In this blog I am going to use the d-word aka
drought; i.e. “A period (long) of dry
weather that is harmful to crops”.
Canterbury (and parts of lower eastern North
Island) has had weather conditions that are having (and have had) an adverse
effect on plant growth since December 2014 - a drought.
Since November (and especially since
December) ET has outstripped rainfall on a daily basis. Setting aside an isolated 27mm rainfall on 22
February there have been only 10 days since 1 December when the daily rainfall
has exceeded ET. In that time a soil
moisture deficit of more than 170mm has developed on a Lismore silt loam or
about 3 months average rainfall.
This season is not abnormal and is very similar to the
El Niño event of
1997-98. Do the seasons 1997-98 and
2014-15 have similar El Niño Southern
Oscillation Indices (SOI)? Comparison of
the SOI for the two seasons shows strong similarity. No question 1997-98 had much stronger SOI
with index values, especially in December-March when they were in excess of -20
in. While
the index alone does not guarantee drought like conditions, most will remember
the 1997-98 drought.
The 2014-15 SOI is similar
because it has been negative since June and greater than -6, the indicator of a
strong event. El
Niño typically
results in the lower than normal rainfall on the east coast - so it has been on
the east coast of Canterbury and parts of the North Island.
If the seasons continue
to mimic each other it would appear there is still a wait till April or May for
a significant rainfall and probably not until May before rainfall might exceed
ET and start to attack to the soil moisture deficit.
This insightful blog post was written by Dr Anthony Davoren from HydroServices Ltd