Short-Term Drought Risk Dynamics: The Impact of Multi-Decadal Climate Variability and Water Supply System Properties

The impact of climate variability and climate change on the security of water supply has significant consequences for water resource planning. It is currently the subject of considerable uncertainty in Australia and around the world. Persistent drought across much of Australia and increasing water demand due to population growth has placed greater stress on water supply systems. This paper represents an alternative approach to assessing water supply system security through the use of short-term drought risks and dynamic conditional simulation techniques. A simulation framework that incorporates multi-decadal climate persistence into stochastic rainfall simulations is presented. This climate-informed multi-time scale stochastic (CIMSS) framework was compared to the widely-used AR(1) model. It was found that the CIMSS framework estimated short-term drought risks that were up to double that estimated by the AR(1) model, dependent on climate regime. However, the long-term risks showed insignificant differences between the models. Conditioning simulations on the climate regime and initial reservoir conditions induced a peak in the dynamic short-term drought risk. Thus, the concept of peak short-term conditional drought risk is introduced. A study using non-dimensional water supply system properties revealed that short-term drought risks can be significantly higher than long-term drought risks, increasing non-linearly as the system becomes more stressed. This work illustrates the value of using stochastic rainfall models that actively capture, climate variability. They can be considered more informative of water supply system risks than models that rely merely on the hydrological record for their calibration. Further, it is argued that the short-term dynamic drought risk approach is a useful strategic planning tool for water authorities. It represents an advance in thinking and provides a more realistic and informative estimation of drought risk than traditional long-term approaches.