TECHNICAL
CANTERBURY SOILS DRYING OUT Words by Georgina Griffiths : MetService meteorologist
Soil moisture deficits (SMD) in Canterbury typically start dropping rapidly through October and November, due to the drying spring westerlies.
-150
-130
-110
-90
-70
-50
-30
-10
10
30
50
70
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.
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 -150
-130
-110
-90
-70
-50
-30
-10
10
30
50
70
Figure 1B: A zoomed-in ‘snapshot’ of soil moisture deficit (SMD) across Canterbury, exactly as described in Figure 1A 54 NZGROWER : DECEMBER 2021
