November 2016 - climate report

4 Nov 2016

The Hydrology team will be posting a monthly report on the various climate data Northland Regional Council collects. The report will include rainfall, river flows and groundwater levels. Every three months we'll have a seasonal report, (very similar to NIWA's) with a regional spin.

The climate models used by NIWA indicate a 53% chance of a moderate La Nina, returning to neutral conditions by February 2017. For Northland this means more low pressure systems and persistent easterly or north-easterly airflow than normal.

Rainfall

Rainfall maps now use the median instead of the mean (average), a detailed explanation is found later in this report.

The median rainfall map for October (below left) compares what rain fell in the region to what was to be expected. The 100% or purple colour mark shows rainfall at normal levels, the yellow colour shows below normal levels.

The map (below right) shows what rain in millimetres fell across Northland during October. The red dots display the locations of the rainfall stations used to record the data. You can view these locations on the council's website at: www.nrc.govt.nz/riversrainfalldata

Rainfall received across Northland for October was regionally close to normal. The upper half of Northland was close to normal, the lower half and eastern catchments were slightly above normal. In general there were several good bursts of rain in the first half of the month. The last half was relatively dry with scattered showers.

Median rainfall map                                               Rainfall in millimetres map

Median rainfall map - October 2016.  Rainfall in millimetres - October 2016.

River flows

River flows across Northland were generally very similar to the rainfall – slightly higher in the southern catchments.  Historically, flows and soil moisture deficits decline from November onwards.  Flows are already beginning to reduce as indicated by more of the light blue in October’s flow map (below).

When there is little rainfall, such as summer months, the groundwater systems supply water to our streams.  This is known as base flow. 

Flow map - September                                            Flow map - October

River flows - flow map September 2016.  River flows - flow map October 2016.

Groundwater

Groundwater levels across Northland were above average, except for the Whangārei area.  Several bores near Whangārei have water levels slightly below average for this time of the year.

Groundwater Systems

Current Status for October 2016

Aupouri

Average

Taipa

Average

Russell

Average

Kaikohe

Average

Whangārei

Average / below average

Mangawahi

Average

Ruawai

Average

LAWA

Land, Air, Water Aotearoa (LAWA), is a central database for local goverments enviromental data.

Initially a collaboration between New Zealand’s 16 regional councils and unitary authorities, LAWA is now a partnership between the councils, Cawthron Institute, Ministry for the Environment and Massey University and has been supported by the Tindall Foundation.

Some of the data collected by the council is transferred to LAWA for everyone to access.

The graph below is an example of data that can be viewed on the LAWA website.  Statistical summaries are also available.

Groundwater levels at Taipa at School Bore

View the graph and information on the LAWA website (Opens in a new window).

Northland Regional Council data

The council operates a hydrometric network which is automated and feeding data back to the Whangārei office every 30 minutes. The network is used for a variety of reasons; monitoring water quantity for central and local government; flood forecasting and helping marine farmers manage harvesting.

Data for our maps and graphs is drawn from the network.

For more information contact the council's Water Resources & Hydrology Manager or check out the network at: www.nrc.govt.nz/riversrainfalldata

Rainfall reporting

The council runs a network of almost 40 automatic rain gauges alongside MetService (four) and NIWA (one).

Monthly rainfall plotting

The three graphs (rainfall maps) below are based on the same amount of rain that fell at each recorder site during the month of October.

The first graph is a plot of actual rainfall in millimetres at each recorder site and contoured so it maps the whole of Northland. However, there is great variability in our rainfall so the further away you live from one of the recorders the less reliable that data may be.

The second graph shows the rainfall spread in Northland as a percentage of what would normally occur; the "average" rainfall at a site.

The third graph is possibly the best way to represent the rainfall; the percentage of the median rainfall.  This sees rainfall by year from highest to lowest and the middle ranking or "median" used.

Three October rainfall maps.

The (above) maps display October rainfall: in millimetres, percentage of the average (mean) and percentage of the median.

The use of the median value is quite useful because it balances out the extreme rainfalls we often get in Northland. (When using the average, we often end up showing that Northland is drier than expected when actually it possibly isn't).