Potential Evapotranspiration (PET)Whakaeto Toruhūnga (PET)

Potential Evapotranspiration (PET) values are a guide to how much water is being lost from the earth’s surface to the atmosphere and on this page you will find measurements of PET recorded at our climate sites in the region. 

What is PET?

Evapotranspiration (ET) is a combination of evaporation and plant transpiration (water vapour released into the atmosphere through plant stomata). PET is the amount of evapotranspiration that would occur from a reference crop, usually pasture, if there is sufficient water available.

PET is highest in sunny, windy conditions because solar radiation provides the energy for evaporation to occur while the wind quickly transports the water vapour into the atmosphere, enabling more to fill its place.

Knowing the difference between rainfall amounts and PET can help when deciding whether or not to irrigate. To convert PET to an actual crop ET, a crop co-efficient and stress coefficient must be applied. However, when irrigating, the stress coefficient is not essential as most irrigation strategies aim to prevent the crop from experiencing stress. An excellent reference for crop ET is the Food & Agriculture Organisation (FAO) Irrigation and drainage paper 56. Crop Evapotranspiration—Guidelines for Computing Crop Water Requirements (Alan et al. 1998).

The Regional Council PET data

Hawke’s Bay Regional Council measures meteorological information at several climate stations across Hawke’s Bay. The parameters monitored vary between sites but normally include air temperature, humidity, rainfall, wind speed and wind direction. Several sites also measure soil moisture, soil temperature and solar radiation. For sites where solar radiation is recorded, PET is calculated using the Penman-Monteith equation. Units are measured in millimetres.

Data from the climate monitoring sites throughout the region are telemetered to the Council office throughout the day. The information collected is vital for flood warning purposes, water management during drought and detecting long-term trends in climate. The climate stations are calibrated on an annual basis and are regularly maintained by Council staff.

Additional climate data is also available from NIWA’s CliFlo website which provides access to the National Climate database. Climate data accessed through CliFlo is free by subscribing on-line, and is subject to NIWA’s terms and conditions of use.