May climate report 2019

Summary

April was another in a long line of dry months. Areas such as Bream Bay and very localised areas around the Bay of Islands, Tapia and Tutamoe Ranges received normal rainfall totals due to some small, isolated rainfall events. Nevertheless, most areas were quite dry, particularly Kaitaia, Aupouri Peninsula and the Parataiko/Tutamoe Ranges. The period of drier than normal weather now includes the previous 10 months and analysis of rainfall over this period gives some useful information. The NRC Precipitation Index rates areas around Kaitaia and the western hills as extremely dry and approaching that of the 2009/2010 drought.

Rivers flows remain low in response to the dry conditions, with many rivers recording flows 20-40% of typical April flows, and some as low as 0-20% of typical April flows. Most notable is the Awanui River in Kaitaia, which is currently recording low flows consistent with a 15-20 year low flow event. Low flows also persist in the Wairua catchment, with around 10-year low flows here. With a dryer than average May forecast by MetService, river flows may remain low or drop further.

There is a large soil moisture deficit in most areas but especially Kaitaia, Kaikohe and Whangārei. With large soil moisture deficits, small rain events have very little impact on river and groundwater levels, as much of the moisture is absorbed and utilised by plants and little infiltration and runoff occurs. Rainfall deficits will continue to influence river flows and groundwater until some significant rainfall is received.

Outlook

May:

Metservice predicts rainfall during May to be below the long-term May average. The first two weeks are likely to be quite dry, with a ridge of high pressure prevailing around Northland. Some rain is predicted around the 12th and 18th/19th of the month but high rainfall totals are not likely to be high and the high-pressure ridge and dry conditions may return for week three and four of the month.

May – July:

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. Rainfall for April to June is predicted to be equally likely to be below normal and near normal, while soil moistures and river flows are most likely to be below normal for this period (50% chance). Air temperatures are quite likely to be above average for this three-month period (55% chance).

Recent sea surface temperature anomalies

NIWA Temperature, Rainfall, Soil Moisture and River Flows probabilities for May to July 2019

	Temperature	Rainfall	Soil moisture	River flows
Above average	55•	25	15	10
Near average	35	35•	35	40
Below average	10	40•	50•	50•

Rainfall

Rainfall for April overall less than expected for the month. There were some areas of normal or slightly above normal rainfall due to the small isolated rainfall events, particularly at the beginning of the month, but region-wide the median rainfall was 71.7mm when we would usually expect 98.8mm. Kaitaia and Aupouri Peninsula have been particularly dry during April. The NRC SPI index rates Kaitaia and parts of the western ranges as “moderately dry” for the month, but “extremely dry” in these areas and “severely dry” in many other areas when the whole period of dry conditions are analysed (previous 10 months).

Comparing the Precipitation Index for the current dry period to the 2009/2010 drought, it is clear that certain areas, such as the Awanui catchment and the Parataiko/Tutamoe Ranges, currently show dryness approaching that of the 2009/2010 drought event, but during 09/10 the area of dryness was much more extensive.

Rainfall deficit values show continuing deficits in all areas except Ruawai, with Kaitaia, Kerikeri, Kaikohe and Raumanga all showing a substantial deficit over the previous 6 months and Kaitaia showing the greatest deficit over the previous year. This rainfall deficit will continue until Northland either experiences some significant rainfall events, or experience wetter than average monthly rainfall over a number of months.

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

SPI Index map for the dry period, July 2018 to April 2019 and compared to the 2009/2010 Northland Drought

One year and six-month rainfall deficit percentage values

River flow

River flows for the month of April continued to show then effects of low rainfall over the previous 10 months. The “River Flows” map below shows that most stations recorded mean monthly flows of 20-40% of typical April flows, and some as low as 0-20% of typical April flows. Similar to the previous two 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. With another dry month predicted for May, river flows are likely to remain low for the remainder of the month.

Most notably, the Awanui River in Kaitaia 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 April 2019

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

Days below 7 day MALF
Site	Feb-19	Mar-19	Apr-19
Awanui at School Cut	13	21	17
Oruru at Saleyards	0	15	22
Punakitere at Taheke	16	31	21
Waipapa at Forest Ranger	24	29	21
Maungaparerua at Tyrees Ford	23	31	7
Rangitane at Stirling	23	31	17
Waitangi at Wakelins	20	31	12
Opouteke at Suspension Br	12	19	4
Kaihu at Gorge	13	22	6
Mangahahuru at County Weir	17	28	22
Mangakahia at Gorge	0	19	5
Mangakahia at Titoki Br	8	22	6
Manganui at Permanent Station	7	13	2
Hikurangi at Moengawahine	15	30	13
Wairua at Purua	26	31	23
Waiotu at SH1 Br	0	17	24
Ngunguru at Dugmores Rock	6	20	22
Waipao at Draffins Rd	21	31	30
Mangere at Knights Rd	12	11	0
Hatea at Whareora Rd	0	14	0
Raumanga at Bernard St	12	27	20
Waiarohia at Lovers Lane	20	28	18
Waihoihoi at St Marys Rd	0	0	0
North at Applecross Rd	0	6	13
Ahuroa at Braigh Flats	3	14	0

Days below 1:5 year low flows
Site	Feb-19	Mar-19	Apr-19
Awanui at School Cut	0	4	0
Oruru at Saleyards	0	0	4
Punakitere at Taheke	0	13	0
Waipapa at Forest Ranger	13	27	15
Maungaparerua at Tyrees Ford	13	29	0
Rangitane at Stirling	0	8	0
Waitangi at Wakelins	0	6	0
Opouteke at Suspension Br	0	0	0
Kaihu at Gorge	0	0	0
Mangahahuru at County Weir	0	0	0
Mangakahia at Gorge	0	5	0
Mangakahia at Titoki Br	0	3	0
Manganui at Permanent Station	0	0	0
Hikurangi at Moengawahine	0	16	0
Wairua at Purua	12	31	13
Wairua at Wairua Br	9	20	2
Whakapara at Cableway	0	15	0
Waiotu at SH1 Br	0	17	11
Ngunguru at Dugmores Rock	0	0	0
Waipao at Draffins Rd	0	0	0
Mangere at Knights Rd	6	5	0
Raumanga at Bernard St	5	10	0
Waiarohia at Lovers Lane	0	0	0
Waihoihoi at St Marys Rd	0	0	0
North at Applecross Rd	0	3	0
Ahuroa at Braigh Flats	0	0	0

Predicted River Flows for Northland from 09 April 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	0	0
Kaihu at Gorge	0	10
Maungaparerua at Tyrees Ford	0	15
Ngunguru at Dugmores Rock	0	2

Groundwater

Water level reading indicates that the Kaikohe, Whangārei, Marsden-Raukaka and Poroti aquifers were below average for April 2019. The groundwater systems at Aupouri, Taipa, Russell and Mangawhai appear OK.

Northland aquifer status

Groundwater Systems	Status for April 2019
Aupouri	OK
Taipa	OK
Russell	OK
Kaikohe	BELOW AVERAGE
Whangārei	BELOW AVERAGE
Marsden-Ruakaka	BELOW AVERAGE
Mangawhai	OK
Poroti	BELOW AVERAGE

Soil moisture deficits

NIWA water balance modelling indicates soil moisture deficit is below average in all areas but especially Kaitaia, Kaikohe and Whangārei, with large soil moisture deficits at these locations for this time of year.

Soil moisture deficit at NIWA climate stations 

NIWA Climate Station	Observed Soil moisture deficit (mm) as of 06/05/2019	Average April soil moisture deficit	Difference between Expected and Average
Kaitaia	-125 mm	-50 mm	-75
Kerikeri	-50 mm	-15 mm	-35
Kaikohe	-90 mm	-15 mm	-75
Whangārei	-100 mm	-25 mm	-75
Dargaville	-90 mm	-50 mm	-40
Warkworth	-90 mm	-50 mm	-40

NIWA soil moisture deficit plots