global, scale. The groundwater models used for theseassessments are dedicated to the global scale and therefore not oftenapplied for studies in smaller areas, e.g. catchments, because of theirsimplifying assumptions. In New Zealand, advanced numerical groundwater flow models have been appliedin several catchments. However, that application is piecemeal: only for alimited amount of aquifers and through a variety of groundwater modelsuites, formats, and developers. Additionally, there are large areas wheregroundwater models and data are sparse. Hence, an inter-catchment,inter-regional, or nationwide overview of important groundwater information,such as the water table, does not exist. The investment needed to adequatelycover New Zealand with high-resolution groundwater models in a consistentapproach would be significant and is therefore not considered possible at this stage. This study proposes a solution that obtains a nationwide overview ofgroundwater that bridges the gap between the (too-)expensive advanced localmodels and the (too-)simple global-scale models. We apply an existing,global-scale, groundwater flow model and improve it by feeding in nationalinput data of New Zealand terrain, geology, and recharge, and by slightadjustment of model parametrisation and model testing. The resultingnationwide maps of hydraulic head and water table depths show that the modelpoints out the main alluvial aquifers with fine spatial detail (200 m gridresolution). The national input data and finer spatial detail result inbetter and more realistic variations of water table depth than the original,global-scale, model outputs. In two regional case studies in New Zealand, thehydraulic head shows excellent correlation with the available groundwaterlevel data. Sensitivity and other analyses of our nationwide water tablesshow that the model is mostly driven by recharge, model resolution, and elevation(gravity), and impeded by the geology (permeability). The use of this first dedicated New Zealand-wide model can aid in provisionof water table estimates in data-sparse regions. The national model can alsobe used to solve inconsistency of models in areas of trans-boundary aquifers,i.e. aquifers that cover more than one region in New Zealand. Comparison of the models, i.e. the national application (National Water Table model: NWT) with theglobal model (Equilibrium Water Table model: EWT), shows that most improvement is achieved by feeding in betterand higher-resolution input data. The NWT model still has a bias towardsshallow water tables (but less than the EWT model because of the finer modelresolution), which could only be solved by feeding in a very high resolutionterrain model that incorporates drainage features. Although this is a modelshortcoming, it can also be viewed as a valuable indicator of the pre-humanwater table, i.e. before 90 % of wetlands were drained for agriculture sinceEuropean settlement in New Zealand. Calibration to ground-observed water level improves model results but can ofcourse only work where there are such data available. Future research shouldtherefore focus on both model improvements and more data-driven,improved estimation of hydraulic conductivity, recharge, and the digitalelevation model. We further surmise that the findings of this study, i.e.successful application of a global-scale model at smaller scales, will leadto subsequent improvement of the global-scale model equations.

Application of an improved global-scale groundwater model for water table estimation across New Zealand