Forecast Uncertain? Improving the Use of Hydrologic Probabilistic Information in Decision-Making

Communicating forecasts effectively requires understanding how intended audiences interpret and use forecast information whether presented deterministically or probabilistically. Prior research has studied how public and professional audiences use and understand probabilistic hydrologic forecast information, but questions remain concerning how uncertainty should be presented most effectively and how to simultaneously present deterministic and probabilistic forecasts without diminishing the value of either.

To investigate these questions, Nurture Nature Center and East Carolina University conducted a mixed-methods social science study of three forecast products to propose improvements to the display and communication of uncertainty and probabilistic information in hydrologic forecasts. Specifically, the Advanced Hydrologic Prediction System and regional hydrographs, outputs from the Hydrologic Ensemble Forecast System (HEFS), including seasonal forecast products, and briefings for impact-based decision support services were presented to residential and professional audiences in three River Forecast Center service areas (Middle Atlantic, Colorado Basin, and California Nevada) using scenario-based focus groups and surveys. An iterative process involving potential variations of the scenario products allowed the project team to assess how deterministic and probabilistic forecasts can best be understood to convey the complexity of certainty and uncertainty in short, medium-term and seasonal hydrologic forecasts. The products were tested in a range of high and low flow conditions in the study areas through a total of 16 in-person focus groups and an online follow-up survey.

Findings show significant differences in understanding and needs among the various stakeholders (professionals and public) and between the different regions, but in general, additional uncertainty information was positively received. A series of user-tested prototype products developed in cooperation with the operational offices in each study area and detailed key findings and recommendations for probabilistic communication will be presented. Studying the three products – hydrograph, briefing, and HEFS - promotes understanding of the issues associated with probability and understanding, and of the relative influence of elements across these products that can improve understandability and utility.