Floods and droughts are the result of markedly different processes. Large-scale floods are usually generated by intense, and long-lasting rainfall, or by snowmelt. Droughts are characterised by persistent large-scale weather circulations causing low rainfall, often in combination with high rates of evaporation. Despite these differences, WATCH has looked at floods and droughts together to develop techniques and methods that allow consistent characterisation of extreme events at the European as well as the global scale.
As the name suggests, extreme events are rare. To make analysis meaningful, we need long data records of daily river flows for several decades or more. The WATCH approach of exploring trends at the pan-European scale means that the data set must also include enough measuring sites to capture the range of topography and climate across Europe.
WATCH has compiled an exceptional pan-European data set (the EWA/WATCH data set) of more than 400 stations with observed river flows covering the period 1962 – 2004. These data have contributed to the production of flood and drought maps for Europe which can be combined with other key data sets to produce figures for the human, economic and environmental consequences of individual historical events.
The data set is also ideal for evaluating models that are being run using the WATCH Forcing Data. In the past, river flow data from global models has been validated using observed data from large continental-scale catchments such as the Danube or the Amazon. The EWA/WATCH data set is compiled from smaller, undisturbed catchments, and the network of measuring stations is dense enough to provide meaningful information over a large area. It is this denseness of data that has allowed WATCH to assess the ability of large-scale hydrological models to reproduce extremes at continental scale. This is a significant addition because, as a general rule, large scale models have focused on annual, seasonal and monthly means for large river basins.
Extreme events in the water cycle cause damage, disruption and loss of life. A changing water cycle will include changing extremes. According to IPCC predictions, there will be increases in the length and severity of droughts, and more seasonal and regional changes in floods. Policy makers and planners need the information to deal with this. WATCH has developed methods and techniques to recognise and record the conditions that have caused floods and droughts in the past, so that we can analyse the likely effects of future changes.