Overseer: modelling nitrogen losses from farmland

Estimating the loss of nutrients from various land uses, and in particular intensive farming land uses, is very topical. The recently amended National Policy Statement for Freshwater Management now requires regional councils to set water quality limits for water bodies based on the value the local community feel the water body holds.

The responsibility to set limits within New Zealand’s water bodies leaves councils with the problem of how to quantify and control non-point source discharges of nutrients, in particular nitrogen. As a potential solution to this, Liburne and colleagues looked at ways to estimate nitrate leaching from rural land uses in Canterbury in 2010. This technical report was produced by Environment Canterbury to facilitate a review of the council’s approach to managing the effect of land use on the region’s water quality. Lincoln University played a key role in providing data to support the development of the report from their research farms as well as Lincoln staff providing technical input for the report itself.

The negative impacts on water quality from farming, and in particular intensive farming are well documented, Forest and Bird include a concise summary on their website. In particular, intensive farming can lead to increased leaching of nutrients such as nitrogen which can degrade both surface and groundwater quality. Erosion and land clearance can also lead to increased sediment and phosphorus entering our waterways (Figure 1) and, of course, more cows means more methane which is a primary greenhouse gas. Increasing demand for irrigation water puts pressure on our waterways and causes tension between agriculture and competing uses, such as recreation, electricity generation, ecosystem services and community drinking water supplies. Removal of water for irrigation results in lower peak and mean flows, exacerbating issues with water quality and potentially decreasing the occurrence of flushing flows.


Figure 1: Sedimentation of a Waterbody During Installation of a Crossing (Source: Isobel Oldfield)
Figure 1: Sedimentation of a Waterbody During Installation of a Crossing (Source: Isobel Oldfield)

The Liburne and colleagues technical report focuses on one aspect of these negative impacts, by attempting to quantify the loss of nitrate nitrogen from rural land uses. A number of models were used to establish what Liburne and colleagues termed a set of look up nitrogen leaching values, or put another way, a table of reference for nitrate leaching values for various land uses. These values were developed through a series of workshops. The first, held in May 2008, concluded that existing information was inadequate and that modelling would be required to provide sufficient information. Four models were proposed: Overseer version 5.2 which was developed by AgResearch; SPASMO which was developed by HortResearch (now Plant & Food); LUCI which was developed by Crop & Food Research (now Plant & Food); and SWatBal developed by SCION, a water balance model for plantation forests and in particular Pinus radiata.

The second workshop, held in October 2008, identified a number of issues with the modelling to date, including discrepancies which arose from using several different models and the need to include standard as well as best practice management scenarios. As a result of these concerns it was recommended that SPASMO be used to estimate nitrate leaching from pastoral farming. The third workshop held in November 2009 included discussion of the results, during which the continued differences between the various models used was raised again and concern was also expressed regarding the criticism of Overseer. Various industry bodies stated that they had committed to supporting the future development of the model and therefore preferred the use of this model to estimate nitrogen leaching.

A caucus meeting in February 2010 took the initial modelling results and extrapolated these out to include dairy farms with higher or lower stocking rates, sheep and beef systems. In September 2010 the revised table was sent to participants for review and approval. The reference table provided general guidance on the quantity of nitrogen that may be expected to leach under various rural land uses, however the technical report concluded that these values were only suitable for use on a regional or large catchment-scale basis not an individual farm basis. This means that the results had limited applicability to the consenting process.

The technical report was updated in September 2013, this update included new modelling using Overseer version 6. Overseer was used in the updated report due to the use of this model as a regulatory tool by a number of councils. The same parameters used in the original study were run through Overseer version 6 with the addition of more soil information, to produce updated values for the reference table.

In the final table non-pastoral land uses such as lifestyle blocks and fruit crops were modelled using LUCI and SPASMO, while pastoral land uses were modelled using Overseer. The amended report also concluded that the values produced continued to be only useful on a regional or catchment wide basis and not an individual property basis.

So while these values are helpful in assessing the impact of wide-spread land use changes, farm based modelling is required to assist the regulatory process. Environment Canterbury has, therefore, included rules in the proposed Land and Water Regional Plan to require farm based modelling of nutrient loss using Overseer. As a result the quantity of non-point sources of nitrogen and phosphorous loss from farm systems, such as the one pictured in Figure 2, are being predicted on a property-by-property basis for the first time.

Figure 2: Cows Stand by an Unfenced Drain (Source: Isobel Oldfield)
Figure 2: Cows Stand by an Unfenced Drain (Source: Isobel Oldfield)

As someone who has undertaken Overseer modelling to support consent applications on behalf of clients, I am well aware that the use of this model is not without its flaws. The Overseer team state that this model was not developed to be used as a regulatory tool and every new version changes the quantities predicted by the model. Despite these flaws, modelling nutrient loss from individual land uses is becoming increasingly important to ensure that targets that have been and will be set under the amended National Policy Statement for Freshwater Management can be met through identification and management of key areas of nutrient loss. The growing use of this modelling tool will increase our knowledge base and lead to improvements within the model itself, which will continue to increase its reliability as a regulatory tool.

Through the new Land and Water Regional Plan Rules farms will be required to undertake Overseer modelling on an annual basis with consent conditions written to allow for changing Overseer models. While these models are currently not supported by on farm testing, Environment Canterbury does require assessments of the concentrations of nitrogen and phosphorous within water bodies that are the vicinity of each farm. The Overseer model continues to be updated and perfected as further research is undertaken and therefore large scale use of the model will continue to support the identification of issues and provide motivation to improve the science behind the model.

In the future therefore, the reference tables first proposed by Liburne and colleagues may be able to be updated using the data currently being collected on a farm-by-farm basis. This more detailed data may then be able to be used to derive values that can be more confidently used on a smaller scale.

The author Isobel Oldfield is a postgraduate student at Lincoln University. She wrote this article as part of her assessment for ECOL 608 Research Methods in Ecology.

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