Under water quality standards streams not wider than a stride are not required to be fenced.
But “wider than a stride and deeper than a Redband” might not prove a sufficient standard if the pastoral sector was to continue to achieve water quality gains.
Professor Richard McDowell and his colleagues modelled streams based on their size classification against the terrain, soil, land cover and contour they flowed through to gauge how great waterway contamination was, something not done before in New Zealand.
They studied the contribution watercourses made to phosphorous, nitrogen, E coli and sediment level. Data was sourced from more than 700 testing sites around the country between 1998 and 2009.
The scientists applied the standard of the Land and Water Forum to their testing.
It said streams less than a metre wide and under 30cm deep on slopes less than 15 degrees were exempt from fencing requirements under the Forum’s standards.
The authors said past research including work done by McDowell in 2008 found fencing headwater streams was extremely effective, reducing phosphate and E coli levels on a trialled deer unit by 90%.
The authors acknowledged fencing all headwaters on a farm could effectively bankrupt it.
They also said while 90% of dairy watercourses over 1m wide had been fenced “far fewer” large streams on sheep and beef properties had been because they were used as sources for stock drinking water.
McDowell said the fact most of the contaminant load had been proved to come from areas exempt from fencing requirements reflected the greater number and area small streams occupied among the country’s watercourses.
That was potentially from steeper country where dairying was unlikely to be present.
His work also indicated a “substantial” proportion of contaminant concentrations might come from natural sources including natural sediment losses and E coli from wildlife.
The report said nutrient loads from smaller watercourses accounted for particularly large proportions of contaminants in the Canterbury, Southland, Nelson and Hawke’s Bay catchments.
They were also catchments that had tourism and recreational use.
“Our data suggests not requiring fencing may significantly delay or reduce the ability to mitigate water quality impairment unless other measures are taken,” the authors said.
McDowell said fencing was not a silver bullet solution and other measures could include a suite of options.
Fencing costs had been estimated as high as $1.4 billion nationally.
McDowell and his colleagues suggested that based on the immediate impact of smaller waterways on nutrient and sediment levels it might prove more cost-effective to lower their nutrient losses rather than trying to deal with those losses in bigger streams further down the catchment.
“These could include minimal tillage, precision fertiliser application avoiding vulnerable areas of nutrient loss, natural seepage wetlands or grazing gullies only in the dry periods.”
The authors noted the challenge their conclusions posed to the pastoral farming sector when it cames to determining how to deal with small streams in steeper country.
“Apart from an assumption that headwaters contribute little to catchment loads, the main reason in NZ for de-emphasising their role is that it is impractical (too steep) and too costly to fence them.”
“Nevertheless high costs may be justified when a large amount of contaminant loading originates from a small area.”
Earlier research had already found 90% of catchment loads for sediment, phosphate and E coli originated from a very small area of a catchment used by red deer for wallowing.
A Federated Farmers-AgFirst report last year based on the Waikato’s Healthy Rivers Plan put the average cost to fence waterways on the region’s sheep and beef properties at about $300,000 and as high as $750,000 a farm.
In Waikato the definition of a “waterway” that would require fencing under the Healthy Rivers Plan included any drain, river or wetland that continually contains surface water.
Beef + Lamb NZ environmental policy manager Corina Jordan said the organisation recognised about 80% of contaminant losses came from 20% of farm catchment areas and farm-specific environmental plans aimed to identify and manage those areas, known as “critical source areas”.
“This tailored, farm-specific approach achieves far greater environmental outcomes, including for biodiversity and greenhouse gas emissions, than blanket rules such as fencing.
“Fencing can fail to address overland flow pathways which, as highlighted by this research, can be financially unviable for some farming businesses.”