Electronic Thesis and Dissertation Repository

Thesis Format



Master of Engineering Science


Civil and Environmental Engineering


Najafi, Mohammad Reza


Recently, an upsurge in the occurrence of hydroclimatic extremes and their temporal swings is observed in several regions around the world. Such transitions to the contrasting extremes such as the drought to flood in California (2016 – 17) has raised concerns about the increasing variability and rapid transitions between hydrological extremes and their associated compounding economic and environmental impacts. The intensification of the global hydrological cycle associated with climate change can further alter the drivers of such extremes and their interactions. Therefore, it is important to understand the characteristics of consecutive flood and drought events, including their spatiotemporal frequency and intensity in a changing climate. In this study, wet-dry swings are investigated based on precipitation and streamflow data in the Northwest North America. To this end, wet and dry conditions, as drivers of hydrologic floods and droughts, are investigated using the Standardised Precipitation Index for multiple accumulation periods (1, 3, and 6-months), calculated based on the downscaled and statistically bias corrected simulations of six Global Climate Models from the 5th phase of the Coupled Model Intercomparison Project with two medium and high emission scenarios for 1.5°C-4 °C global warming levels. Further, we use the Variable Infiltration Capacity hydrologic model simulated streamflow to show the hydrological response of the study area to the lagged compound floods and droughts under global warming. We identify future hotspots for the lagged compound hydroclimatic events. Our findings assert the necessity of integrating mitigation measures targeting such events into Disaster Risk Reduction strategies at the identified hotspots.

Summary for Lay Audience

Floods and droughts have profound impacts on a wide range of sectors such as water, agriculture, energy production, infrastructure, and ecosystem health. Although the characteristics, changing behaviour, and social impacts of floods and droughts have been previously investigated in many studies, traditionally floods and droughts have been treated separately. Therefore, the intersection between these two extremes has been overlooked, even though their rapid transitions can lead to greater economic and environmental impacts than the sum of each individual type of event. Historically, several catastrophic instances of transition between drought and flood have been recorded, such as the widespread floods in 2016 – 17 that occurred on the back of the multi-year drought of California. Such sudden swings can place substantial strain on emergency response teams and exacerbate tensions between stressed resources for flood relief or long-term water resource management. On top of all, scientists have shown that the global warming, a phenomenon occurring due to emission of greenhouse gasses that is warming up the atmosphere, can cause climate change, which could bring about changes to the weather patterns. Climate change has drastically contributed to intensification of weather and climate related extreme events (including floods and droughts) in some locations and is expected to continue this intensification in the future. Therefore, understanding the characteristics of flood-drought transitions as well as their projected future behaviour under climate change is of particular importance for a multitude of water-resources dependant sectors. In this study, we investigate how often floods and droughts swing, how long their transition takes, and how do their severity alter in a changing climate. Our results indicate that more frequent flood-drought alterations are expected in the future. Future flood-to-drought transitions are projected to occur more swiftly and between more intense floods and droughts if the global warming is not limited. Our findings illustrate the future hotspots for flood-drought transitions and provide valuable insights for decision makers to develop policies and take actions to cope with such disastrous compound natural hazards and adapt to their changing behaviour under a changing climate.