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Incentives for Change

Reasons for Water Quality Improvement

Investigating the reasons for decreasing phosphorus concentrations in many waterways

Fenced and planted stream


Challenge funding: $50,000

Research duration: January 2017 – December 2018

What Did We Do?

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?

  • 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.

In the Media

Phosphorus levels have fallen

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

Science omnivore takes on the tough questions

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.

Why are phosphorus concentrations decreasing in our rivers?

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

Our Land and Water takes on agricultural challenges

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."


Research Updates

Cows on irrigated pasture. Photo by Dave Allen, NIWA

Aug 11 2021

Measurements Add Confidence to Analysis of Mitigation Effectiveness

Contaminant loss estimates from modelling aiming to capture the diversity of New Zealand pastoral farms ...
Best-case scenario: applyiing all on-farm mitigations by 2035

Dec 7 2020

How Much Difference Has Good Farm Management Made?

Farmers have been taking action to improve water quality for years. Despite much hard work ...

Team Snapshot

Research Outputs


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

R.W. McDowell, M.J. Hedley, P. Pletnyakov, C. Rissmann, W. Catto, W. Patrick
Journal of the Royal Society of New Zealand, January 2019

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 strategies were mitigating P loss from land, guidelines were directing where to best use strategies, and policy instruments were including P management. We found little evidence that this improvement was caused by a decrease in fertilizer use or change in fertilizer form, or that greater nitrate loads were assimilating P from groundwater or sediments.

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

R. W. McDowell, R. M. Monaghan, W. Dougherty, C. J. P. Gourley, R. Vibart and M. Shepherd
Animal Production Science, March 2017

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 implementing mitigations. In 4 regions in New Zealand we found that P could be mitigated quite cheaply, but N reductions required more measures, some of which are costly. Conversely, prioritising on the basis of mitigation cost-effectiveness for a specific nutrient will lead to more rapid reductions in losses of the target nutrient, but with fewer co-benefits for non-target nutrients or pollutants, such as faecal microorganisms and sediment. This information will assist farmers in deciding how to meet a catchment target at least cost.

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

McDowell RW, Elkin KR, Kleinman PJ
Journal of Environmental Quality, March 2017

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 unknown how these factors influence P concentrations in streams. We sought to evaluate differences in biotic and abiotic processes affecting stream sediment P dynamics under different temperature and N-enrichment regimes. The rate and magnitude of P uptake by sediment was greater at 19°C compared with 26°C, and also when N was added compared with no N added. Because microbial biomass is a temporary store of P, these data suggest that more P may be available with increasing temperatures during the growing season, especially under baseflow, implying that strategies to mitigate P losses from land to water should be strengthened to prevent potential water quality impairment.

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

McDowell RW, Dils RM, Collins AL, Flahive K, Sharpley AN
Nutrient Cycling in Agroecosystems, September 2015

The improper use of phosphorus (P) on agricultural land in developed countries is related to P losses that impair surface water quality. We outline policy in New Zealand, the UK, and the US, contrast the strategies used in each country and discuss their likelihood of being able to decrease P losses and improve surface water quality. A poor understanding of catchment processes and critical source areas of P loss has resulted in some areas where regulation has had minimal effect on P discharges. Furthermore, we give several examples where social and economic challenges are of equal if not greater relevance to P discharges (e.g. subsidies). Other policy instruments (e.g. trading schemes) may be needed at larger scales, but should be flexible and encourage innovation over a culture of dependence. There is increasing recognition among all three countries that while targeting good management practices can substantially decrease P losses from existing land use, to achieve ‘good’ water quality in catchment, policy may have to consider land use change.

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