March-April 2021 climate report

Summary

• Broadly speaking, March 2021 was a dry month, while April 2021 was a wet month for the region. Rainfall over longer timeframes (the previous 6 months and previous 12 months) can be categorised as near normal overall, with an SPI indicating “Moderately Dry” to “Near Normal” conditions over these periods (see rainfall section below).
• River flows were generally lower than normal for March, particularly in the Mangakahia and middle Wairua catchments. Recorded river flows indicated some recovery in April, with river flows in the top half of the region in the “Normal” range. Rivers have recovered or are continuing to recover from summer low flows.
• Groundwater levels were normal or above normal in the Aupouri, Taipa, Russell and Ruawai, but well below normal in the Whangarei, Poroti, and Mangawhai groundwater systems (0 to 10th percentile). We would typically expect to see these groundwater levels increase during May.
• Soils are either wetter than average or around average for this time of year at all NIWA climate stations.
• The NIWA three-month outlook is for near normal rainfall.

Outlook

May - July

The long-range outlook for May to July 2021 is for near normal rainfall, soil moisture and river flows for Northland. Temperature is still likely to be above normal during this period. La Niña has transitioned to ENSO neutral conditions during April.

NIWA Temperature, Rainfall, Soil Moisture and River Flows probabilities for May to July 2021

Rainfall

March was a dry month for Northland overall, although the northern half of the region, from Bay of Islands and Hokianga northward, received rainfall close to that expected for the month (averaging around 80% of typical rainfall). Southern areas received much less, with around 45% of the typical rainfall received.

April began with a relatively wet day on the 1st of the month, but was followed by a week of fine weather until the 10th. Our Brynderwyn site had the highest hourly rainfall on both the 1st and the 10th April with 24mm/hr on the 1st and 33mm/hr on the 10th. April continued with sporadic rain falls and fine days. On average Northland received about 114mm of rain, which equates to about 116% of the expected rainfall or slightly wetter than expected in April.

NRC Standardised Precipitation Index is an indicator used to compare rainfall received over extended periods compared to typical rainfall over those periods. This index indicates that rainfall across the region has been close to typical over 6 months and 12 months with most stations being classified as “Moderately Dry” to “Near Normal” over these periods.

Rainfall % median maps for March and April 2021

SPI Index map for 6 and 12 months to the end of April 2021

Rainfall deficit values (mm)

River flow

Flow map March and April 2021

Groundwater

During April 2021 Groundwater levels were normal in the Taipa, Russell and Ruawai, and well below normal in the Whangarei, Poroti, and Mangawhai groundwater systems (0 to 10th percentile). Groundwater levels were above normal in the Aupouri groundwater system.

Northland aquifer status table

Northland aquifer status map April 2021

Soil moisture deficits

NIWA water balance modelling indicates soil moisture deficit is slightly below average at the Whangarei and Kaikohe NIWA climate stations, with the other stations recording normal or higher soil moisture than average 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 Soil Moisture Deficit Plots