Uncertainty in three dimensions: the challenges of communicating probabilistic flood forecasts maps
Abstract. Real time operational flood forecasting most often concentrates on issuing streamflow predictions at specific points along the rivers of a watershed. Those points often coincide with gauging stations, and the forecasts can eventually be compared with the corresponding observations for post-event analysis. We are now witnessing an increasing number of studies aimed at also including flood mapping as part of the forecasting system, by feeding the forecasted streamflow to a hydraulics model. While this additional new information (flood extent, depth, velocity, etc.) can potentially be useful for decision makers, it also has the potential to be overwhelming. This is especially true for probabilistic and ensemble forecasting systems. While ensemble streamflow forecasts for a given point in space can be visualized relatively easily, the visualization and communication of probabilistic forecasts for water depth and extent brings additional challenges. The uncertainty becomes three dimensional and it becomes difficult to convey all the important information to support decision-making, while a confusion that could arise from too much information, counter-intuitive interpretation, or simply too much complexity in the representation of the forecast. In this paper, we synthesize the results of a large-scale survey across multiple categories of users of hydrological forecasts (28 government representatives, 52 municipalities, 9 organizations, 37 citizens and farmers, for a total of 139 persons) regarding their preferences in terms of visualizing probabilistic flood forecasts over an entire river reach. Those users have different roles and realities, which influence their needs and preferences. The survey was performed through individual and group interviews during which the interviewees were asked about their needs in terms of hydrological forecasting and their preferences in terms of communication and visualization of the information. In particular, we presented the interviewees with four prototypes representing alternative visualizations of the same probabilistic forecast in order to understand their preferences in terms of colour maps, wording, and the representation of uncertainty. Our results highlight several issues related to the understanding of probabilities in the specific context of visualizing forecasted flood maps. We propose several suggestions for visualizing probabilistic flood maps in order to convey all the relevant information while limiting the confusion of decision makers, and also describe several potential adaptations for different categories of end users.
Valérie Jean et al.
Status: final response (author comments only)
RC1: 'Comment on hess-2022-305', Helen Hooker, 24 Oct 2022
- AC1: 'Reply on RC1', Marie-Amélie Boucher, 01 Dec 2022
RC2: 'Comment on hess-2022-305', Anonymous Referee #2, 29 Oct 2022
- AC2: 'Reply on RC2', Marie-Amélie Boucher, 01 Dec 2022
Valérie Jean et al.
Valérie Jean et al.
Viewed (geographical distribution)
The research aims to understand and improve the important challenge of communicating three-dimensional flood map uncertainty to various end-users through a series of qualitative surveys. The manuscript is well written and structured, there are several wordy tables that could be presented differently (see suggestions below). The probabilistic visualisation prototypes presented represent a significant step-forward in terms of communicating uncertainty to forecast end-users. The work would benefit from more emphasis on which uncertainties are being represented in the visualisations and how the prior knowledge of the surveyed participants is assessed and how this impacts their opinions and the conclusions drawn from the results. The research questions proposed in the introduction are reasonable, they should be re-addressed again in conclusion. Consideration of the limitations of the survey approach and applications of this approach outside of Quebec would enhance the manuscript. Once these concerns are addressed, I feel that the article would make a valuable contribution to HESS.