Reasons for Water Quality Improvement

Investigating the reasons for decreasing phosphorus concentrations in many waterways

Project Details Ngā taipitopito

Project Status:
Challenge funding:
Research duration:
January 2017 – December 2018

Collaborators Ngā haumi

AgResearch | Ballance Agri-Nutrients | DairyNZ | Land & Water Science | Massey University | Ministry for the Environment

You are here: Home / Projects / ­ Reasons for Water Quality Improvement

What are we doing?E aha ana mātou?

Fertiliser and farm dairy effluent contain phosphorus, which can enter freshwater via runoff or leaching from agricultural land. In freshwater, phosphorus can stimulate algal growth, leading to impairment of water for swimming, fishing, drinking, and reduced biodiversity.

Algal growth in New Zealand streams and rivers is widespread due to agricultural phosphorus, but in many areas the concentration of phosphorus in our waterways is decreasing.

Phosphorus has decreased at over 40% of measured sites in streams and rivers since 1994, and 65% of sites since 2004, despite an increase in national dairy cow numbers by 26% and the expansion of dairying into areas previously used for sheep farming.

The research investigated the possible factors contributing to reductions in phosphorus concentrations at these sites. We found the 3 most probable causes for improvement were that on-farm strategies were mitigating phosphorus loss from land, industry guidelines were directing where to best use strategies (for example, in critical source areas), and phosphorus was being mentioned more in policy instruments.

How can the research be used? Ka pēhea e whai take ai te rangahau?

  • This research tells us that farmers and growers should continue to implement strategies to mitigate phosphorus (P) losses.
  • Previous research indicates that strengthened strategies to mitigate P losses will be required as temperatures increase under climate change, because of the decreased rate and magnitude of phosphorus uptake by stream bed sediment at higher temperatures.
  • This research found little evidence that decreasing P concentrations are caused by a decrease in fertiliser use or change in fertiliser form. However, this doesn’t mean these practices won’t help – more likely, it’s because these practices aren’t yet widely adopted across whole catchments. Using low-water-soluble-P fertilisers, such as reactive phosphate rock, in critical source areas remains one of more than 40 mitigations Our Land and Water research recommends to decrease nutrient losses.
  • Industry guidelines can help direct where to best use on-farm mitigation strategies. Advances in farm mapping (eg MitAgator) can allow these practices to be targeted even more precisely to critical source areas of phosphorus loss, improving their cost-effectiveness.
  • Where losses haven’t been caught in time or where changes have not been made through voluntary means, policy instruments can help in decreasing losses.

Participation & engagement Te hunga i whai wāhi mai

  • DairyNZ’s environmental science team revisited dairy farms in five catchments to assess whether on-farm actions have helped improve water quality over time. The five catchments – Waiokura (South Taranaki), Toenepi (Waikato), Waikakahi (Canterbury), Bog Burn (Southland) and Inchbonnie (West Coast) – are dairy-dominated and were part of the 2001-2010 Best Practice Dairy Catchments project, which monitored water quality and environmental work on-farm.

Research team Te hunga i whai wāhi mai

Research Lead
Richard McDowell
Mike Hedley
Massey University
Peter Pletnyakov
Clint Rissmann
Land and Water Science
Warwick Dougherty
NSW Department of Primary Industries
Cameron Gourley
Victoria Department of Primary Industries
Ross Monaghan
Mark Shepherd
Ronaldo Vibart

Tools & resources Ngā utauta me ngā rauemi

Journal Article

Linking the uptake of best management practices on dairy farms to catchment water quality improvement over a 20-year period

Intensive land use, such as dairying, can impair water quality. Although many guidelines exist on how to mitigate the loss of dairy-associated contaminants from land…
View Journal Article

Academic outputs He whakaputanga ngaio

Journal Article

Why are median phosphorus concentrations improving in New Zealand streams and rivers?

Between 2004 and 2013 phosphorus concentrations decreased at 65% of the 304 sites monitored. Our research found the most probable causes of improvement were that…
View Journal Article
Journal Article

A review of the policies and implementation of practices to decrease water quality impairment by phosphorus in NZ, the UK, and the US

We outline policy in New Zealand, the UK, and the US who have imposed limits for P. We contrast the strategies used and discuss their…
View Journal Article
Journal Article

Temperature and Nitrogen Effects on Phosphorus Uptake by Agricultural Stream-Bed Sediments

Climate change will likely increase the growing season, temperatures, and ratio of nitrogen (N) to phosphorus (P) loss from land to water. However, it is…
View Journal Article
Journal Article

Balancing water-quality threats from nutrients and production in Australian and NZ dairy farms under low profit margins

We summarise the losses of N and P from Australasian dairy farms, examine the policy drivers used for mitigating losses and evaluate the effectiveness of…
View Journal Article

In the media Mai i te ao pāpaho

Farmers Weekly, February 2019
“We are now at a stage where we can provide some guidance on the strength of factors such as land use practices put forward in the scientific and public literature as likely causes of phosphorus trends in stream flow” – Rich McDowell
Royal Society research update, 28 February 2019
The newness of national policy direction for phosphorous in Aotearoa such the National Policy Statement for Freshwater Management (NPS-FM) and the lead-in time for its implementation suggest that the changes seen in water quality are not likely to be stemming from the NPS-FM
Dairy Farmer, 18 February 2019
Probable causes are that on-farm strategies are stopping phosphorus loss from land and that together with policy, the actions are being directed to areas on-farm or in the catchment where phosphorus losses are most concentrated. They are called critical source areas and farm environment plans are a way to find and manage them.
Irrigation NZ News, Autumn 2019 (page 36)
“Sales of ‘alternative’ fertilisers are still low,” says Professor McDowell, “but targeted use of low-water-soluble-P fertilisers, such as reactive phosphate rock, can decrease phosphorus losses."
Coast and Country News, 17 October 2023

Incentives for Change Projects

Tauutuutu Diagram For Website

Kaitiaki Intelligence Platforms

Positioning Māori as first movers in next-generation environmental intelligence
View Project
This project has produced
Information Type Icon Infographic OutlineInformation Type Icon Summary OutlineInformation Type Icon Technical Reports Outline

Eutrophication Product Footprinting

Eutrophication indicators will help New Zealand agribusiness reduce environmental impacts and meet expected new European environmental standards
View Project

Indicators Working Group

Investigating the use of indicators to help the primary and public sector meet local, national and international reporting requirements
View Project
The Matrix Of Drivers 2022 1200x800

The Matrix of Drivers

A foundational understanding of international and domestic drivers and issues, and their importance to the primary sector
View Project
This project has produced
Information Type Icon Interactive Tool OutlineInformation Type Icon Summary OutlineInformation Type Icon Technical Reports Outline
New Models Of Collective Responsibility

New Models of Collective Responsibility

Developing new ways to strengthen the connection between people, land and water
View Project
Register Of Land Management

Register of Land Management Actions

What's been done? A national record of land management actions to improve water quality and restore catchments
View Project
This project has produced
Information Type Icon Guidance OutlineInformation Type Icon Infographic OutlineInformation Type Icon Interactive Tool OutlineInformation Type Icon Journal Article OutlineInformation Type Icon Video Outline
Scroll to Top