Submitted on July 19, 2017

Inflows into the South Island’s major hydro lakes since February rank as the lowest on record, by quite a margin.

By Greg Sise, 19 July 2017

I’m fond of saying that we can see patterns in the 86 years of hydro inflow data we have access to, dating back to April 1931, but that I’d feel much more confident in these patterns if I had 860 years of data.

Eighty six years of data sounds like a lot, but this year has once again proven that in an ideal world we would have access to a more accurate dataset.  Looking at the period from mid February to 11th July, inflows into the South Island hydro lakes were 13% lower than any other year on record, the next lowest being 1970 and then followed by 2011.

Which explains why South Island hydro output has fallen, the HVDC (inter-islandf) link is running southward almost all of the time, the North Island thermal generators are running flat out and spot prices are running high.

The chart shows the cumulative inflows in GWh into the South Island since 14th February and 2017 can be seen sitting well below all other years.

2017 storage

The last time a new record was set during a dry year was in 2012 – yes, that’s only five years ago - when the period from December 2011 to May 2012 was the driest in the South Island by a margin of around 10%.

This is all the more surprising because there is a 20 year trend showing in the South Island with inflows being lower in February-March and wetter in May-June-July relative to the first 66 years of data we have access to.  These two trends haven’t produced an overall change in the average annual inflow, just a change in their timing. But so far this year, we’ve missed out on the top-up of the lakes that has occurred in the last few years in late autumn or early to mid-winter. 

But I’m also fond of saying that just when you think you’re in a new inflow trend, that’s when it will all change…