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Assessing Contaminants with Stream Order

Determining whether excluding livestock from large streams in flat catchments would substantially decrease the contaminant load

This stream may not be as deep as a Redband, but it's important


Challenge funding: $50,000

Research duration: January 2017 – December 2017


What Did We Do?

The New Zealand dairy industry’s Water Accord requires farmers to exclude livestock from all large fourth-order streams – “wider than a stride and deeper than a Red Band gumboot” – by committing to a riparian planting plan (due by 31 May 2020, with 50% of planting complete). Government is considering implementing this as national policy.

Our research looked at whether excluding livestock from large streams (over 1 metre wide, over 30cm deep) would substantially decrease the load of contaminants (nitrogen, phosphorus, sediment and E.coli) entering waterways.

The researchers found that loads from large streams in flat catchments dominated by pasture accounted for, on average, 23% of the national load of all contaminants. This research suggests additional mitigations should be implemented to reduce the 77% of contaminants that enter waterways from smaller streams in rolling to steep land.

Further Our Land and Water research is now developing a simple tool for farmers to identify mitigations that are more cost-effective than fencing.

How Can The Research Be Used?

  • We discovered that fencing only large streams to exclude stock would have less effect on freshwater quality than originally thought. Small, steeply sloping streams contribute, on average, 77% of the load of freshwater contaminants. This work was covered in all major newspapers, and radio and TV stations.
  • Farm environment plans and tools such as MitAgator can help farmers target strategies to mitigate contaminant losses from critical source areas. This means that cheaper strategies than fencing can be used to keep livestock out of smaller streams, for better effect.
  • The results challenge proposed government stock exclusion regulations.
  • This research is now being used by regional government to determine policy to improve the quality of freshwater. For example, Taranaki Regional Council has committed to fencing more streams than those covered by central government’s proposed stock exclusion regulations.
  • This research is being used internationally. The paper Assessing the Yield and Load of Contaminants by Stream Order was awarded JEQ Best Paper 2019, for the best paper produced in the prior three years, as awarded by the editors of the American Tri-Societies, whose members number over 20,000 North American scientists covering the primary and environmental sectors.

Team Snapshot

Research Outputs


Assessing the Yield and Load of Contaminants with Stream Order: Would Policy Requiring Livestock to Be Fenced Out of High-Order Streams Decrease Catchment Contaminant Loads?

McDowell RW, Cox N, Snelder T
Journal of Environmental Quality, September 2017

This paper won the JEQ Best Paper Award 2019. Concentration and flow data for 1998 to 2009 were used to calculate catchment load and yields of nitrogen (N) and phosphorus (P) species, suspended sediment, and Escherichia coli at 728 water quality monitoring sites. On average, the yields of all contaminants increased with increasing stream order in catchments dominated by agriculture. Loads from low-order small streams exempt from potential fencing regulations accounted for an average of 77% of the national load (varying from 73% for total N to 84% for dissolved reactive P). This means that to substantially reduce contaminant losses, other mitigations should be investigated in small streams.

Anthropogenic increases of catchment nitrogen and phosphorus loads in New Zealand

Snelder T, Larned S, McDowell R
New Zealand Journal of Marine and Freshwater Research, May 2017

Spatial regression models were used to predict yields of nitrogen (N) and phosphorus (P) discharged from catchments throughout New Zealand. The models were derived using loads (kg yr−1) of TN, NO3-N, TP and DRP calculated for 592 river water quality monitoring sites. Anthropogenic increases in loads of TN, NO3-N, TP and DRP exported to the ocean were 74%, 159%, 48% and 18%, respectively. Increases in loads varied considerably at smaller scales, between 4- and 26-fold for N and 6- to 9-fold for P. Predictions of yields and loads have utility in the development of strategies to manage nutrients.

Estimation of nutrient loads from monthly water quality data

Snelder TN, McDowell RW, Fraser C
Journal of the American Water Resources Association, December 2016

Causes of variation between nutrient loads estimated using alternative calculation methods and their repeatability were investigated using 20 years of daily flow and monthly concentration samples for 77 rivers in New Zealand. The L7 method generally produced more realistic loads with the highest precision and representativeness. Short‐term load estimates poorly represented the long‐term load estimate, and deviations frequently exceeded estimated imprecision. Regulatory authorities should be aware that the precision of loads estimated from monthly data are likely to be “optimistic” with respect to the actual repeatability of load estimates.

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