Simulation of precipitation in the upper Waitaki catchment, New Zealand, and its relation to the Interdecadal Pacific Oscillation: Interannual and intraseasonal variability

We aim to simulate time series of daily precipitation amounts within a season over many years in the upper Waitaki catchment New Zealand and find its relationship with the Interdecadal Pacific Oscillation. Such simulations are highly desirable in hydroclimatic research for New Zealand, particularly in predicting the potential capacity of hydroelectricity generation two to three decades in the future. A rainfall generator, based on a mixture model for precipitation that is conditioned on a low frequency climate state, is proposed. The daily rainfall amounts at three rain stations in the upper Waitaki catchment and the Interdecadal Pacific Oscillation Index during 1950–2000 austral summer are fitted to a stochastic rainfall generator by an EM algorithm to derive the parameters of the model. The proposed rainfall generator together with a time series of the Interdecadal Pacific Oscillation Index are then used to simulate the daily rainfall amounts for summer. We find the simulated interannual variability, given by the mean and variance of seasonal precipitation totals and the correlation between seasonal precipitation and the Interdecadal Pacific Oscillation Index, are close to the observed values. The simulated intraseasonal variability, given by distributions of dry and wet-day durations, and the means and tails of the distribution of daily precipitation, are also close to that observed in the data.