How the data are USED

Most West Thames local stakeholders present at the focus group reported using seasonal hydrological forecasts in their everyday job and believed that they could possibly forecast flood and drought risk. The stakeholders also indicated that the forecasts could be used to support decision-making and to increase preparedness. Still, many noted that the forecasts cannot yet be used in this way as the products that are available in the UK show too much uncertainty and have a coarse spatiotemporal resolution (i.e. Hydrological Outlook UK).

In December 2017, a focus group was held at the University of Reading in collaboration with the UK Environment Agency (EA; IMPREX stakeholders)¹. It included a decision-making activity designed to capture how different water sector users currently interpret and act on SHFs to inform decisions in the West Thames. Participants were shown progressively confident and locally tailored SHF out to four months to focus on a period of extreme stormy weather and flooding (i and ii, Figures 1 and 2, respectively). To avoid participant bias through prior knowledge, they were not told which time period was covered by the SHFs.

Ensemble hydrographs for streamflow and groundwater levels indicated the predicted trajectory of the hydrological conditions for the next four months in each of the ten Thames catchments.²
Exceedance thresholds – the median and the 90th percentile of climatology – were provided for context. The maps showed the maximum probability that the full forecast ensemble for a catchment exceeded the 90th percentile of climatology in a given month.

Based on the EFAS-Seasonal forecasts (Fig. 1), the consensus was for above-average streamflow and groundwater levels, but participants requested clearer information to take action. Most participants opted simply to communicate internally and to keep an eye on the situation. Based on the improved EFAS-Seasonal forecasts (Fig. 2), the consensus was for confident forecasts that showed a high risk of streamflow and groundwater flooding in about 6 weeks' time. Participants also increased their decision/action choice in response to these more confident, locally tailored forecasts. They, for instance, adopted a “watch and wait” approach, requested further information about the situation (i.e. reliability of the forecasts), informed other organisations and implemented actions for preparedness.

Local knowledge, risk appetite, and experience of previous flood events played an important role in informing decisions. Discussions highlighted that forecast uncertainty does not necessarily pose a barrier to use, but that SHFs need to be presented at a finer spatial resolution than that of currently available operational products to aid local decision-making. The discussions also revealed that SHF information that is visualized using combinations of maps, text, hydrographs, and tables are more easily interpreted and that better communication of SHFs that are tailored to different user groups is needed.

Currently, the EA use SHFs predominantly as supporting information and rely on shorter-range forecasts for action. As a co-developer of this focus group, they expressed the ambition to make conscious decisions based on SHF as part of routine incident management strategies though. They suggested this could be achieved by helping staff to interpret this type of information and by identifying suitable low-consequence actions that could be taken.