June climate report 2019

25 Jun 2019, 8:30 AM

Summary

May 2019 was yet another month that was much drier than normal, with the region recording only around 50% of the rainfall expected for the month. There was some showery rainfall towards the end of the month as the positive ‘Southern Annular Mode’ made way to the negative phase, leading to less predominate high pressure systems and more destabilised weather. This showery, low intensity rainfall was not enough to push monthly rainfall recordings up to the monthly totals typical of this time of year.

Autumn 2019 has been unusually dry, with only around 50% of the expected rainfall received for the season. Looking back at the whole period of dry weather so far (previous 11 months, starting June 2018), the NRC SPI index rates the Awanui catchment and upper Wairua catchment as “extremely dry” and most other catchments as “severely dry”. Accordingly, 1-year rainfall deficits are large in Kaitāia, Kerikeri and Whangārei, with deficits of -40%, -31% and -28% respectively.

River flows for the month were very low with that most stations recorded mean monthly flows of 0-20% of typical May flows, and many were below 7 day MALF, which is unusual for this time of year.

Soil moisture would typically be at capacity at this time of year, but large soil moisture deficits remain at Kaitāia, Kaikohe and Whangārei and Warkworth. Similarly, groundwater levels are quite low for this time of year, with most aquifers below or approaching the 20th percentile of the long-term average for this time of year. This is a reflection of the long period of below normal rainfall received.

Outlook

June:

This is typically an unsettled and wet month, and Met Service predict this to be the case for June 2019. So far there have been three periods of rainfall on the 1st/2nd, 5th/6th and 7th June. Northland will likely receive similar rainfall events throughout the month, and rainfall and air temperature for the month should be near normal overall. This bucks the trend of the previous monthly forecasts which have predicted below average rainfall for many of the previous eleven months.

July – August:

NIWA predict the weak central pacific El Niño conditions to continue for this three-month period, but are likely to weaken later in the year. For winter months this often means, more Southerly and South-westerly winds across the country, but for this weak El Nino, no obvious impacts are expected in the short term. Rainfall for June to August is predicted to be equally likely to be below normal and near normal, while soil moistures and river flows are quite likely to be below normal for this period (55% chance). Air temperatures are likely to be near average or above average for this three-month period.

NIWA Temperature, Rainfall, Soil Moisture and River Flows probabilities for June to August 2019

  Temperature Rainfall Soil moisture River flows
Above average 45 25 20 20
Near average 40 35 25 25
Below average 15 40 55 55

Rainfall

May 2019 was much drier than expected based on long term rainfall records, with the region only receiving around 50% of the expected rainfall for the month. The NRC SPI Index rates the Awanui catchment from “severely dry” to “extremely dry”, and most of the region as a whole as “severely dry”.

Looking at a wider time period, Autumn 2019, was unusually dry. The region received around 50% of the rainfall that would typically be received for the season, with the NRC SPI index rating the region as “severely dry”.

For the long period of observed dry weather (11 months, from June 2018 to May 2019), the NRC SPI index rates the Awanui catchment and upper Wairua catchment as “extremely dry”. This is consistent with very low flows observed in these catchments towards the end of May. Most of the east coast, as well at the Tutamoe Ranges and part of the Pouto Peninsula are rated as “severely dry” for the 11-month dry period. Kaitāia, Kerikeri and Whangārei all have large rainfall deficits for the previous year of -40%, -31% and -28% respectively. For Kaitāia we would expect around 1370 mm, while only 820 mm has been recorded. This is a -547 mm deficit for the previous year.

Rainfall mm, % median and SPI Index maps for May 2019

Rainfall May 2019 in MM.

Rainfall percentage of median May 2019.

SPI Map May 2019.

Rainfall Median Map and SPI Index Map for Autumn 2019

Rainfall median map - Autumn 2019.

SPI Map Autumn 2019.

SPI Index map for the dry period, July 2018 to May 2019 and a comparison to the 2009/2010 Northland Drought

SPI map July 2018-May 2019.

SPI map November 2009 - April 2010.

One year and six-month rainfall deficit percentage values

One year and six-month rainfall deficit percentage values.

River flow

River flows for the month of May continued to show then effects of low rainfall over the previous 11 months. The “River Flows” map below shows that most stations recorded mean monthly flows of 0-20% of typical May flows. Similar to the previous three months, many stations recorded flows below “7-day MALF” for most of the month, and some in the Wairua catchment spent a good proportion of the month below “1:5 year low flow” levels. Although MetService predict rainfall to be near normal for the month of June, river flows will likely remain below the long-term average for this month.

Most notably, the Awanui River in Kaitāia  had flows measured at around 360l/s at the beginning of May. This is a considerably low flow, with similar flows only recorded during five other dry periods since the 1950’s. This can be considered a 15-20 year low flow event.

Flow Map May 2019

River flows May 2019.

Count of days below 7 Day MALF and 1:5 year low flows at river recorders during the previous year 

DAYS BELOW 7 DAY MALF

Site

Jun-18

Jul-18

Aug-18

Sep-18

Oct-18

Nov-18

Dec-18

Jan-19

Feb-19

Mar-19

Apr-19

May-19

Awanui at School Cut

0

0

0

0

0

0

0

0

13

21

17

15

Oruru at Saleyards

0

0

0

0

0

0

0

0

0

15

22

29

Punakitere at Taheke

0

0

0

0

0

0

0

4

16

31

21

25

Waipapa at Forest Ranger

0

0

0

0

0

0

0

23

24

29

21

28

Maungaparerua at Tyrees Ford

0

0

0

0

0

0

0

17

23

31

7

10

Rangitane at Stirling

0

0

0

0

0

0

0

25

23

31

17

26

Waitangi at Wakelins

0

0

0

0

0

0

0

20

20

31

12

20

Opouteke at Suspension Br

0

0

0

0

0

0

0

0

12

19

4

3

Kaihu at Gorge

0

0

0

0

0

0

0

0

13

22

6

3

Mangahahuru at County Weir

0

0

0

0

0

0

0

1

17

28

22

30

Mangakahia at Gorge

0

0

0

0

0

0

0

0

0

19

5

4

Mangakahia at Titoki Br

0

0

0

0

0

0

0

0

8

22

6

8

Manganui at Permanent Station

0

0

0

0

0

0

0

0

7

13

2

0

Hikurangi at Moengawahine

0

0

0

0

0

0

0

0

15

30

13

20

Wairua at Purua

0

0

0

0

0

0

3

25

26

31

23

31

Waiotu at SH1 Br

0

0

0

0

0

0

0

0

0

17

24

29

Ngunguru at Dugmores Rock

0

0

0

0

0

0

0

0

6

20

22

26

Waipao at Draffins Rd

0

0

0

0

0

0

0

6

21

31

30

31

Mangere at Knights Rd

0

0

0

0

0

0

0

4

12

11

0

8

Hatea at Whareora Rd

0

0

4

0

0

0

0

0

0

14

0

2

Raumanga at Bernard St

0

0

0

0

0

0

0

0

12

27

20

26

Waiarohia at Lovers Lane

0

0

0

0

0

3

0

9

20

28

18

25

Waihoihoi at St Marys Rd

0

0

0

0

0

0

0

3

0

0

0

0

North at Applecross Rd

0

0

0

0

0

0

0

0

0

6

13

21

Ahuroa at Braigh Flats

0

0

0

0

0

0

0

0

3

14

0

5

Days below 1:5 year low flows

Site

Jun-18

Jul-18

Aug-18

Sep-18

Oct-18

Nov-18

Dec-18

Jan-19

Feb-19

Mar-19

Apr-19

May-19

Awanui at School Cut

0

0

0

0

0

0

0

0

0

4

0

7

Oruru at Saleyards

0

0

0

0

0

0

0

0

0

0

4

27

Punakitere at Taheke

0

0

0

0

0

0

0

0

0

13

0

0

Waipapa at Forest Ranger

13

0

0

0

0

0

0

2

13

27

15

23

Maungaparerua at Tyrees Ford

0

0

0

0

0

0

0

2

13

29

0

0

Rangitane at Stirling

0

0

0

0

0

0

0

0

0

8

0

0

Waitangi at Wakelins

0

0

0

0

0

0

0

0

0

6

0

0

Opouteke at Suspension Br

0

0

0

0

0

0

0

0

0

0

0

0

Kaihu at Gorge

0

0

0

0

0

0

0

0

0

0

0

0

Mangahahuru at County Weir

0

0

0

0

0

0

0

0

0

0

0

0

Mangakahia at Gorge

0

0

0

0

0

0

0

0

0

5

0

0

Mangakahia at Titoki Br

0

0

0

0

0

0

0

0

0

3

0

0

Manganui at Permanent Station

0

0

0

0

0

0

0

0

0

0

0

0

Hikurangi at Moengawahine

0

0

0

0

0

0

0

0

0

16

0

0

Wairua at Purua

0

0

0

0

0

0

0

0

12

31

13

26

Wairua at Wairua Br

0

0

0

0

4

2

4

11

9

20

2

15

Whakapara at Cableway

0

0

0

0

0

0

0

0

0

15

0

17

Waiotu at SH1 Br

0

0

0

0

0

0

0

0

0

17

11

20

Ngunguru at Dugmores Rock

0

0

0

0

0

0

0

0

0

0

0

0

Waipao at Draffins Rd

0

0

0

0

0

0

0

0

0

0

0

0

Mangere at Knights Rd

0

0

0

0

0

0

0

4

6

5

0

2

Raumanga at Bernard St

0

0

0

0

0

0

0

0

5

10

0

0

Waiarohia at Lovers Lane

0

0

0

0

0

0

0

0

0

0

0

0

Waihoihoi at St Marys Rd

0

0

0

0

0

0

0

0

0

0

0

0

North at Applecross Rd

0

0

0

0

0

0

0

0

0

3

0

0

Ahuroa at Braigh Flats

0

0

0

0

0

0

0

0

0

0

0

0

Worst Case Scenario Predicted River Flows for Northland from 13 June 2019 estimated from recession curves (assuming no rainfall):

Water Level Recorder

Days to reach Mean Annual Low Flow (MALF)

Days to reach 1  in 5 year low flow

Awanui at School Cut

6

30

Kaihu at Gorge

17

30

Maungaparerua at Tyrees Ford

20

33

Ngunguru at Dugmores Rock

0

15

Groundwater

Water level reading indicates that the Taipa, Russell, Kaikohe, Whangārei, Marsden-Raukaka, Mangawhai and Poroti aquifers were below average for May 2019. The groundwater system at Aupouri appears OK. The “One-year groundwater trend” plots below show that most groundwater systems are below long-term averages for this time of year, and where the red line is within or approaching the bottom grey bar, the systems are at around 20th percentile levels.

Northland aquifer status

Groundwater Systems

Status for May 2019

Aupouri

OK

Taipa

BELOW AVERAGE

Russell

BELOW AVERAGE

Kaikohe

BELOW AVERAGE

Whangarei

BELOW AVERAGE

Marsden-Ruakaka

BELOW AVERAGE

Mangawhai

BELOW AVERAGE

Poroti

BELOW AVERAGE

One-year groundwater trends (red) relative to the long term average (black) and upper and lower percentiles (bottom grey area denotes 0-20th percentiles)

One-year groundwater trend Aupouri.

One-year groundwater trend Taipa.

One-year groundwater trend Kaikohe.

One-year groundwater trend Maunu.

One-year groundwater trend Mangawhai Heads.

One-year groundwater trend Whatitiri.

Soil moisture deficit

NIWA water balance modelling indicates soil moisture deficit is below average in Kaitāia, Kaikohe and Whangārei and Warkworth. These areas show large soil moisture deficits at these locations for this time of year. “SMD is calculated based on incoming daily rainfall (mm), outgoing daily potential evapotranspiration (PET, mm), and a fixed available water capacity (the amount of water in the soil 'reservoir' that plants can use) of 150 mm” (NIWA).

Soil moisture deficit at NIWA climate stations

NIWA Climate Station

Observed Soil moisture deficit (mm) as of 13/05/2019

Average April soil moisture deficit

Difference between Expected and Average

Kaitāia

-60 mm

0 mm

-60

Kerikeri

0 mm

0 mm

0

Kaikohe

-100 mm

0 mm

-100

Whangārei

-60 mm

0 mm

-60

Dargaville

0 mm

0 mm

 0

Warkworth

-50 mm

0 mm

-50

NIWA Soil Moisture Deficit Plots

Soil Moisture Deficit Kaitaia.

Soil Moisture Deficit Kerikeri.

Soil Moisture Deficit Kaikohe.

Soil Moisture Deficit Whangarei.

Soil Moisture Deficit Dargaville.

Soil Moisture Deficit Warkworth.