  The KnnCAD Version 4 weather generator algorithm for nonparametric, multisite
simulations of temperature and precipitation data is presented. The K-nearest
neighbour weather generator essentially reshuffles the historical data, with
replacement. In KnnCAD Version 4, a block resampling scheme is introduced to
preserve the temporal correlation structure in temperature data. Perturbation
of the reshuffled variable data is also added to enhance the generation of
extreme values. A case study of the Upper Thames River Basin in Ontario, Canada
is performed and the model is shown to simulate effectively the historical
characteristics at the site. The KnnCAD Version 4 approach offers a major
advantage over parametric and semi-parametric weather generators as it can be
applied to multiple sites for simulation of temperatures and precipitation
amounts without making assumptions regarding the spatial correlations and the
probability distributions for each variable.
