In Switzerland, groundwater is the most important water resource. On the one hand, groundwater is used for provision of drinking water for the populace (80% of the drinking water comes from the groundwater) and, on the other hand, groundwater is important in maintaining ecosystems, for the basic drainage of rivers during dry periods and is increasingly important in the irrigation of agricultural land. The main aim of the present project was to study the sensitivity of various aquifers to climate change, and to assess the consequences arising from this for groundwater availability. The most significant question in this regard was whether higher temperatures and lower levels of rainfall in summer could lead to a fall in the groundwater level and the density of water flow. This issue is particularly important as the drinking water supply in Switzerland usually comes from aquifers near the surface, which could react sensitively to periods of drought.
The study of a range of aquifers concentrated on the Swiss Plateau. On the one hand, the aim was to ascertain how the feeding of the aquifers by infiltration of precipitation (direct groundwater regeneration) and of river water changes. On the other hand, analysis was carried out into how these changes manifested themselves in the groundwater levels and the density of water flow. To do this, data from past periods of drought (e.g. 2003) was studied and possible future developments were simulated with mathematical models.
According to our calculations, the average annual change in direct regeneration of groundwater (infiltration by precipitation) is relatively low, and the results for different climate models can vary widely. The seasonal amounts of direct groundwater regeneration, however, could differ more significantly in future: while in winter and spring direct groundwater regeneration increases due to greater levels of precipitation, these levels drop in summer and autumn due to increased evaporation/transpiration and the greater need of plant life for water.
The feeding of the aquifers from river water will probably change in a similar way: other projects (CCHydro) have shown that rivers on the Swiss Plateau and in the Jura regions will, in future, carry less water in late summer or, in the case of streams, run dry on certain stretches. This would lead to a decrease in the feeding of aquifers by infiltration from river water. The reduction in direct infiltration has the consequence that permeable minor aquifers (e.g. fluvio-glacial deposits) could empty rapidly, which could lead to local water shortages. Nonetheless, not all groundwater aquifers would be similarly endangered during dry periods: for example, groundwater aquifers with greater groundwater-retention potential (e.g. molasse aquifers), large unsaturated zones or delayed inflows across boundaries are more resistant to water shortages and are better equipped to withstand dry periods. Furthermore, our observations and calculations indicate that a reduction in the infiltration of river water in valley floor aquifers has differing effects depending on geographical location. In valleys that stretch from the Swiss alpine foothills to the Swiss Plateau (e.g. Emmental or Tösstal), the groundwater level drops sharply during dry periods in the upland valley area, while hardly changing in the central section. In this way, groundwater aquifers in the upland areas work as dynamic “buffers” and stabilise drainage and groundwater levels further down the valley. However, water levels can fall rapidly in valley-floor aquifers on the Swiss Plateau as well if infiltration decreases rapidly at low water, which is the case in highly canalised rivers in particular. Whether or not that would lead to a water shortage depends on how much water has been stored in the aquifers in order to span periods of dry weather.
Aquifers fed by rivers recover relatively quickly after dry spells, as rivers generally collect water from a large catchment area and feed that into the aquifers. Aquifers fed by infiltration from precipitation (direct groundwater regeneration), in contrast, need more time as dried out soil will absorb a lot of water before it can infiltrate deeper zones. Should several periods of dry weather follow each other, the impact of these could soon add up.
The present project has led to the development of new methods, particularly in the twin fields of groundwater/surface water modelling. This new knowledge and the concomitant improvement in the understanding of the process of aquifer sensitivity during dry spells will help us to better assess the importance of different factors when it comes to the impacts of climate change on groundwater regeneration. Using the results, aquifers that respond particularly sensitively to climate change can be identified, appropriate measures can be taken in good time and targeted monitoring programmes can be implemented.
Groundwater resources under changing climatic conditions