Application of an improved global-scale groundwater model for water table estimation across New Zealand
Authors: Westerhoff Rogier , White Paul , Miguez-Macho Gonzalo 
Published: December 2018

Abstract. Many studies underline the importance of groundwater assessment at the
larger, i.e. global, scale. The groundwater models used for these
assessments are dedicated to the global scale and therefore not often
applied for studies in smaller areas, e.g. catchments, because of their
simplifying assumptions. In New Zealand, advanced numerical groundwater flow models have been applied
in several catchments. However, that application is piecemeal: only for a
limited amount of aquifers and through a variety of groundwater model
suites, formats, and developers. Additionally, there are large areas where
groundwater 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 adequately
cover New Zealand with high-resolution groundwater models in a consistent
approach would be significant and is therefore not considered possible at this stage. This study proposes a solution that obtains a nationwide overview of
groundwater that bridges the gap between the (too-)expensive advanced local
models and the (too-)simple global-scale models. We apply an existing,
global-scale, groundwater flow model and improve it by feeding in national
input data of New Zealand terrain, geology, and recharge, and by slight
adjustment of model parametrisation and model testing. The resulting
nationwide maps of hydraulic head and water table depths show that the model
points out the main alluvial aquifers with fine spatial detail (200 m grid
resolution). The national input data and finer spatial detail result in
better and more realistic variations of water table depth than the original,
global-scale, model outputs. In two regional case studies in New Zealand, the
hydraulic head shows excellent correlation with the available groundwater
level data. Sensitivity and other analyses of our nationwide water tables
show 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 provision
of water table estimates in data-sparse regions. The national model can also
be 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 the
global model (Equilibrium Water Table model: EWT), shows that most improvement is achieved by feeding in better
and higher-resolution input data. The NWT model still has a bias towards
shallow water tables (but less than the EWT model because of the finer model
resolution), which could only be solved by feeding in a very high resolution
terrain model that incorporates drainage features. Although this is a model
shortcoming, it can also be viewed as a valuable indicator of the pre-human
water table, i.e. before 90 % of wetlands were drained for agriculture since
European settlement in New Zealand. Calibration to ground-observed water level improves model results but can of
course only work where there are such data available. Future research should
therefore focus on both model improvements and more data-driven,
improved estimation of hydraulic conductivity, recharge, and the digital
elevation model. We further surmise that the findings of this study, i.e.
successful application of a global-scale model at smaller scales, will lead
to subsequent improvement of the global-scale model equations.