When is a drought not a drought? When it's a hydrological drought instead of a meteorological drought, according to a Northland Regional Council scientist and her manager.

Resource scientist (surface water) Hoa Pham has been investigating the differences between, and local implications of, the two drought types in Northland, and with the support of her manager, Jean-Charles Perquin, has written an article on the issue for the New Zealand Hydrological Society, which she will present to the society's upcoming conference in Rotorua this week.

In very broad terms, Ms Pham said, the simplest explanation of meteorological drought was what most people understood a drought to be — a lack of rain over a reasonably long time that made things very noticeably dry.

"It's pretty easy to measure low rainfall and for how long this has been going on," she said, but hydrological drought, which was more complex, was what happened to the region's actual hydrological processes; its rivers, lakes, reservoirs and groundwater, especially over the longer-term.

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In some cases the impacts of meteorological drought would be displayed by stream flows, which could remain lower than usual more than a year after rain returned.

She and Mr Perquin were investigating the relationship between the two types of drought, including the historical impacts (including severity) of meteorological droughts, their influence on stream flows, and how they could be used to model current and future impacts.

Ms Pham said with Northland experiencing a number of droughts in recent years, their research was expected to provide valuable and useful information, noting that over the period studied for the presentation (July 2018-June 2019) the amount of water in some Northland streams had reduced dramatically.

Meanwhile the regional council is already keeping a close eye on the water situation in Northland as the region inches ever closer to another potentially dry summer, prompting local authority water restrictions in some areas already.

Figures released by the council in October showed the Mid and Far North had received a third to 40 per cent less rain than usual over the previous 12 months.

The problem had been made worse by consecutive dry periods leading up to winter this year. In the first six months of this year Kerikeri and Whangarei were the driest they had been in more than 80 years (since 1935 and 1937 respectively), and the situation had not really improved since then.

River and rainfall data can be found at www.nrc.govt.nz/riversandrain, while restrictions on public water supplies provided by Northland's three district councils are at www.bewaterwise.org.nz