By Bill Vant and Jon Palmer
More than 10 years ago, the Waikato Regional Council became concerned about the effects of increasing catchment loads of nitrogen on the water quality of Lake Taupo. Calculations showed that most (>90%) of the manageable nitrogen entering the lake came from areas of pasture in the catchment. A variation to the Waikato Regional Plan was developed, and was finally confirmed by the Environment Court in July 2011. The Plan aims to maintain the lake’s current water quality well into the future (“by 2080”), by managing loads of nitrogen in the catchment. It aims to do this by (1) capping all manageable sources at their 2001 levels, and (2) reducing some of the manageable sources to offset much of the historic load from land development that is stored in groundwater and is thus still in transit to the lake. The Resource Management Act provides the authority for the Plan’s rules that cap the existing loads of nitrogen, while a public trust – the Lake Taupo Protection Trust – was set up under the Local Government Act to use public monies to offset the historic loads.
The farm-scale nutrient budget model OVERSEER® was used to determine the recent (2001–05) discharges of nitrogen from areas of developed land in the catchment, resulting in “Nitrogen Discharge Allowances” for each farm. These vary – generally depending on the intensity of land use in each case – with the average value for pastoral land in the catchment being about 17 kg N/ha/yr. The combined Discharge Allowance for pastoral land in the catchment is about 880 t N/yr.
Each farm in the catchment has now been issued with an individual “consent to farm” that specifies the Discharge Allowance for it. And farms must be operated within this Allowance. About 80 consents to farm have been issued. The Council has developed a risk-based ranking, and is auditing the 60 highest priority farms each year, with a further 20 farms being audited every three years. This is done using the farm accounts that are routinely prepared for tax purposes. The Office of the Auditor General oversees this auditing.
Under the Plan, Nitrogen Discharge Allowances may be traded, with the resulting changes being recorded against the land title. The Lake Taupo Protection Trust has purchased Discharge Allowances totalling about 150 t N/yr, while a dairy operation has purchased 17 t N/yr. The Trust’s purchases have enabled it to pursue its strategy of reducing the future load of nitrogen entering the lake so as to offset much of the historic load that is yet to enter the lake. As part of this, about 58 km2 of pasture has been converted to forestry, and recorded on the relevant consents. The Auditor General pays particular attention to operations where public money has been spent.
Over the past 10 years (2003–12), the load of nitrogen entering the lake in the inflowing streams has increased at a rate of about 1% per year (while the load of phosphorus has decreased). Increases like this were anticipated in the Regional Plan, and are consistent with our understanding of the storage of “old” nitrogen from historic pastoral land uses in the groundwater. However, about half of the overall increase in nitrogen loads was carried by streams draining areas of mostly native bush and forestry, and this is somewhat unexpected.
We have recently commissioned two analyses of historic aerial photos of the land in the eastern part of the catchment that is now covered in pine forest. Photos from the 1950s and 1960s of land that is now covered with more than 450 km2 of pine forest – about 60% of the total area of pine forest in the lake’s catchment – show that about one-third (31%) of this is planted on land that was previously in pasture. It is likely that the pre-plantation history of land use in this area is at least partly responsible for the recently-observed increases in the nitrogen loads carried by the streams that drain it. These increases suggest that the nitrogen load that is still to enter the lake from historic land use practices is likely to be larger than was previously predicted.
Bill Vant is a water scientist and Jon Palmer is a senior resource officer at Waikato Regional Council.