New project to assess on-farm detainment bunds and phosphorus mitigation 

A new Rotorua-based research project has been initiated, supported by the Sustainable Farming Fund (SFF), to investigate the ability of detainment bunds (DBs) to mitigate on-farm phosphorus losses.

Every year Lake Rotorua receives (on average) 23.4 tonnes of phosphorous from human activity which is about half of the total phosphorus inputs (48.7 T). This is about 0.36 kg of phosphorus lost from every hectare in the catchment. Detainment bunds can help reduce this flow of nutrients to the lake by temporarily detaining storm water run-off, allowing phosphorus laden particles to settle out. The SFF project aims to validate the performance of DBs in terms of kilograms of phosphorus retained on farm and prevented from entering waterways. DBs are not farm ponds as no pasture is taken out of production.

The 20^th Rotorua District DB was completed on a Rerewhakaaitu farm last month. Six of these existing DBs, including several in the Lake Rotorua catchment, will be the trial sites for SFF project over the next three years.

An inaugural meeting of the landowners who initiated the SFF bid was held recently. An executive committee and a local project manager were selected and the project was renamed the Phosphorus Mitigation Project. The new governance group, chaired by Lachlan McKenzie, felt the former ‘The P-Project’ name had too many other connotations! The next task of this governance group is to form a Science Advisory Team that will finalise the project methodology.

Lachlan says “Rotorua Farmers need to have practical and proven tools to reduce nutrient losses from their farms.  It’s great to have a local project that focuses on phosphorus as most recent research in the Lake Rotorua catchment has been about nitrogen”.

The project is to build on past research including a 2013 Master’s Thesis on Rotorua DBs. The new project will provide much more quantitative DB performance data. A PhD student is being sought to work on the project for 3 years.  The aim is to have a six fold increase in DB data sets (from 15 to 90) allowing close scrutiny of DB performance.

There are some very specific design criteria for DBs to achieve adequate particle settling. GIS analysis of farm topography is needed to identify sites with a DB storage ratio of at least 120 cubic meters of storage per hectare of contributing catchment. DBs must be managed carefully during storms with the outflow plugged to catch the first flush of storm water that contains about 80% of the nutrients washed off during a storm event.

Project co-funders are matching the SFF grant 50:50 with significant contributions from DairyNZ, BOPRC, Ballance Agri-Nutrients, ECan, Beef + Lamb and the Deer industry. The total cash budget is over $400,000 and additional in-kind contributions are valued at $200,000. The project is expected to start in July when the contract gets the final sign-off from the SFF administrators.