21 Sep 2022
 | 21 Sep 2022
Status: this preprint is currently under review for the journal HESS.

Uncertainty in three dimensions: the challenges of communicating probabilistic flood forecasts maps

Valérie Jean, Marie-Amélie Boucher, Anissa Frini, and Dominic Roussel

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)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • 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.


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Short summary
Flood forecast are only useful if they are understood correctly. They are also uncertain, and it is difficult to present all the information about the forecast and its uncertainty on a map, because it is three dimensional (water depth and extent, in all directions). To overcome this, we interviewed 139 persons to understand their preferences in terms of forecast visualisation. We propose simple and effective ways of presenting flood forecast maps so that they can be understood and useful.