Canterbury soils drying out

By Georgina Griffiths : Meteorologist, MetService

Soil moisture deficits (SMD) in Canterbury typically start dropping rapidly through October and November, due to the drying spring westerlies.

This year was no exception, although modest rain at the time of writing in South Canterbury has briefly put the brakes on this decline. Ashburton recorded 20mm of rainfall in mid-November, while Rangiora recorded just 7mm over the same period. As of 15 November, a ‘snapshot’ of estimated soil moisture (fig. 1a) highlights how Hurunui and the Kaikoura coast, as well as Nelson, Blenheim, and central Hawke’s Bay, are showing significant soil moisture deficits. The definition of ‘significant’ soil moisture deficit is more than 110mm of deficit. These maps are useful to compile a national picture of where it is dry and where it is not. A zoomed-in version of the same map (fig. 1b) shows some of the detail available in the 4km resolution SMD estimates at the time. However, estimates are estimates, so always ‘ground truth’ the soil state-of-play for yourself. Figures 2 and 3 tell an interesting story about just how hard 2021 has been for Canterbury farmers. The plots show the unusually prolonged drought during summer and through autumn of 2021. Soils remained extremely dry until the end of May, when extreme flooding in Canterbury on 31 May caused widespread damage. New Zealand really breaks a drought savagely, doesn’t it?

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Valid MON 15.11.2021 13:00 local time

Figure 1a: A national ‘snapshot’ of soil moisture deficit (SMD), shown in mm of deficit (orange, yellow, green colours) and mm of surplus (blue colours), from mid-November. (The snapshot is estimated as at 1pm, 15 November 2021). This is an estimated SMD based on 4 km resolution weather modelling, based on estimated daily rainfall (mm), outgoing daily potential evapotranspiration (PET, mm), and a fixed soil available water capacity of 150 mm. ‘Significant soil moisture deficit’ is defined as more than 110mm of deficit, and ‘severe soil moisture deficit’ is defined as more than 130mm of deficit.

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Valid TUE 16.11.2021 12:00 local time

The rapid change in soil moisture levels at both Rangiora and Ashburton during the first half of November also caught my eye (figures 2 and 3). By mid-November, soil moisture deficits had accelerated higher than five of the six years shown, and were comparable to 2017 at the same time of year. The year 2021 sure has been a rollercoaster for Canterbury! With La Niña forecast for the coming summer and the associated risk of higher-than-usual pressures over the South Island, there is a very real likelihood that this weather rollercoaster is not yet over for the region. As always, you should keep up to date with the MetService long-range forecast at http://metservice.com/rural/monthlyoutlook, or ask us questions on the MetService Facebook or Twitter feeds.

Rangiora Soil Moisture Deficit

Figure 2: Rangiora soil moisture deficit (SMD), shown in mm of deficit, for the last six years (2016 to 2021-so-far). SMD is calculated based on incoming daily rainfall (mm), outgoing daily potential evapotranspiration (PET, mm), and a fixed available water capacity of 150 mm. (This is the amount of water in a theoretical soil ‘reservoir’ that plants could utilise). Soil moisture deficit in Rangiora in 2021 remained in ‘extreme’ soil moisture deficit (more than 130mm of deficit) through most of summer and all of autumn, until the extreme Canterbury floods on 31 May shot soils straight to run-off (saturation) for winter and early spring. Since mid-October, soil moisture levels have started to drop quickly again.

Ashburton Soil Moisture Deficit