Fitsum Woldemeskel

Natural hazards in Australia: droughts

Droughts are a recurrent and natural part of the Australian hydroclimate, with evidence of drought dating back thousands of years. However, our ability to monitor, attribute, forecast and manage drought is exposed as insufficient whenever a drought occurs. This paper summarises what is known about drought hazard, as opposed to the impacts of drought, in Australia and finds that, unlike other hydroclimatic hazards, we currently have very limited ability to tell when a drought will begin or end. Understanding, defining, monitoring, forecasting and managing drought is also complex due to the variety of temporal and spatial scales at which drought occurs and the diverse direct and indirect causes and consequences of drought. We argue that to improve understanding and management of drought, three key research challenges should be targeted: (1) defining and monitoring drought characteristics (i.e. frequency, start, duration, magnitude, and spatial extent) to remove confusion between drought causes, impacts and risks and better distinguish between drought, aridity, and water scarcity due to over-extractions; (2) documenting historical (instrumental and pre-instrumental) variation in drought to better understand baseline drought characteristics, enable more rigorous identification and attribution of drought events or trends, inform/evaluate hydrological and climate modelling activities and give insights into possible future drought scenarios; (3) improving the prediction and projection of drought characteristics with seasonal to multidecadal lead times and including more realistic modelling of the multiple factors that cause (or contribute to) drought so that the impacts of natural variability and anthropogenic climate change are accounted for and the reliability of long-term drought projections increases.

A network-based analysis of spatial rainfall connections

This study presents an analysis based on recent developments in network theory to examine the spatial dynamics of rainfall. The concepts of clustering coefficient and degree distribution are employed to study the spatial connections in a rainfall network across Australia. The clustering coefficient is a measure of local density, while the degree distribution is a measure of spread. Monthly rainfall data over a period of 68 years from a network of 230 gaging stations across Australia are analyzed, and different correlation thresholds are considered. The clustering coefficient results help identify actual neighbors and actual links in the network as well as stations/regions with high and low connectivities. The results from both methods also suggest that the network is not a purely random graph but may be an exponentially truncated power-law network. The connectivity and type of the raingage network are also influenced by the correlation threshold used for identifying connections. A network-based approach for studying spatial variability of rainfall is presented.Clustering coefficient and degree distribution methods are employed to study connections in a rainfall monitoring network.The influence of correlation thresholds on network properties is examined.The outcomes have implications for rainfall interpolation and classification purposes.

Should flood regimes change in a warming climate? The role of antecedent moisture conditions

Assessing changes to flooding is important for designing new and redesigning existing infrastructure to withstand future climates. While there is speculation that floods are likely to intensify in the future, this question is often difficult to assess due to inadequate records on streamflow extremes. An alternate way of determining possible extreme flooding is through assessment of the two key factors that lead to the intensification of floods: the intensification of causative rainfall and changes in the wetness conditions prior to rainfall. This study assesses global changes in the antecedent wetness prior to extreme rainfall. Our results indicate a significant increase in the antecedent moisture in Australia and Africa over the last century; however, there was also a decrease in Eurasia and insignificant change in North America. Given the nature of changes found in this study, any future flood assessment for global warming conditions should take into account antecedent moisture conditions.