Monthly water balance ratio 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).
This layer differentiates between regions that have minimal water deficit, but vary markedly in the total amount of rainfall they receive. Based on a water balance model using monthly estimates of mean daily temperature, mean daily solar radiation, and mean rainfall. This layer is the annual average of the monthly ratios of rainfall to potential evaporation calculated using the method of Priestley and Taylor (1972). The resulting 100 metre layer (The temperature, solar radiation and rainfall layers were all 100m) was then interpolated to 25 metres using bilinear interpolation.
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 The data layer was derived from surfaces fitted to monthly data describing daily average temperature, daily solar radiation and monthly rainfall. The rainfall surface was fitted using NZMG coordinates, elevation, and a model describing relationships between topography and westerly winds. Having derived the 36 input climate layers, an empirical model was first used to calculate the monthly potential evaporation from the monthly estimates of solar radiation and temperature. These estimates of evaporation were then compared with the monthly rainfall estimates. To calculate the monthly water balance ratio, the ratio of rainfall to evaporation was computed for each month, and then the average of these twelve ratios was calculated.
Additional details such as the climate station locations used in the creation of the layer are defined in the attached LENZ Technical Guide.