The model intercomparison of extremes has helped to reveal the strengths and weaknesses of the individual models. This exercise provided invaluable feedback to the modellers and has improved our understanding of the water cycle.
The ability to reproduce extremes will improve as this feedback is incorporated into revised models. This is highly encouraging as we look to model extremes using the WATCH Driving Data for the period to 2100.
Access to up-to-date data is essential when we are trying to identify trends over periods that include the immediate past. Unfortunately, access to hydrological data is not as easy as it should be, and our ability to make further progress will be hindered unless accessibility is improved.
Two highly visible outputs of WATCH are the Flood and Drought Atlases. They capture the characteristics of droughts and floods over the 20th century across Europe.
Similar work has been done in the past, but the WATCH atlases use newly developed, more visual, and more intuitive ways to present the timing and location of events, their duration and extent, plus their severity. The atlases are also very good at presenting rainfall and streamflow anomalies. For example, they can highlight when and where a particular amount of rainfall has triggered an unexpectedly severe flood. The reasons for this can then be investigated.
The atlases have established a comprehensive reference of historical European floods and droughts, viewed in a continental context rather than national or regional. This larger scale approach allows policy makers to see beyond their own region and to see how their responsibilities link to neighbouring areas that may be experiencing similar conditions, and to judge when coordinated management is needed. The atlases also provide a benchmark of extremes against which to compare future modelled data, and future observed events.
The EWA/WATCH data set has been used to detect changes in observed data over time. The analysis concurs with climate records for Europe, showing higher mean annual flows in the wetter north, and lower annual means in the drier south.
Analysis also revealed that in those regions that experience low flows during the summer, we are now seeing lower low flows. But in contrast, in regions that experience low flows during winter (due to freezing conditions) there are now higher low flows. This is because increased temperatures are causing winter snowmelt. Seasonal changes within years are also apparent. In northern Europe, the period December-February is becoming wetter, and there is a noticeable shift towards drier conditions in March and April. Further work will reveal whether this is the result of trends in temperature or rainfall. Our objective is for the models to reproduce subtleties such as these.