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).