A drought is when there's a rainfall deficit over an extended period of time which restricts or prevents a human activity such as farming or power generation.

Extended dry periods are experienced in New Zealand (NZ) most years but whether or not they are classed as drought depends on a range of different factors. Soil moisture deficit, measured in millimeters (mm), is used as one index of drought. It measures the amount of soil moisture available to the roots of pasture plants and is generally higher in summer when evaporation rates are higher. In turn, this can lead to the ground drying up and can also reduce the volume of water in rivers/streams, lakes/dams and subterranean reservoirs.

The impacts of drought can be economic, environmental and social. Drought can affect farmers by shriveling crops or drying out grass so that livestock don't have enough to eat (or drink). These losses then flow through into downstream production and other sectors, such as retail, where shortages can cause price increases. Drought also increases the risk of fire, as well as depleting water flow in storage which, in turn, can cause problems for the production of hydroelectric power. Drought can also mean that rural and urban populations have water restrictions imposed on them.

Drought can occur almost anywhere. Apart from the West Coast the whole of NZ, particularly the northern and eastern areas of both islands, is likely to become a little drier over the next few decades if climate change progresses as predicted. Consequently, stocking up in the hope of a wet season with an abundance of feed is most likely asking for trouble! It is better to have fewer stock and, if you do end up with a surplus, it will provide good insurance for the rest of the season or can be conserved and sold.

Monitoring daily rainfall records and doing regular pasture cover assessments should be part of your routine farm assessment programme. Lack of rainfall, low soil moisture, poor pasture growth rates and dwindling feed supplies are all obvious indicators of a longer than usual dry spell and should act as triggers for action.