title: A new approach to navigate uncertainty in climate-related hydrological drought risk
Creators: name: Chan, Wilson C. H.
Publisher: name: University of Reading
Year: 2024

description: Hydrological droughts threaten public water supply and significantly impact the natural 
environment and other sectors. Quantifying the plausible worst case drought in the present day 
climate and understanding the impacts of climate change on future droughts help decision-makers 
formulate plans to enhance resilience. However, existing studies are often dominated by a top-down approach characterised by a cascade of uncertainty which particularly suffers from an 
insufficient exploration of low-likelihood, high-impact outcomes. This thesis aims to navigate
uncertainty in the hydrological impacts of climate change by introducing a physical climate 
storyline approach. Drought storylines created from process understanding, pooling of ensemble 
reforecasts and sampling from large ensemble climate model simulations enhances risk awareness 
and aids robust decision-making for water resources planning. 
Retrospective storylines of past droughts created by perturbations made to observed 
events embraces downward counterfactual thinking through identification of different ways in 
which events could have turned out worse. A more severe precipitation deficit in autumn 1975 for 
east England or an even drier winter 1975/76, both of which could have arisen from natural 
climate variability, could have led to a reduction in accumulated river flows by up to &gt;50% at slow-responding catchments in East Anglia. Storylines of the 1976 drought could have surpassed the 
maximum intensity of the 1921-22 drought by up to &gt;30% to become the most severe post-1891 
drought for the region. Similarly, drier preconditions of the 2010-12 drought and a plausible third 
consecutive dry winter could have led to significantly more severe conditions across the UK. A 
third dry winter, in particular, could have led to conditions matching the benchmark multi-year 
drought for east England (1989-1993), highlighting the fact that the drought could be seen as a 
near miss. Placing the observed drought in a 2°C warmer climate is estimated to exceed mean 
deficit of the benchmark 1989-93 drought by &gt;60% at some of the worst affected catchments.
Initialized large ensemble model simulations is an emerging way to create high-impact, 
low-likelihood storylines. Storylines of the 2022 drought in East Anglia created by combining 
observations with pooled ensemble reforecasts enable decision-makers to explore the likelihood 
of worst-case river flow trajectories, identify high impact combination of physical climate drivers 
to increase risk awareness during an on-going event. Further, application of the UNprecedented 
Simulated Extremes using ENsembles (UNSEEN) technique estimates that the chance of 
unprecedented high (low) summer temperature (rainfall) increases from 5.7% (8.8%) in the present 
day to 58.3% (18.1%) in a 3°C warmer world. The larger sample simulations are ideal for searching 
for high impact drought sequences where the physical credibility of simulated events may be
verified more easily compared to statistical methods. The utility of the current generation of large 
ensemble simulations to sample for multi-year drought storylines is further discussed with 
drawbacks due to spatial resolution, the need for bias adjustment and the under-estimation of 
weather system persistence (such as atmospheric blocking). 
The various methods presented in this thesis provide evidence of the magnitude of present 
and future extreme droughts. Outstanding research gaps to ensure the physical credibility of 
drought storylines include the need to improve quantification of observational data uncertainty 
and better characterisation of hydrological model realism during extreme droughts. The 
demonstrated value of the storyline approach contributes to the diversification of approaches used 
in water resources planning and could form a core part of planning to assist in stress-testing water 
resources systems and enhancing resilience to future droughts.