Impact of Interdecadal Hydroclimatic Variability on the Management of Water Supply for Melbourne, Australia

This paper presents the implications of interdecadal hydroclimatic variability on the management of Melbourne's water supply system. Melbourne is the second largest city in Australia with a population of over three million. Streamflow and reservoir inflow data from seven water supply catchments and rainfall data from 18 stations are used in this study. The empirical mode decomposition (EMD) method is used to identify cycles in the long rainfall and flow time series. The EMD analysis is also applied to bootstrap samples from the original time series to test the statistical significance of the identified cycles. In EMD analysis, a time series is decomposed into a set of intrinsic mode functions that are mutually independent. The decomposition is based on the direct extraction of energy (variance) associated with various intrinsic time scales that are automatically and adaptively selected from the time series. The EMD is a relatively new technique that is able to deal with both non-linear and non-stationary data, and has several advantages over other spectral analysis techniques. The EMD analyses of data from Melbourne's water supply catchments show statistically significant interdecadal cycles in many, but not all, of the time series data. The study also shows that the rainfall, runoff and storage characteristics are different in different interdecadal periods. The region is considerably wetter, with the storages spilling more often (or less drawn down) during the negative IPO phase compared to the positive IPO phase. The El Niño/Southern Oscillation versus hydroclimate teleconnection is also stronger during the negative IPO phase. The Interdecadal Pacific Oscillation (IPO) represents a low frequency ocean-atmosphere fluctuation in the Pacific region. However, the wet and dry cycles in the rainfall and flow data, and the cycles identified in the EMD analysis, are not necessarily in phase with the IPO periods. Nevertheless, although natural cycles are evident in the historical data, their future characteristics are difficult to predict. In addition, the future hydroclimatic characteristics are likely to be modified by global warming. This paper will discuss the implications of interdecadal hydroclimatic variability and climate change on the concept of reliability/security of water supply and the management of water resources.