October vapour pressure deficit data layer used in the creation of Land Environments of New Zealand (LENZ) classification. The classification layers have been made publicly available by the Ministry for the Environment (see koordinates.com/#/layers/?q=LENZ for to access these layers).
October vapour pressure deficit, recorded in kPa, is used to estimate the effects of variation in the dryness of the air. The climate station data used in the development of this climate surface were derived from summaries of climate observations published by the New Zealand Meteorological Service, using data collected over the period from 1950-1980. Estimates of the vapour pressure deficit for each month were derived by coupling a 100 m DEM with a thin-plate spline surface fitted to an irregular network of 287 meteorological stations, using humidity and temperature data. The resulting 100 metre layer was then interpolated to 25 metres using bilinear interpolation.
Calculation of the vapour pressure deficit first required estimation of the temperature at 0900 hours, the time at which humidity measurements are made. This was calculated from the measured mean daily minimum and maximum temperatures for each month using a function that simulates the temperature course through the day. The estimated temperature at 0900 hours was then used to calculate the saturation water vapour pressure , which indicates the maximum amount of water vapour able to be held in the air given its temperature. Vapour pressure deficits in October were used in LENZ as this is the month when westerly winds are generally most persistent, resulting in strong geographic variation in vapour pressure deficits across New Zealand.
This layer has been multiplied by a factor of 100 (i.e. converted into an integer grid) to save space and make the grids more responsive. A value of 33 is actually 0.33 kPa.
Additional details such as the climate station locations used in the creation of the layer and error maps are defined in the attached LENZ Technical Guide.