This year's drought and the promise of more contain a message for land-based industries, writes Jim Salinger.

Most of the North Island is currently in the grip of a historic drought that is the most severe in over 70 years of climate records. It is hard to imagine how drought could occur in this cluster of damp islands immersed in a huge ocean. But the cogs of the Southern Hemisphere weather machine do get stuck at times, and droughts have disrupted New Zealand life at various times since humans first settled the land. Indeed, they are our costliest of climate disasters. Drought is sneaky. Unlike the other climate hazards, it tends to creep up on us unobtrusively until suddenly a disaster is upon us.

When it rains, water tops up the soil for plant growth and fills up our water-supply reservoirs and hydro lakes. As time passes, plants conduct moisture out from the soil to the air, home-owners and industries use the water in the reservoirs, farmers use the water on their land for their animals or to irrigate their crops, and water from the hydro lakes is used to generate electricity. Eventually, if there is no rain to replenish the water that is being used, the lakes and reservoirs run dry. It is as simple as balancing your spending against your income in your cheque book. Although droughts in New Zealand are not as frequent as heavy rainfall, they have a huge impact.

The number one cause of droughts is anticyclones, which bring fine weather and light winds. When highs get stuck in the Tasman Sea as they have this summer, droughts occur in the North Island and the upper South Island. So how does the current agricultural drought rate historically?

The regional spread is borne out by the dramatic soil moisture deficit status maps for March 9 from the National Institute of Water and Atmospheric Research. For the North Island as a whole, the present drought as in early March ranks as the most severe for the 70 year record - and for New Zealand by the end of the month it will probably rank as the second most severe. For the north of the North Island it ranks as the second most severe and for the east of the North Island the third.


And how are agricultural droughts changing and expected to change in the future? In Niwa's latest report on drought released last year, New Zealand can plan for around 10 per cent additional time spent in deficiency by the middle of this century for key agricultural regions in the east of both islands. Their projections from climate models show that by the 2080s much of the country's agricultural zone will experience some increase in drought, even under lower climate warming scenarios. Chris de Freitas of Auckland University also notes that in the Far North and Auckland major droughts could become more frequent with climate warming. The trend towards a hotter climate overall and drier in parts raising the incidence of a droughtier climate is clear.

And this is exactly what has been happening recently. Mean temperatures have now increased 1C since the 1900s. The diagram shows the potential soil moisture deficit over each growing season - the amount of water that would be required to keep pastures topped up with moisture for grass growth. The higher the deficit, the more severe the soil moisture deficiency is. In the North Island overall the nine year period from 2004/05 has recorded the highest potential soil moisture deficit record in observations from 1941/42. Fortunately last season was wet. Similar statistics for New Zealand record 2000/01, 2007/08, 2009/10 and 2012/13 in the top six years for potential soil moisture deficit.

The economic impacts of this drought are very likely to be huge. Earlier droughts have forced farmers to send livestock to the works early.

Dairy farmers have had to dry off their herds early, and feed out or buy in feed. Drought also affects other farming activities, with fewer crops such as maize are being harvested. Financially, earlier droughts last century pushed some farmers to the wall, and they walked off their land.

This last point brings us to the fact that major droughts have knock-on effects. For instance, those farm animals that have survived are left very weak, and in the following spring, many fewer lambs and calves are born than usual.

The direct financial toll of earlier droughts has been huge. The total costs of the 1997-8 El Nino and 1998-9 La Nina droughts have been estimated at $2.5-$3 billion (2010 dollars). Similarly the nationwide drought between spring 2007 and autumn 2008 cost the economy $2.8 billion. Already initial estimates of the 20012/13 drought are $1 billion, and rising.

The message is clear: nature is showing the trend towards a future where major droughts are more common. Those involved in land based industries will need to adapt their decisions and management to fit the trends towards more water and soil moisture deficiency in a warmer world.

Dr Jim Salinger is an Auckland climate scientist.