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Groundwater modelling on the upper Waikato catchment: stage 2

TR 2012/13
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Report: TR 2012/13

Author: J Weir (Aqualinc) and C Moore (ESR)

Abstract

Groundwater in the upper Waikato catchment is a valuable resource for agriculture, water supply, forestry and industry. Groundwater quality is naturally high, but there are indications that this quality is deteriorating as a result of existing land use intensification and deforestation. Compounding this concern is the very substantial lag time between land use changes and the realisation of subsequent effects on groundwater and surface water quality. It is expected that the effects of land use changes have not yet fully manifested, and additional intensification may take years to fully develop, further compounding the deterioration.

Waikato Regional Council has consequently proposed a long-term programme to develop a groundwater model to assist managing water quality and appropriate policy development within the catchment.

The first stage of the investigation was completed in 2010 and marked the beginning of the longer-term investigative and modelling programme. The stage 1 work focused on choosing a suitable modelling platform for the long-term investigative programme. The purpose of the stage 2 investigation was to construct a numerical groundwater model (both flow and transport) for the study area. This report documents work completed on stage 2 and makes recommendations for future investigations.

Groundwater modelling on the upper Waikato catchment: stage 2 [PDF, 9.3 MB]

Contents
  Executive summary 1
1 Introduction 5
1.1 Project purpose 5
1.2 Stage 1 investigation 5
1.3 Stage 2 investigation 6
1.4 Study area 7
1.5 Project collaboration 10
1.6 Key personnel 10
1.7 Disclaimers, acknowledgemetns and copyright statements 10
1.7.1 Data supplied by Waikato Regional Council 10
1.7.2 Land resource inventory 10
1.7.3 Geological information from GNS Science 10
1.8 Data collation and analysis 10
1.9 Report structure and objectives 11
2 Grid discretisation and transport time step trials 12
2.1 Grid sizes and transport time steps trialled 12
2.2 Results 12
3 Model development 14
3.1 Re-assessment of the geological representation 14
3.2 Groundwater level data 17
3.3 Refinement of the river boundaries and measured river gains 17
3.4 Aquifer test data 18
4 Model calibration 19
4.1 Parameterisation and observation weighting 19
4.2 Calibration results 19
4.2.1 Fit to measured groundwater levels 19
4.2.2 Fit to measured flow gains in the Waikato River 23
4.2.3 Model groundwater budgets 24
 4.2.4 Calibrated parameter values 24
4.2.5 Parameter sensitivities: calibration 27
4.2.6 Parameter uncertainty 32
4.2.7 Parameter confidence intervals and parameter uncertainty 36
4.2.8 Parameter correlations and Eigen values 37
4.2.9 Calibration of groundwater age, travel times and model porosity 38
4.2.10 Comparison of grounwater nitrate-nitrogen concentrations 39
4.2.11 Comparison of Waikato River nitrate-nitrogen concentrations 41
5 Predictive transport simulations 44
5.1 Predictive scenario 1: intensive dairying everywhere 44
5.2 Predictive scenario 2: timing of regional effects on rivers 46
6 Considerations for future work 48
7 References 49
  Appendix A: Data collection and analysis 51
  Appendix B: Hydrogeological descriptions supplied by GNS Science 71
  Appendix C: MODFLOW grids for the grid discretisation trials 84
  Appendix D: Reproduction of figure 3.2 from Collier et al (2010) 87