13 February - climate report

15 Feb 2017

Current situation

The dry weather is persisting in Northland with soil moisture deficits down to 135mm in Dargaville. The brief rain in late January and February did provide some relief to the mid-north.

Kaeo south through Whangārei were the main beneficiaries of the rain.  Rainfall totals ranged from about 40-70mm, with Oakura receiving the most rain recording 67.5mm.

The rain would have helped fill water tanks and provide relief to the critically low river levels for the next few days. Those areas probably still required similar amounts of rain over the next week to make a lasting impression on the drought.

The MetService has advised the 7 week cycle of the high pressure system sitting just North of Cape Reinga has finally broken with a south easterly flow now expected which will bring some cooler dry weather. Currently the weather models indicate rain beginning from 15 February, both coasts looking to receive 25-50mm possibly over a four-day period.

Outlook

The MetService has advised that the models are uncertain for the last two weeks of February 2017. With the tropics heating up, the conditions are favourable for the formation of a tropical cyclone. The MetService have forecasted Northland to expect below average rainfall and warm temperatures continuing through February.

Rainfall

January was looking very similar to December for low rainfall. The rain event in late January bumped the figures up for the mid-north, but the remainder of Northland had a dry January (refer to figures 1 and 2 below).

January summary

Figure 1 : Median rainfall map                              Figure 2 : Actual rainfall map

Median rainfall map - January 2017.   Actual rainfall map - January 2017.

Both rain events did little to help the western and southern regions of Northland (refer to figures 3 and 4 below).

Rain event summary

Figure 3 : January rain event                                  Figure 4 : February rain event

January rainfall event map in millimetres.   February rainfall event map in millimetres.

River flows

Flows for January were below average for the entire region.  The Far North and Kerikeri catchments have reaped the benefits of the two rain events, but are still below the expected flows for January (refer to figure 4 below). For the flow status as of 13 February 2017 refer to figure 5 below.

  • Red dots indicate rivers that are currently below expected drought flows (1:5-year Design Drought Flow), which include The Mangakahia, Kaihu, Hikurangi and Ahuroa
  • Orange dots indicate rivers approaching drought flows
  • Yellow indicate s river above mean annual low flows
  • Green indicate river above mean annual low flow

Figure 4 : January flow map                                      Figure 5 : River status as of 13 February 2017

 

River flows map January 2017.   River flow status map.

Using the prediction curves (recession curve) the approximate days to reach mean annual low flow and drought flows are detailed in Table 1 below.

Table 1 : Predicted river flows from 14 February assuming no rain for the next 35 days

Water Level recorder

Days to reach Mean Annual
Low Flow (MALF)

Days to reach
1 in 5 year low flow

Awanui at School Cut

12

25

Kaihu at Gorge

BELOW

BELOW

Maungaparerua at Tyrees Ford

5

16

Ngunguru at Dugmores Rock

19

32

Groundwater

Some aquifers are beginning to hit low levels, particularly in the Kaikohe, Whangārei and Ruawai area. The Maunu groundwater station in Whangārei recorded the fifth lowest water level for January since records began in 1983.

Table 2 : Summary of Northland Groundwater levels

Groundwater Systems

Current Status for February 2017

Aupouri

OK

Taipa

OK

Russell

OK

Kaikohe

BELOW AVERAGE

Whangarei

BELOW AVERAGE

Mangawhai

OK

Ruawai

BELOW AVERAGE

The Ruawai and Maunu groundwater stations are at similar water levels observed in the 2013 and 2014 droughts (refer to figures 6 and 7 below), the colors indicate:

  • Blue - actual water level
  • Red -  average/ expected water level
  • Dark green - normal range 90th percentile
  • Light green - extreme range 10th percentile

Figure 6 : Puriri Park at Maunu

Groundwater graph - Puriri Park at Maunu.

Figure 7 : Ruawai at Wallace Road

Groundwater graph - Ruawai at Wallace Road.

Soil moisture deficits

Soil moisture deficits levels at the NIWA climate stations have varied across the region ranging from 75mm recorded Kerikeri to approximately 137mm at Dargaville.

The westerly wind pattern has abated slightly allowing some easterly flows combined with the recent rain and decrease in soil temperatures have helped alleviate soil moisture deficits along parts of the east coast (refer to table 3 below).

Table 3 : Northland Soil moisture deficits (NIWA Climate Stations)

NIWA Climate station

Current Soil moisture deficit (mm)

Kaitaia

-125

Kaikohe

-125

Kerikeri

-75

Dargaville

-137

Whangarei

-112

Warkworth

-130

Soil moisture deficit remain high in the western region, at similar levels seen in the 2010 drought (refer to figures 8 and 9).

Figure 8 : NIWA Climate Station Dargaville 2010 Drought

Soil moisture deficit graph - NIWA Climate Station Dargaville 2010 Drought.

Figure 9 : NIWA Climate Station Dargaville 2017 Drought

Soil moisture deficit graph - NIWA Climate Station Dargaville 2017 Drought.