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Source: Environment Canterbury Regional Council

1. Look Up table for Waimakariri nitrate modelling

The look up table below was not released as part of the supporting material when proposed Plan Change 7 was notified. At the request of an interested party during the submission period, this has now been made available.

The look up table reflects the adjustments described in Section 2.4.3 of “Preparation of land use and nitrogen-loss data for the Waimakariri Zone limit-setting process” (Lilburne et al, 2019), and only includes the look up table rows associated with Matrix of Good Management Practice Overseer files – the rows describing the arable, sheep, beef and deer, and dairy land uses.

Waimakariri modelling

The two reports below were not released as part of the supporting material when proposed Plan Change 7 was notified. At the request of an interested party during the submission period, these have now been made available.

2. Nitrogen concentrations

A request was made by the Waimakariri Primary Industry Network Group quarterly meeting held on 23 August 2019 for a table that sets out, for each waterbody (including groundwater) the current measured nitrate concentration, predicted nitrate concentration (under Current Pathways) and the predicted nitrate concentration under proposed Plan Change 7 provisions.

Compiling all the data into one table is difficult because groundwater and surface water have different data sources and assumptions. However, we have provided the relevant information for private water supply well areas, Waimakariri District Council (WDC) community supply wells, Christchurch deep aquifer areas and surface water bodies in the following tables.

These tables were collated from tables included in the Waimakariri Land and Water Solutions Programme Options and Solutions Assessment Nitrate Management report (Kreleger and Etheridge, 2019, Environment Canterbury). We note that the predicted nitrate concentration under Plan Change 7 included in the following table is the target nitrate concentrations provided in Plan Change 7.

Regarding the request that the table identify the trend of concentrations, nitrate concentration trends for surface water bodies were calculated and are included in the tables.

Trends in nitrate concentration for the private water supply areas and the WDC community supply wells were not calculated. Monitoring results for the Christchurch deep aquifer show that nitrate concentrations are increasing over time, but remain very low – between 0.1-0.8 mg/L.

Our Current State of Groundwater Quality in the Waimakariri CWMS zone report (Scott et al., 2016, Environment Canterbury) noted that groundwater nitrate concentrations in two of our long-term monitoring wells, at Eyrewell and Ohoka, are increasing.

Concentrations have increased from around 6.5 mg/L to 7.5 mg/L nitrate at our monitoring site in Ohoka and from 4.5 to 7 mg/L at Eyrewell over the past 10 years. Data from the Springbank monitoring well near the Cust River show a decreasing trend in nitrate concentrations from nearly 16 mg/L to below the drinking-water Maximum Acceptable Value (MAV). Nitrate concentrations are generally increasing in the Kaiapoi River catchment.

The report further notes that a declining or increasing nitrate trend should be interpreted with caution, and within the context of the other processes (such as climate variability and lag times) which affect groundwater and stream nitrate concentrations.

GMP and Current Pathways – Nitrate modelling results for Private Well Supply Areas (PWSAs)

Adjusted from Table 4-8 Nitrate management report (Kreleger and Etheridge, 2019)

Concentrations are presented in 50th percentile model results, with 5th and 95th percentile results between brackets.

Private Well Supply Areas Current measured Nitrate-N (mg/L) Current pathways calculated Nitrate-N (mg/L) PC7 Nitrate-N target(mg/L) Predicted Nitrate-N under PC7 (mg/L)
Clarkville 4.4 8.2 5.65 5.65
  (0.5 – 9.4) (5.0-11.7)    
Cust 4.4 6.7 5.65 5.65
  (0.05 – 8.8) (3.9-9.7)    
Eyreton (shallow) 5.2 12.3 5.65 5.65
  (0.6 – 9.6) (8.3-16.6)    
Eyreton (deep) 5.2 15.2 5.65 5.65
  (0.6 – 9.6) (7.4-24.0)    
Fernside 3.7 4.9 5.65 <4.9</td>
  (0.04 – 8.8) (2.2-7.8)    
North East Eyrewell (shallow) 3.6 4.9 5.65 5.65
  (0.7 – 7.0) (2.5-13.6)    
North East Eyrewell (deep) 3.6 6.6 5.65 5.65
  (0.7 – 7.0) (4.0-11.5)    
Flaxton 4.4 3.5 5.65 <3.5</td>
  (0.05 – 8.8) (2.0-6.3)    
Horellville 3.7 4.6 5.65 <4.6</td>
  (0.04 – 8.8) (2.2-7.2)    
Mandeville 4.4 4.8 5.65 <4.8</td>
  (0.05 – 8.8) (2.3-8.9)    
North West Eyrewell (shallow) 3.6 6.3 5.65 5.65
  (0.7 – 7.0) (2.0-12.5)    
North West Eyrewell (deep) 3.6 7.7 5.65 5.65
  (0.7 – 7.0) (2.1-14.5)    
Ohoka (shallow) 4.4 6.3 5.65 5.65
  (0.05 – 8.8) (4.0-8.7)    
Ohoka (deep) 4.4 7.5 5.65 5.65
  (0.05 – 8.8) (4.4-10.9)    
Rangiora 0.5 2.7 5.65 <2.7</td>
  (0.3 – 0.7) (0.4-6.7)    
Springbank 3.7 6.6 5.65 5.65
  (0.04 – 8.8) (4.0-9.5)    
Summerhill 3.7 6.6 5.65 5.65
  (0.04 – 8.8) (5.0-16.1)    
Swannanoa (shallow) 3.7 7.1 5.65 5.65
  (0.04 – 8.8) (3.0-12.1)    
Swannanoa (deep) 4.4 8.4 5.65 5.65
  (0.05 – 8.8) (4.4-12.5)    
Waikuku 0.8 1.3 5.65 <1.3</td>
  (0.03 – 3.8) (0.6-3.5)    
West Eyreton (shallow) 3.7 5.6 5.65 <5.6</td>
  (0.04 – 8.8) (2.8-11.1)    
West Eyreton (deep) 3.7 6.3 5.65 5.65
  (0.04 – 8.8) (3.7-9.3)    
Woodend – Tuahiwi 0.8 2.8 5.65 <2.8</td>
  (0.03 – 3.8) (0.8-6.4)    

3. Particle tracking modelling

The modelling files below were not released as part of the supporting material when proposed Plan Change 7 was notified. At the request of an interested party during the submission period, these have now been made available. The reports which this modelling informed are available under ‘Supporting documents and disciplines’.

What is NetCDF as a groundwater model output format?

The USGS MODFLOW code has become the most widely used groundwater flow code throughout the world since its release in 1989. Because MODFLOW is a plain FORTRAN code with no graphical user interface (GUI) or visualisation capabilities, model results visualisation and analysis is usually done with commercial or open-source packages, and self-made FORTRAN snippets.

The output format of MODFLOW is a FORTRAN binary which may vary depending on compilers and platforms. NetCDF, on the other hand, is a standardised, sharable and compact format which can be read and visualised with numerous free and commercial packages including R. It is also possible to embed useful geospatial information like coordinates, projection and grid discretisation in the NetCDF which are absent in the FORTRAN binary.

Using NetCDF as a standard model output format would allow modelers and non-modelers to easily share, visualise and plot model results using readily available software (R, ArcGIS, MS Excel, Paraview, GRASS GIS, SAGA GIS…etc). NetCDF is a particularly good format for storing large, multidimensional datasets.

Many NetCDF tools were designed for the climate community, whose datasets are often orders of magnitude larger than datasets typically used in groundwater modeling. In this study R was used to generate a NetCDF file from a MODFLOW binary output and example analyses and visualisations were implemented. R has extensive statistical and plotting capabilities which are available to the user once MODFLOW outputs are available in NetCDF format.

Source: The case for NetCDF as a groundwater model output format using R: Example using USGS MODFLOW
Authors: Coulibaly, K. M.; Barnes, M.; Barnes, D.
Affiliation: AA (Schlumberger Water Services, Fort Myers, FL, USA;, AB (Chesapeake Research Consortium, Chesapeake, MD, USA;, AC (Schlumberger Water Services, Fort Myers, FL, USA;
Publication: American Geophysical Union, Fall Meeting 2011, abstract id. H11B-1059

4. Nitrate loading

The folder below was not released as part of the supporting material when proposed Plan Change 7 was notified. At the request of an interested party during the submission period the raster products showing estimates of diffuse nitrate (N) losses and drainage rates beneath the root zone, for the current, Plan Change 5 and Zone Implementation Programme Addendum scenarios described in Lilburne et al. (2019) have been made available.

A readme file provides some metadata linking the raster files to the modelled scenarios and outputs.