Feature article

Information on Drought

Stuart Burgess and Ashmita Gosai, NIWA Climate Applications and Research Group

The issue of drought can be a major concern in some Southwest Pacific countries. Drought occurs during a sustained period of low rainfall, which has persisted long enough to produce serious hydrological imbalances. These may result in shortages or restrictions on water supplies, crop failure and loss of productive land, damage to horticulture and lack of feed and consequences for agriculture, as well as increased potential for wild fire, and other extreme events.

There are many ways of measuring or defining drought. Agricultural drought is based on modelled estimates of soil moisture balance, where drought is said to occur when there is insufficient soil moisture to sustain plant growth. Crops become stressed as the readily available water capacity of the pasture root-zone becomes depleted and incipient wilting occurs. On tropical Southwest Pacific islands it can take as little as two or three weeks with insufficient rainfall to meet this criterion, even in the wet season. The drought ends when rainfall finally restores the soil moisture levels.

Hydrological drought occurs when rainfall is well below expected levels in any large catchment area for an extended period of time. Hydrological drought is usually associated with dry spells, which are temporally variable in regards to long-term averages.

Another useful climatological or meteorological definition of drought assumes that water shortages may often affect an area for short periods, in which case provisions are usually made or are already in place to cope with those events. Hazard conditions might be said to occur when at least the seasonal (3-month) rainfall in the area affected falls below the ten-percentile value, and a severe event falls below the five-percentile value, i.e. an event that occurs on average less than once in twenty years.

Droughts tend to be prolonged events in comparison to other extreme events which may cause a similar magnitude of havoc yet be over within a few hours or days. Droughts tend to break when there is substantial rainfall to recharge the moisture levels in the rivers, lakes and in some cases even soils.

Regular reporting and monitoring of rainfall and its relationships to ENSO characteristics play an important role towards improving drought prediction for Southwest Pacific Island countries, and also towards mitigating potential hazards caused by drought, by assisting people and organisations to become better prepared for drought and its consequences.

The Governments of the Pacific Island countries have established National Disaster Management Units under the umbrella of South Pacific Geosciences Commission (SOPAC) which not only deals with tropical cyclones, floods, earthquakes, and tsunamis, but also provides advice and national warnings during droughts.