Compound droughts in Switzerland under current and future climate

Periods with very low discharge, anomalously low precipitation or anomalously low soil moisture content are all considered droughts, although different types. The differentiation is relevant from an impact perspective. For example, low discharge (hydrological drought) is challenging for transportation and fish, low precipitation (meteorological drought) is challenging for energy production and agriculture, and low soil moisture for agriculture and soil subsidence. If the different types of droughts occur simultaneously impacts may compound, e.g. if discharge is low irrigation water can no longer be taken from rivers.

We quantify the co-occurrence of the three different drought types in Switzerland in the extended summer season under current and future climate conditions using the Swiss climate change scenarios (CH2018, CH2018-hydro). These scenarios are a set of 44 GCM-RCM model chains based on CMIP5. Temperature and precipitation information from these scenarios is then used to run a hydrological model for more than 80 catchments in Switzerland to generate discharge time-series but select only catchments that are not strongly influenced by glaciers and snow melt.

Compound drought days are prevalent under current climate conditions both north and south of the Alps (on average more than 3 days per year).  Compound drought days tend to affect several catchments across the country at the same time and the simultaneously affected area increases with climate change. We find a significant and substantial (more than a doubling) increase in the number of compound drought days in all catchments both north and south of the Alps under RCP8.5 forcing conditions. Under an effective mitigation forcing (RCP2.6) the increase is not significant.