This case study outlines a pilot AquiferSim case study that was set up for the mid-Mataura Basin (Southland). AquiferSim is a model designed to help regional councils determine the likely long-term effects of land-use change (e.g. conversion from sheep to dairy) on groundwater in their region and how long before these cumulative effects will become problematic. This report was prepared by Landcare Research, ESR and Lincoln Ventures for Environment Southland and builds upon some preliminary data preparation work (Envirolink grant 963-ESRC147).

The objective of this project was to apply the AquiferSim groundwater model in a case study area in Southland to determine whether it adequately represents the groundwater processes in the Mataura Basin.

Methods:

  • GIS layers for the boundary, soil, land use, climate, aquifer transmissivity and river–aquifer fluxes were created from various sources including Environment Southland, S-Map and a parallel aquifer modelling study
  • Leaching rates were obtained from an Environment Canterbury report
  • An area of 45 x 35km was used with cell resolution of 50m
  • The Toposouth soils layer provides spatial information for most of the area of interest
  • Only one climate zone of 850mm rainfall was used
  • A land use layer was supplied by Environment Southland which was derived primarily from LCDB2 plus identification of dairy farms
  • A GIS layer of irrigated areas (parcels) was supplied by Environment Southland.  There was also some point information from a June 2011 survey for the location of winter grazing areas
  • The Environment Canterbury nitrate lookup table was taken as an approach in the first instance

Outputs:

The Generate Nitrate and Recharge tool (which is a stand-alone component used in AquiferSim) combines land use (farm type), soil, and climate layers together with the lookup table of leaching rates. The outputs from this tool (run in ArcGIS) are layers of predicted nitrate-N leached from below the root zone.

Mataura

 

 All outputs from the AquiferSim model in this project provide a long-term steady-state view of nitrate-N transport. The set of outputs include:

  • Flowpaths showing the horizontal movement of water/contaminants
  • 1D and 2D vertical slices below a point or flow path
  • Horizontal layers of average or maximum concentration of nitrate-N over
  • specific depth ranges (including the root zone)
  • Groundwater catchments of the lowland streams and gaining parts of the main
  • river
  • Estimates of nitrate-N loads to the lowland streams and gaining parts of the
  • main river

The design parameters of AquiferSim (which does not explicitly model any temporal dynamics) along with the data uncertainties mean that it is unrealistic to expect predictions at a specific time, location and depth to be accurate. However, results can be used to understand the general spatial pattern of the long-term mean annual nitrate-N contamination. At this stage we note that the modelled long-term distribution of nitrate-N in the mid-Mataura basin using AquiferSim is not inconsistent with the observed distribution of nitrate-N. 

As part of ESR’s Groundwater Assimilative Capacity research programme, a comparison of the AquiferSim with a steady-state version of the Modflow/MT3D model will be undertaken. This project will investigate the differences between the model outputs and determine the impact of the assumptions and simplifications underlying AquiferSim, as well as assess the advantages/disadvantages of using AquiferSim over Modflow/MT3D. If the evaluation confirms that AquiferSim adequately represents the groundwater processes in the Mataura Basin then it could be used to explore the impact of further land intensification (by running some ‘what if’ simulations), and to help council and stakeholders to better appreciate and understand the dynamics of the mid-Mataura groundwater system.

Recommendations

Further evaluation of the results is recommended (and planned for within the parallel study) before AquiferSim is used to explore alternative land-use scenarios.

Associated Models

  • AquiferSim

    AquiferSim is a regional-scale model of nitrate transport in groundwater and is part of the research programme ‘Integrated Research for Aquifer Protection (IRAP)’.

    More info

Links

Envirolink Report - http://www.envirolink.govt.nz/Envirolink-reports/1-NLCC1/1004-ESRC238/ 

Mataura (Southland) Landcare Research, ESR, Lincoln Ventures Ltd. Lilburne, L., Moore, C., Bidwell, V. and Burbery, L., 2012. AquiferSim modelling of the mid-Mataura Valley. Envirolink 1004-ESRC238, Prepared by Landcare Research for Environment Southland.

References

Nitrate discharge to groundwater from agricultural land use: an initial assessment for the Canterbury Plains (May 2009)

Contributors: Vince Bidwell, Lincoln Ventures

Linda Lilburne, Landcare Research

Mike Thorley, Environment Canterbury

David Scott, Environment Canterbury

Report:  http://ecan.govt.nz/publications/Reports/report-on-nitrate-discharge.pdf