Mean annual solar radiation data layer used in the creation of Land Environments of New Zealand (LENZ) classification. Development of surfaces for annual and monthly solar radiation required substantially more data preparation than for the other climate surfaces, reflecting the small number of stations at which solar radiation has traditionally been measured. Monthly estimates of average daily solar radiation to 1980 were available for 22 meteorological stations, but measurements of sunshine hours were available for a total of 98 stations, including 18 of the stations for which solar radiation measurements were available. To extract as much information as possible from these data, a surface was fitted first that predicted for each month the ratio of solar radiation reaching the earth's surface to that reaching the top of the atmosphere, with the latter calculated from solar geometry equations. In fitting this surface, only the 18 data points where measurements were made of both solar radiation and sunshine hours were used. In addition to NZMG coordinates, it used as an additional predictor the ratio of measured sunshine hours for each month to the maximum possible sunshine hours given no cloud. This surface was then used to estimate the monthly solar radiation received at each of the 80 sites for which measurements of sunshine hours alone were available. Using a total of 98 sites for which solar radiation data were either measured directly or estimated from sunshine hours, surfaces predicting annual and monthly solar radiation were then fitted. Data describing monthly humidity was used as a surrogate measure of cloudiness to improve the fit of the surface to the underlying data. This also increases the local accuracy of the surface predictions, as the number of meteorological stations used to fit the humidity surface is more than three times greater than the number of sites used to fit the solar radiation surface. 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.
All climate layers used in LENZ were derived either directly or indirectly from mathematical surfaces (thin-plate splines) that use information about the climate, location and elevation of a number of meteorological stations. Locations are described either in terms of their latitude and longitude or their coordinates on a map projection such as the New Zealand Map Grid (NZMG). Each surface is calculated using a process in which data values for each climate station are omitted in turn and its climate is predicted from the surrounding stations. This process is repeated until no further improvement can be made to the fit of the surface to the raw data. Surfaces can be simultaneously fitted to up to 12 variables, typically monthly data for various climate parameters, e.g., monthly estimates of temperature or rainfall. Additional details such as the climate station locations used in the creation of the layer and error maps for this layer are defined in the attached LENZ Technical Guide. Once the surface has been fitted, predictions can be made for any point of known location and elevation. For example, coupling the surface with a digital elevation model, a regular grid of elevation values, allows the generation of digital climate maps as used in the creation of LENZ. Finally, maps that show the standard errors of the predicted values can be derived using results from more sophisticated analyses of the errors associated with the climate surfaces. The majority of the climate station data used in the development of our climate surfaces were derived from summaries of climate observations published by the New Zealand Meteorological Service. Temperature and rainfall data were collected over the period from 1950 to 1980, while data describing humidity and solar radiation consisted of averages of all data collected up until 1980. Some additional short-duration records of rainfall, including information from storage rain gauges, were used to describe geographic variation in high-rainfall mountainous areas of the South Island.