Modelling the feasibility and cost-effectiveness of edge-of-field mitigations for reducing nitrogen and phosphorus loads in the Waituna Lagoon Catchment, Southland

ABSTRACT

Edge-of-field mitigations (EoFM) can reduce agricultural nutrient losses to surface water. To determine the feasibility and cost-effectiveness of EoFM, we developed a spatial–economic modelling approach and applied it in the Waituna Lagoon Catchment (Southland, New Zealand) to evaluate reductions in nitrogen (N) and phosphorus (P) loads, utilising three EoFMs: constructed wetlands, woodchip bioreactors and aluminised zeolite filters. Cost-effectiveness per targeted nutrient was calculated using a strategy that considers nutrient reduction potential and implementation costs of EoFM across all farms (catchment-collective approach), and a strategy requiring each farm to mitigate individually using their most cost-effective EoFM mitigations (farm-based approach). Our modelling showed that the load received from potentially EoFM eligible sub-catchments is 46% and 27% for overall N and overall P load, respectively. EoFM can achieve substantial reductions using a catchment-collective approach (max. 24% overall N load; max. 15% overall P load), but cannot on their own provide the reductions required to achieve the desired outcome (59% overall N and 53% overall P load). Catchment-collective approaches for EoFM placement become more cost effective than farm-based approaches when larger nutrient reductions are required (N >33%; P >30%; N&P combined load >16%, based on the received load from potentially EoFM eligible subcatchments).

KEYWORDS:

• Dairy
• agriculture
• cost–benefit
• water quality
• catchment model
• nutrients
• edge-of-field mitigation
• constructed wetland
• denitrifying bioreactor
• phosphorus filter

Acknowledgements

The modelling presented here was developed during a secondment of MPW to DairyNZ. The authors acknowledge the use of output from the Waituna Lagoon catchment water quality model, developed by Deltares and DairyNZ. We wish to express our appreciation for the valuable information on tile drainage provided Dr. Ross Monaghan (AgR); bioreactor/filter field-trial results by Dr. Lucy McKergow and Dr. Neale Hudson (NIWA); Dr. Craig Depree is thanked for improving the manuscript, Dr. Thomas W. Stephens for R support and Dr. Simon Groot for coordinating the secondment of MPW to DairyNZ which resulted in this study. We acknowledge and thank the Waituna farming community for background data used as input to these studies.

Disclosure statement

DairyNZ is funded by the New Zealand Dairy Industry via a levy on dairy farmers to invest in research, resource development, extension, and advocacy to promote sustainable dairy farming.

Additional information

Funding

This work was funded by DairyNZ Limited [grant number PRF8056] and the New Zealand Ministry of Business, Innovation and Employment (MBIE) [grant number C01X1818]. DairyNZ Limited funding is paid for by NZ dairy farmer levy payers.