Measuring drought

Irrigation near Luggate, Central Otago, an area of New Zealand that is prone to drought. [NIWA]

Irrigating near Luggate, Central Otago.

Potential evapotranspiration deficit (mm) that is likely to be equalled or exceeded once every 20 years.

Drought is often hard to quantify, and its severity and duration affect different enterprises in different ways.

One way of measuring drought is to calculate the amount of water typically available for a particular purpose, and then estimate the reduction in water available for that purpose during low rainfall seasons.

For example, we can estimate the amount of water that is 'consumed' during normal pasture growth in an average season. We can quantify production loss during dry seasons by estimating reduction in outputs like dry matter production or lower stock numbers. When we compare these data with the levels of water deficit that caused them, we can establish critical water deficit thresholds.

Potential evapotranspiration refers to the estimated use of water by pasture when there is no shortage of water available for pasture growth, both through transpiration and evaporation.

When water is short, pasture growth is limited. This water shortage can be referred to as a potential evapotranspiration deficit (PED), or the amount of water needed (by irrigation or rainfall) to keep pasture growing at its potential rate for the season or time of year.

Using a soil water balance, we can calculate PED from daily rainfall (input) and expected water use and losses (output). Most east coast regions of New Zealand, for example, experience a PED of 200 to 400 mm each year.

This approximately equates to 50 to 100 days of reduced pasture growth per year.

The map above shows the level of PED that can be expected about once every 20 years or so, the kind of drought farmers might expect, for example, two or three times during their working lives.

Travelling irrigator, Canterbury Plains. (Photo: Alistair McKerchar)