Cox, S.C. 2025 Tauranga shallow groundwater monitoring, spatial observations and forecast conditions under sea-level rise. Lower Hutt, N.Z.: GNS Science. GNS Science report 2024/47. 57 p.; doi: 10.21420/GDWW-6Z50
Abstract
Tauranga City in the North Island of New Zealand has assets and built infrastructure sitting on low-lying coastal land, where groundwater is unconfined and close to the surface. Shallow groundwater limits the volume of unsaturated ground available to store rain and runoff, promotes flooding and creates opportunities for infiltration into stormwater and wastewater networks. Groundwater levels are expected to rise as sea-level rises, causing greater frequency of flooding and/or direct inundation once it nears the ground surface and becomes emergent. Spatial analysis has been carried out on data gathered between 20 August 2015 and 13 August 2021 from Tauranga City Council's shallow groundwater monitoring network. A series of statistical surfaces represent the ‘present-day’ water-table elevation and depth to groundwater, as well as the response to tidal forcing. Simple geometric models have been developed using the shape and position of the water table, combined with tidal fluctuations, to represent median (p50), mean high water spring and storm-surge groundwater levels. Future forecast elevations for these surfaces are provided at 10 cm increments of sea-level rise (up to 1 m), then 20 cm increments (from 1 to 2 m). The geometric models are strongly empirical, with many implicit assumptions and caveats – particularly, these do not account for groundwater flow nor possible changes in water-budget mass balance related to changing climate and pattern of precipitation. Many variables and controlling processes are simplified into a single parameter of groundwater elevation. The future forecasts assume that groundwater will rise at the same rate as sea level, which is likely to be a ‘worst case’ scenario of the situation under sea-level rise. Future models provide a first-pass conservative analysis of groundwater elevation and its range, based on current observations and levels of precipitation, likely to be a maximum groundwater-related contribution to hazard and how this will evolve over time.The aim of this report is to provide some background documentation on the methodology and geospatial analysis used to generate data in an accompanying archive. These data map and define the present state of shallow groundwater in Tauranga and its forecast future condition, which is documented here, and is likely to become a useful tool for planning and natural-hazard analysis. The report is not intended to provide a comprehensive review of the shallow groundwater system(s) (e.g. source, flow direction and flux volumes) nor of the relationships between groundwater levels and rainfall or local infrastructure, which may be addressed in future work. The GIS data to accompany this report are provided under a Creative Commons Attribution 4.0 (CC-BY-4.0) license without warranty freely available from a Zenodo data archive (Cox and Easterbrook-Clarke 2025) (auth)
