Estimating design hydrologic extremes in a warming climate: alternatives, uncertainties and the way forward

Abstract

It is now well established that our warming planet is experiencing changes in extreme storms and floods, resulting in a need to better specify hydrologic design guidelines that can be projected into the future. This paper attempts to summarize the nature of changes occurring and the impact they are having on the design flood magnitude, with a focus on the urban catchments that we will increasingly reside in as time goes on. Two lines of reasoning are used to assess and model changes in design hydrology. The first of these involves using observed storms and soil moisture conditions and projecting how these may change into the future. The second involves using climate model simulations of the future and using them as inputs into hydrologic models to assess the changed design estimates. We discuss here the limitations in both and suggest that the two are, in fact, linked, as climate model projections for the future are needed in the first approach to form meaningful projections for the future. Based on the author's experience with both lines of reasoning, this invited commentary presents a theoretical narrative linking these two and identifying factors and assumptions that need to be validated before implementation in practice. This article is part of a discussion meeting issue ‘Intensification of short-duration rainfall extremes and implications for flash flood risks’.

Towards advancing scientific knowledge of climate change impacts on short-duration rainfall extremes

“…Although CPM analyses have so far mainly concentrated on ‘peak intensity’ changes over fixed durations, e.g. daily, multi-day, hourly, etc., likely structural changes to different storm types in the future are important to understand for both impacts and for updating of design guidance (see [83]). Changes to event clustering are also likely important, e.g.…”

Intensification of short-duration rainfall extremes and implications for flood risk: current state of the art and future directions

“…To understand how best to adapt flood management practice for climate change, Wasko et al [41] review current practice around the world, finding that although globally efforts are emerging to manage flood impacts of climate change, there remains a focus on extreme rainfall changes with little consideration given to other factors such as antecedent moisture conditions. Based on the evidence of a two-stepped change in rainfall extremes (both thermodynamic and dynamic), Sharma et al [42] then put forward a possible strategy for engineering design and water management in a changing world. Finally, Dale [43] presents a pragmatic practitioner's view of what can be done to manage the effects of increasing extreme subdaily rainfall and their impact of raising the risk of flash flooding.…”

“…Based on the evidence of a two-stepped change in rainfall extremes (both thermodynamic and dynamic), Sharma et al . [ 42 ] then put forward a possible strategy for engineering design and water management in a changing world. Finally, Dale [ 43 ] presents a pragmatic practitioner's view of what can be done to manage the effects of increasing extreme subdaily rainfall and their impact of raising the risk of flash flooding.…”

A synthetic view of rainfall intensification in the West African Sahel

“…Hence, we stress that the figures reported here should be considered by engineers and decision-makers for planning adaptation strategies as far as water resources and water-related risks are concerned, though how to use this information to project strategies remains an open question (see e.g. Sharma et al 2021). Moreover, because these figures are not to remain constant in a rapidly changing climate, it is essential to regularly update them; it is thus crucial to continuously monitor the evolution of the daily rainfall regime at the regional scale.…”