School of Earth Sciences - Theses

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    Hydrological extremes and consequences of climate change
    Jakob, Dörte ( 2013)
    In the design of infrastructure, risk has been – and often still is – assessed on the basis of long-term averages. Information on variation in hydrological extremes is required as the basis for informed decision-making, preparedness and possible adaptation. Long-term trends are fairly well understood for temperature but less well for precipitation. While climate models have become sophisticated tools for projecting future changes in our climate, their ability to replicate observed variations in precipitation is limited and it is therefore prudent to complement climate models through analysis of historical observations. Design rainfall is used as one of the required inputs for hydrological models in the design of structures such as dams and bridges. Design rainfall estimates are supplied in form of intensity-frequency-duration curves. Rainfall frequency analysis is almost invariably based on the assumption of a stationary climate. Sub-daily durations are of particular interest for urban applications. This thesis was strongly driven by the motivation to provide guidance to decision makers who have to account for non-stationarity in rainfall extremes. Non-stationarity in rainfall extremes comes about as a conflation of climate change and climate variability. Unlike for temperature extremes, rainfall extremes for Australia as a whole exhibit no clear increase or decrease in intensity over time but strong association with the El Niño-Southern Oscillation (ENSO). This has implications for the choice of suitable analysis techniques, e.g. sophisticated non-parametric techniques. Depending on the planning horizons both climate change and climate variability may have to be accounted for. The association of rainfall extremes with ENSO leads to an opportunity to develop statistical models to support decision-making on shorter time scales. Analysis of seasonality in frequency and magnitude of rainfall extremes revealed considerable variation across a set of sites in the southeast of Australia, implying different dominating rainfall-producing mechanisms and/or interactions with local topography. The strongest signal for an increase in extreme precipitation is found for short durations. Changes in rainfall extremes come about through a combination of changes in thermodynamical and dynamical variables. To assess large-scale changes in circulation, a classification technique (self-organising maps, SOM) was applied and synoptic types were identified. Rainfall extremes were then related to the synoptic type under which they occurred, to assess observed changes in the frequency of rainfall extremes. Rainfall extremes are typically preceded by conditions that are much wetter (both in absolute and relative terms) and warmer than the climatological average. These anomalies tend to be larger for shorter durations, and for rarer events. Given that increase in humidity exhibits strong regional variability and that it may be counteracted by changes in dynamics, it appears simplistic to state categorically that climate change will lead to an increase in extreme rainfall events and observed trends in rainfall extremes show a picture that is more complex. In summary, the combination of changes in thermodynamic and dynamic variables will define the change in frequency and intensity of rainfall extremes. The factors that are most relevant for the effect of climate change on rainfall extremes depend on geographical location.