Lake Rotorua

Lake Rotorua

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Reference Files

Reference files provide a point of reference for farms within the Lake Rotorua groundwater catchment and are used to describe the position of any farm against the average (for the sector). Reference files allow a target to be described as a percentage and allow changes in OVERSEER® modelling to be accommodated in these targets.

What are Reference Files?

Reference files have been created for a hypothetical dairy and a drystock properties that:

·         Have geophysical characteristics that are representative of the catchment (soil, slope and rainfall)

·         Represent the average nitrogen loss for the sector

·         Are modelled as economically viable systems.

The reference file outputs in OVERSEER® equal the average nitrogen loss for the sectors (dairy and drystock) under the allocation version – 6.2.0. The reference file output is 100%. The exact OVERSEER® inputs used for the reference files are detailed in Methodology for creation of NDA reference files and stocking rate table August 2015 (Perrin Ag)

How are they used?

All blocks within the catchment can then be “referenced” against this average as a percentage rather than a fixed number to give a NDA.

For example, if under OVERSEER® 6.2.0 Farm A has an output of 50 and the reference file output under the same version is 40 then Farm A’s NDA is 125% of the reference file. For any future changes to OVERSEER® the NDA target remains constant for Farm A (at 125%).


OVERSEER® version

Sector Reference File

Block NDA

% of Reference File = NDA expressed as a %

Allocation version




Version X




Version Y




Version Z
















The start point (benchmarks) and managed reduction targets can all be expressed as a % of the reference file. A further example is provided in Schedule Five of the rules.

Why are they being used in the rules?

The purpose behind reference files is threefold.

1.     To have a methodology that means we don’t have to continually be re-setting farm targets

2.     To have a mechanism that dampens down the potential for change to a property’s nutrient target as a result of OVERSEER® version changes.

3.     The reference file methodology allows us to take advantage of enhanced OVERSEER® functionality as it develops and is enhanced into the future.

Changes can arise if a new version of OVERSEER® brings in new information that impacts more on one part of the catchment than another. For example, a change to OVERSEER® may see the catchment loss increase by 10% but farms on soil type X increase by 30%. The uncertainty that this would create is not acceptable – nor is fixing an OVERSEER® value at one point in time. Using the reference file approach shares the risk that an individual farm might go up or down more than the average between all the farms.

While the nitrogen loss OVERSEER® value may change, the relative effort for mitigation should also change in a proportional way. Using a simple example, if the nitrogen loss from stock on soil type X goes up by 10% - the mitigation through reducing stock numbers also goes up by 10%.

The required catchment nitrogen reductions do not change through OVERSEER® version changes.

What happens with future OVERSEER® change?

If in the future new information is added (such as new soil information is added or drainage is recalculated) then the reference file changes its OVERSEER® output. This table shows the outputs of the reference files in the different versions of OVERSEER®. 


OVERSEER® version


Released April 2015


Released November 2015


Released May 2016 


Released November 2016

Dairy 65.41 58.3 70.83 70.49
Drystock, grazed trees    25.35 24.1 27.40 27.41
House 78.0 78.0 78 78
Bush and scrub, wetland          3.0 3.0 3.0 3.0
Forestry 2.5 2.5 2.5 2.5

At any point a farm can know where they stand (by looking at the current OVERSEER® output in relation to the reference file) and what their mitigation is worth in relation to meeting their required reductions.