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Tile drain wetland guidelines

These practical guidelines have been developed to assist farmers to construct wetlands to treat tile drainage flows from grazed pastures.

Tile drainage can act as a significant route for nutrient losses, particularly of nitrogen, from intensively grazed pastures to waterways. Constructed wetlands are a simple, practical tool farmers can use to intercept tile drainage water to reduce these nutrient losses.

The New Zealand guidelines for constructed wetland treatment of tile drainage are intended to guide farmers, farm advisors, rural contractors and regional council staff to appropriately locate, size, design and construct effective wetlands.

New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage (PDF 26.7 MB)

They also provide guidance on wetland planting, weed control and maintenance.

The guidelines are built on international experience as well as monitoring and modeling results from field-scale trials carried out on farms in the Waikato, Northland and Southland over 3-5 year periods. They should ideally be employed in combination with good fertilizer, grazing and effluent management practices.

NIWA Principal Scientist Chris Tanner is studying the effectiveness of constructed wetlands in capturing nitrogen and phosphorus that might otherwise leach off farms and into waterways. The right mix of rushes, reeds and shrubs, says Tanner, is critical – even when it’s dead. Plant litter, he explains, fuels the primary nitrogen removal process – microbial denitrification. Credit: Dave Hansford