Unravelling nitrogen delivery to waterways

The shallow and relatively fast pathways that are often crucial for nitrogen transfers at the sub-catchment scale. Image: Lincoln Agritech.

Catchments covering areas of 100s or even 1000s of km2 tend to be the smallest spatial units used in current freshwater management. However, such large catchments inevitably have variable natural and land use characteristics within them. This makes it very difficult to link an observed nitrogen load at the catchment outlet to the many past and present activities within the catchment, that collectively have caused it.

If it was possible to defensibly establish the link between activities on the land and their effects on freshwater quality at the sub-catchment scale (10s of km2), land use, land management, mitigation and policy options could all be better targeted spatially. Developing methods that allow establishing this sub-catchment scale link is therefore exactly what the recently initiated Critical Pathways research programme aims to achieve, says Dr Roland Stenger, the programme’s leader from Lincoln Agritech. The programme is funded by the Ministry of Business, Innovation and Employment (MBIE) and co-funded by Waikato Regional Council and DairyNZ.

The research team from Lincoln Agritech, Aqualinc Research, Manaaki Whenua Landcare Research, Lincoln University, GNS Science, AgFirst, and Ian Kusabs & Associates will introduce an innovative multi-scale measurement, data analysis, and modelling approach in two pilot catchments. Collectively, it will enable the team to unravel the typically shallow and relatively fast pathways between a parcel of land and a local stream or small river.

Key innovations include a novel suite of airborne and ground-based geophysical measurements and innovative techniques for the analysis of the resulting ‘Big Data’ that, in combination, allow gaining currently unavailable information on structural, hydrological, and chemical characteristics of the shallow subsurface (particularly the upper 20 m). The resulting new understanding will be integrated into advanced nested models that make it possible to use high-resolution data in those sub-catchments.

This new approach will be tested in two intensively farmed catchments with contrasting characteristics. The Waiotapu Stream (Wp) catchment (approx. 300 km2) on the North Island’s Central Plateau represents a baseflow-dominated upland catchment with large groundwater store in young volcanic deposits. In contrast, the Piako Stream (Pi) headwater catchment is a lowland catchment (approx. 100 km2) in the upper part of the Hauraki Plains with aquifer deposits of lower transmissivity and a high quickflow fraction in the stream hydrograph.

Given that dairy land alone accounts for more than 17,000 km2, Dr Stenger acknowledges that the two pilot catchments represent only a tiny fraction of intensively used land in New Zealand. However, he expects substantial uptake of the approach once the research team has been able to demonstrate its merits.

Date posted: 12 December 2018

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