Abstract

The poor water quality of many of the shallow lakes of the Waikato region is of concern. Many of the lakes have intensive land use within their catchments, which has contributed to an overall decline in water quality and a loss of indigenous biodiversity.

We undertook a scoping exercise to determine the extent to which the sediment and nutrient loads within the catchments of 44 shallow lakes of the Waikato region could be reduced. A geographic information system (GIS) was utilised to determine the catchments' extents and land cover.

Water quality data from the regional rivers monitoring programme was used to develop a multiple linear regression model of the relationship between land cover and nutrient load (R² = 0.96 for both nitrogen and phosphorus). The resulting coefficients were used with the lakes' catchment land cover data to estimate the nutrient loads to the studied lakes. The modelled nutrient loads were then modified according to hypothetical 'best practice' and 'potential practice' farm management regimes to determine the reductions they would achieve.

Overall, an average reduction of 7% in the nitrogen load was found when moving from 'average' to 'best practice'. Under the more rigorous 'potential practice', this would increase to 36%. For the phosphorus load to the lakes, the 'best practice' management would mean an average reduction of 18% across all lakes. The use of a 'potential practice' farming regime would result in an average reduction of 39% in phosphorus over all the lakes. The results obtained enable the lakes to be identified based on potential reduction in nutrients under the hypothetical farm management regimes.

An estimate of current sediment load to the lakes was calculated based on a national sediment model. Possible reductions in sediment loads to the lakes under different land management practices have not been quantified, so any potential reduction in loads were not calculated for the lakes.

Potential for Reducing the Nutrient Loads from the Catchments of Shallow Lakes in the Waikato Region [PDF, 331 KB]

Contents
	Abstract	i
1	Introduction	5
1.1	Background	5
1.2	Objectives	5
1.3	Approach and limitations	5
1.3.1	Approach	5
1.3.2	Limitations	6
2	Sources of information	8
2.1	Lakes and their catchments	8
2.2	Catchments	10
2.3	Land cover	10
2.4	RERIMP specific yields	11
2.5	Suspended sediment model (NIWA)	14
2.6	Best or potential management	15
3	Analysis	16
3.1	RERIMP nutrient load and land cover model	16
3.2	Calculation of lakes' nutrient loads	17
4	Conclusions	20
	Bibliography and/or references	21
	Appendices	23
Appendix I:	LCDB2 data types and the categories they were grouped into for analysis	23
Appendix II:	Agribase classes aggregated to form 'Dry stock'	23
Appendix III:	All farm types and their notation in Agribase	24
Appendix IV:	AGRIBASE data – Different farming types in hectares – Appendix III contains a key.	25
Appendix V:	LCDB2 data – Landcover in hectares	26
Appendix VI:	AgResearch information sources and assumptions from Ledgard and Power 2006	28