Lake Rotorua Primary Producers Collective The ‘Phosphorus Mitigation Project’ – now hitting the red line - Lake Rotorua Primary Producers Collective

NIWA installing automating measuring equipment at the Paterson’s Detainment Bund

July 26, 2017
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The ‘Phosphorus Mitigation Project’ – now hitting the red line

The Phosphorus Mitigation Project (PMP) has reached an exciting milestone. After some delays with installation of 3 identical sets of leading edge measuring instruments at all 3 Detainment Bund trial sites – the project has just snared its first sets of data from a red-line rainstorm run-off event on the 20^th of July. Samples of Detainment Bund ponding area inflow and outflow volumes and loads of phosphorus and sediment have been collected and analysis at Massey University’s lab will have just commenced by the time you read this update.

While PMP is up and running now, it’s a little unfortunate that it wasn’t quite ready to sample the extremely wet February to June period which had a high number of high intensity (red-line) rain events. High intensity rain, i.e. over 10mm per hour, is the type of rain that usually results in run-off events than can affect downstream water quality.

An exceptional year for rain storm events – will there be consequences?

Figure 1. June 2016 – June 2017 hourly rainfall at Oturoa Road, Rotorua (red line at 10mm/hour).

Figure 1 shows that our catchment experienced 14 runoff events in the past year, more than twice as many as usual. From 2012 to 2016 the average number of high intensity storms was 6 per year. You can access this and related Oturoa Road data from BOPRC’s live monitoring web page. As run-off events are the main conduit for sediment and P-loss from farmland to waterways, the doubling of these types of rainstorms may result in significant increases in sediment and P (assuming all other factors being equal) reaching Lake Rotorua over the last year. It will be interesting to see if this has consequences for water quality issues such as algal blooms going into Summer and if ‘more storms’ is the new ‘normal’ as Climate Change scientists have indicated.

If high intensity rainstorms are to become an increasingly important driver of water quality issues, validation of storm water based mitigations, such as PMP’s Detainment Bunds, will become more important. The PMP also features in the new P mitigation video here.