In summer 2003, we experienced a period of distinct drought in Europe, accompanied by an enduring heat wave. This had an effect on nature, ecosystems, the economy and society at large. It became clear that Switzerland is not sufficiently prepared for occurrences of this nature. According to climate scenarios, such extreme events could occur with greater frequency in the future. Thus it is important to better understand drought and how periods of dry weather in Switzerland can be forecast.
The following questions were studied:
What physical mechanisms could lead to drought in Switzerland?
How do water resources react to climate extremes and what, ultimately, causes drought conditions?
What role do social and economic factors have to play in the impact of periods of drought in Switzerland?
What benefits do hydrological forecasts have in comparison with normal weather forecasts?
The dynamics of periods of dry weather and the capacity of water resources to offset dry conditions were studied with hydrological models and land-surface models. The role of the soil and groundwater content in the resistance to run-off during periods of dryness was studied using stable isotopes. In addition, workshops and surveys helped to identify the information needs of various sectors in relation to water resources and resource forecasts. Based on the above, the researchers developed a prototype for an information platform with regularly updated information on dryness in Switzerland.
Dry weather is a complex phenomenon in which a range of environmental factors play a role: on the one hand, for instance, properly replenished natural water storage , such as soil moisture, snow or groundwater could "take the sharp edges" off precipitation deficits or heat waves. Dried out soil and vegetation, on the other hand, can accentuate dryness and heat waves. Other than that, the properties of soils and vegetation have an impact on the water cycle (processes between water supplies and the atmosphere). In mountainous regions, snow and snow melt also contribute to the water reserves.
As water resources are very varied, there are various forms of dryness with different properties and specific physical processes. Moreover, different ecosystems and branches of the economy can be affected by dry periods.
The main result of the project is the understanding of how long different water resources can balance out periods of dry weather. The most important environmental influences in this respect are the processes between land and the atmosphere and soil properties. As soil moisture and run-off or drainage have a delayed response to weather situations, they may reflect past extreme situations. What was clarified in particular was that the soil’s capacity to retain water is of paramount importance to forecasts.
The results of the research show that today’s climate could even surpass the levels for extreme dry weather experienced during the heat wave of summer 2003. Then, the damp spring weather took the edge off the dryness and high temperatures of the summer that followed.
The project otherwise studied low water run-off in Swiss catchment areas and developed corresponding forecasts for the prototype information platform. Low water run-off can be forecast with some certainty, particularly in the upper section of the rivers and directly below large lakes on the Swiss Plateau. These forecasts are important for shipping and water power.
In general, this project increased the public awareness of the problems that dry weather brings with it. The researchers conducted two workshops and one survey on dry weather. They determined that practitioners and the public at large were becoming increasingly interested in obtaining information on the hydrological situation in Switzerland and the respective forecasts. Of particular interest, apparently, are soil moisture and evaporation via vegetation. Such information is not, as yet, collected by meteorological agencies for forecasting purposes. The Drought-CH prototype information platform (www.drought.ch) created to this end is already receiving positive feedback and appears to be very useful for those affected or other interested users.
Important new insights were gleaned for the research behind this project, particularly in relation to the role of water resources in the initiation and persistence of periods of dry weather. This latest knowledge may be included in the information platform in future, with the purpose of improving forecasts in view of possible operational performance. All in all, the results of the project were important for both research and practice, and represent decisive steps in the early identification of periods of dry weather in Switzerland.
Evaluation of drought risk in Switzerland (DROUGHT-CH)