Thin plate smoothing spline interpolation of daily rainfall for New Zealand using a climatological rainfall surface

This study presents a method for estimating daily rainfall on a 0.05 degrees latitude/longitude grid covering all of New Zealand for the period 1960-2004 using a second order derivative trivariate thin plate smoothing spline spatial interpolation model. Use of a hand-drawn (and subsequently digitised) mean annual rainfall surface as an independent variable in the interpolation is shown to reduce the interpolation error compared with using an elevation surface. This result is confirmed when long-term average annual rainfall data, derived from the daily interpolations, are validated using long-term river flow data. Copyright (C) 2006 Royal Meteorological Society.