The aim of this study is to explore the future relationship between land use and nitrogen and phosphorous; two nutrient pollutants that have considerable effects on water quality throughout New Zealand.
The Land Use in Rural New Zealand (LURNZ) model was developed by researchers at Motu and is used in this study in conjunction with the CLUES model, developed by NIWA, AgResearch and others. The results in this study were produced by the model developers and later used by the Parliamentary commissioner for the environment and her staff to prepare a national report on water quality.
LURNZ role in this case study was to assess the increasing need to address growing water quality issues that are consequence of various land use types in New Zealand. Particular concern is placed on recent intensification of dairying and an increase in the amount of dairy conversions occurring.
More directly LURNZ has been used to predict how land usage will change in various parts of rural New Zealand over time. The predictions are made based on economic variables such as commodity prices and physical variables such as slope of the land. This study then aimed to assess the question of how water quality may change in the future as a result of growing industries and the resultant land use changes.
This study utilised both LURNZ, a model of land use change, and CLUES, a model of nutrient losses from land into water to predict the total annual amounts of nitrogen and phosphorous that will be lost from land into water in various parts of the country in 2020. Assumptions that were included in this model prediction were those such as the decline in Sheep/Beef farming would continue for the foreseeable future and that this trend would take place in virtually every region throughout the country. Assumptions were also made that future land use changes would occur within practical limitations such as the physical characteristics of different areas of land i.e. Dairy intensification would not occur on low quality land with a high gradient.
A high-level schematic depiction of the modelling exercise on which this study is based
The LURNZ model was based heavily on a set of high-resolution satellite photographs taken in three different years showing land cover in different pixels. The modelled information was supplemented by data from a number of sources. Land cover was divided into 11 different pixel types;
At the heart of LURNZ is a set of satellite photographs taken in three different years. This multi-spectral image shows green grass as orange, exotic forest as red, and so on, and gives much more information than a ‘natural colour’ image would. The town of Hanmer Springs is near the centre of the image.
Modelling of future nutrient movement and the effects that land use changes would have on the nature and magnitude of this movement was undertaken using CLUES. In this study CLUES modelling occurred based on the results from LURNZ modelling that had occurred prior; these predicted changes in land use were the inputs on which CLUES then modelled changes nutrient loads in associated years.
Findings and Conclusions
The modelling results predicted that the area of land used for Sheep/Beef would continue to decline throughout all areas leading up to 2020, while at a fairly proportionate rate with each other Scrubland, Forestry and Dairy would continue to increase. At the time that the report was written, midway between observed and modelled periods, the predicted results surrounding dairy seemed to hold true while modelled expansion of Forestry was not as prevalent as anticipated.
The final outputs from the combination of LURNZ and CLUES allow a comparison between changes in land use and changes in nitrogen loads. From this study it was clear that in regions with significant increases in dairy farming there was also a large increase in in nitrogen loads. This relationship became more complex when conversions of sheep & beef to forestry and also increases in dairy were consecutively occurring in a catchment, in these cases the improvements of conversion back to forestry were often outweighed or masked by the impact of dairy conversions.
Basic conclusions were as follows;
All models are, of course, approximations of reality and some are better than others and it is noted within this study that certain relevant material is not covered, highlighting potential limitations of this modelling approach as;
Land Use in Rural New Zealand (LURNZ) is a computer model that simulates land-use change at a fine spatial scale over the whole country.