Rapporteur: Clemens Mathis
(Text nur auf Englisch verfügbar)
The presentations and discussions within Theme Block III aim to give answers to the following questions:
Ensemble Prediction Systems (EPS) in flood forecasting can give an indication that an extreme flood may occur, but do not forecast how extreme the flood will actually be. Nevertheless, flood early warning systems based on ensemble technique help to reduce flood damage. Safety measures (e.g. evacuation) can cost a lot of money, therefore the person or group of persons who has to make a decision is under considerable pressure. The difficulty for a human forecaster is to translate the uncertainties, which are inherent in the forecasting systems, for the end-user (water resource manager, rescue service, general public), who only wants to know a yes or a no.
EPS show a wide range of possible discharge outcomes and the communication of this diversity has not yet been solved. It is less a question of flood forecasting than of communication sciences. The correct way depends on many factors: characteristics and size of the river catchment, people's experiences of previous floods, and costs of the safety precautions are but a few. Therefore, there are many different solutions. The respective characteristics require adapted solutions.
At the Royal Netherlands Meteorological Institute (KNMI) an expert team determines the Weather Warning [1]. Frank Lantsheer showed in his presentation that this ensemble approach leads to better results. Furthermore, he presented the European Multiservice Meteorological Awareness System (EMMA) project by highlighting the problem of harmonisation of warning levels.
The EPS of the European Commission's Joint Research Centre produce 51 discharge curves. With box representations combining numbers and colours, the uncertainty of the medium-range forecasts is transferred into uncertainty measures. It is an effective way of visualizing the deterministic and probabilistic results of the European Flood Alert System (EFAS) in one diagram [2]. The end-users addressed here are the national regional flood forecasting and warning centres.
Forecasting in small catchment areas (< 100 km²) is very difficult. The lead time is very short. Nevertheless, the Swiss Federal Institute for Forest, Snow and Landscape Research has developed the IFKIS Hydro project, which was explained by Christoph Hegg [3]. It has been tested in two case studies. He sees the advantages of the ensemble predictions but stressed that additional information in crisis situations is needed and no added value will be reached without training of decision makers/end-users or members of their staff.
Extreme events are rare. The return period is in the range of about 100 years or more. Therefore, there has been little experience with forecasts of extreme precipitation events. In his written abstract Gerd Tetzlaff [4] showed the physics of extreme precipitation and the problem of validation of the forecast quality. He argues that, in addition to the mathematical and statistical validation of precipitation which has occurred and been recorded, criteria for good forecasts related to economical values should also be defined.
In this connection, he gave a summary of the Third International Conference on Early Warning (EWC III) held in Bonn, Germany on 27-29 March 2006. He highlighted the symposium findings that early warning is a major element of disaster risk reduction. It helps to prevent loss of life and reduces the potential economic impact of disasters. To be effective, early warning systems need to actively involve the communities at risk, facilitate public education and awareness, communicate and disseminate warnings and messages, and ensure that there is a constant state of preparedness.
The flood forecasting practices in the Netherlands with 55 different water level forecasts were presented by Leonie Bolwidt [5]. The question to the crisis manager is: How can the best of all this information be used? Ben Zweverink has had years of experience as a crisis manager. His decision tree for evacuation shows the importance of well-founded decisions. The authorities in the Netherlands have well-prepared flood management plans.
The ensuing discussion could not solve all the problems of the uncertainties. The participants felt that the human forecaster is needed in order to communicate the uncertainties to the end-user. Due to the uncertainties inherent in meteorological forecasts, communication between meteorologists and hydrological forecasters is necessary, especially in the case of extreme events. One advantage of the medium-range EPS is getting earlier information about a flood. The authorities have thus more time to prepare. The major benefit of ensemble prediction systems is the high probability that the right discharge curve is contained. The solution to finding the right one in every case is still missing. More case studies have to be carried out and lessons must be learned from them. Also, our knowledge of user needs has to be improved.
As a general conclusion it can be stated that probabilistic forecast products have to be tailored to the user-dependent level of decision-making.
[1] Frank Lantsheer, Severe weather warnings and focus on awareness.
[2] Jutta Thielen, Added value of ensemble prediction systems products for medium-range flood forecasting on European scale.
[3] Christoph Hegg, IFKIS-Hydro, an information system for flood risks at local and regional levels.
[4] Gerd Tetzlaff, Uncertainties of forecasts of extreme precipitation events.
[5] Leonie Bolwidt/Ben Zweverink, Dealing with uncertainties during high discharge conditions in The Netherlands.