Monday 30 March 2015

"A period (long) of dry weather that is harmful to crops"

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