Development of hydrogeological information to evaluate national-scale hydrological parameters
Creators: name: Westerhoff, R
name: Tschritter, C
name: White, PA
Publisher: name: GNS Science
Year: 2017

description: The National Institute for Water and Atmospheric Research established, and is leading, the ‘National Hydrology Program’ (NHP) research project. NHP, is a five-year project that aims to develop hydrological understanding across the New Zealand landscape, through combining surface water, soil, and geological information. GNS Science has been sub-contracted to provide subsurface and groundwater information, with the long-term aim to enable a better coupling of surface water to groundwater in the National Hydrological Model (TopNet). A major initial task of the NHP project was to set up and test the geospatial framework, including combination and compilation of different datasets in a framework suitable for input to TopNet. In this report, the work carried out by GNS Science in the geospatial framework task in Year 1 of the NHP project is presented. This includes the development of the first version of national-scale datasets of three geologically derived hydrological parameters as spatial data layers (e.g., hydraulic conductivity (K); effective porosity (φe); and depth to hydrogeological basement). Furthermore, methods for estimation of K and φe over depth are presented. At the national scale, the resulting datasets of K and φe follow the expected patterns, e.g., high values in the alluvial plains, medium values in the central volcanic region and low values in basement and Tertiary rocks. Also, the calculated depth to hydrogeological basement provides a general idea of the potential basement depth. These parameters are accurate in areas where basement is at the ground surface and seems adequate in some coastal plains such as the Hauraki and Canterbury Plains. However, in other areas, e.g., coastal plains and the central volcanic region, the calculated basement depth appears too shallow. The results were evaluated in the Southland region using a 3D geological model previously developed by GNS Science, and field data provided by Environment Southland. This evaluation showed that the estimates of K and depth to hydrogeological basement of this study followed the same spatial pattern as the 3D geological model, but showing more spatial detail. However, field-observed K values were magnitudes higher than the NHP K values estimated in this study, and depths to hydrogeological basement in this study were shallower than earlier estimates of a 3D groundwater flow model. The nationwide and regional findings lead to the conclusion that the methods used for this nationwide approach are warranted, but that input parameters should be adjusted. Specifically, future research is recommended to include testing of increased values of near-surface K for gravels and sand. These increased K estimates will also lead to improved estimates of depth to hydrogeological basement. These estimates can be further improved using additional information on geological system knowledge and possibly with calibration to field-observed values in a national groundwater model. (auth)