<p>Probabilistic flood forecasts often concentrate on streamflow, but water depth and extent might convey more tangible flood information for some people. Water depths and extent can also be used more directly than streamflow as part of an impact-based forecasting set-up. However, within a probabilistic or ensemble approach, the uncertainty inherent to water extent and depth applies to all three spatial dimensions: the depth itself is uncertain, and so is the extent in terms of latitude and longitude. The notion of forecast uncertainty is generally well accepted by users, and on the one hand, the addition of new information (flood extent, depth, velocity, etc.) has the potential to be useful for decision makers. On the other hand, it also has the potential to be overwhelming and confusing. Therefore, visualising probabilistic flood forecast maps and communicating the information to the general public and to decision-makers poses multiple challenges. In this presentation we will synthesise the results from a large-scale survey of forecast users, including 28 government representatives, 52 municipalities, 9 organisations, as well as 37 citizens and farmers. Those different groups have different roles, realities, and perspectives. They also have different needs and preferences in terms of hydrological forecasts. The survey consisted of individual and group interviews. The participants were asked a variety of open questions regarding their needs and preferences for hydrological forecasts and also for the visualisation and the communication of those forecasts. One key element of the interviews was the presentation of four alternative visualisation prototypes for probabilistic forecasts of flood depth and extent. The participants were asked to compare those prototypes, to express their preferences in terms of colour maps, wording and the representation of uncertainty. They also provided useful comments on potential modifications to those prototypes and sometimes suggested ideas for entirely new prototypes. Our results highlight that most participants, regardless of their role or background, had the same overall preference in terms of the proposed prototypes, with prototype number 2 the overall favorite (all prototypes will be shown and explained during the presentation). Nevertheless, we also found several specificities among the respective preferences of different user groups. Our results also highlight specific issues related to the understanding of probabilities in the context of flood forecast maps.  The results of this research are currently being used to inform the design of the new forecast communication and visualisation platform in the province of Quebec, Canada.</p>

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.

No abstract

No abstract