Zeroing in on contaminant hotspots is proving invaluable for one Southland Catchment Group to help better understand and manage farming practices and improve water quality.

About 40 people attended a field-day at the Otapiri & Lora Gorge Memorial Hall on July 26 to learn about the results of the Makarewa Headwaters Catchment Group LUCI-Ag Project identifying nitrogen (N), phosphorous (P) and sediment loss hotspots across the catchment.

Key findings (listed below) include that the intensity of the farming system impacts both the nitrogen and phosphorous loads, particularly for dairy farms, but fencing off waterways and retiring steeper land near waterways led to significant reductions.

The group engaged Ravensdown Environmental to model N and P and soil losses from the catchment to look at possible ways to reduce contaminant losses and better understand the dynamics around these losses.

LUCI-Ag is based on geographic software that uses readily available national data, supplemented by on-farm management data and knowledge. It uses this to develop spatially detailed maps and analyses specific to the property so farmers can see nutrient hotspots on their farm maps.

Makarewa Headwaters Catchment Group chair Alexis Wadworth said six farms were used in the modelling - five sheep and one dairy - with the results extrapolated across the rest of the catchment using all available land information databases.

“It’s been a long journey to get here, and we are really pleased with the work that’s been done. We’re really grateful to Thriving Southland, as this could not have happened without their funding and support,” she said.

Presentations from Ravensdown staff were complemented with thought provoking panel and Q&A sessions.

A key focus is building increased understanding around how much activity different types of soils are capable of supporting, while discussions reiterated the importance of looking at the overall ecological health in a catchment.

Presentations centred around practical on-farm mitigations using LUCI-Ag modelling, including fencing of waterways, forestry, aligning farming intensity and timing to land use, and taking a catchment-based approach.

The potential impacts of land retirement of steeper areas and intensification of low slope scrub land was also discussed.

Otapiri Gorge farmers Dan and Brett Frew talked about the on-farm changes they have made to keep their soil in one place since severe damage caused during 2018 thunderstorms.

Key findings include:

• The intensity of the farm system impacted the N load; with the highest N load areas on dairy farms, particularly the dairy farms on the free draining Brown soils (compared with the poorly

draining Gley soils). The less intense sheep/beef/deer farms generated lower N loads than the dairy farms, while the lowest N loads were generated under native or exotic forestry.

• The intensity of the farm system also impacted the P load; with the highest P load areas on farmed sloped Dairy land underlain by Pallic or other slower draining soils.

• Areas of highest soil loss are associated with harvested forest on steeper topography.

• It has been shown through individual farm reports (LUCI-Ag modelling), that work farmers have done fencing off waterways has resulted in significant reductions in N & P losses of 5-28% and

4-70%, respectively.

• Through increasing fencing, modelling showed over 50% of waterways saw reduced P concentrations, with over 20% of waterways showing a reduced P concentration of over 10%. Modelling also showed over 33% of waterways saw reduced N concentrations, with over 13% of waterways showing a reduced N concentration of over 10%. There are also other additional benefits of fencing/riparian planting including biodiversity gain and providing shade to waterways which reduces water temperature and excessive plant growth.

• In a catchment scenario, 990 ha of flat scrub land was intensified, and 990 ha of steeper farmed land was retired, this reduced catchment P losses by 9%. Individual farm reports have shown

that retiring land around streams can offset N & P losses of intensification of better land.

A copy of the full report can be found here: https://www.thrivingsouthland.co.nz/makarewa-headwaters/