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How extreme? Integral assessment of the 2018 drought

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Background

How extreme was the summer drought 2018 in Switzerland?  The answer this question is not straightforward since droughts show diverse spatial and temporal patterns. Furthermore, these events can be characterized by different variables including precipitation, discharge, soil moisture, or groundwater level. An integral assessment approach is therefore needed to put  the 2018 drought into perspective to other recent past drought events.

 

Analyses

The analysis consisted of two parts: 1) water balance analysis, and 2) drought frequency analysis.

 In the first part, we looked at the water balance components precipitation, evapotranspiration, discharge, and storage changes for the large hydrological basins in Switzerland.

In the second part, we looked at different types of drought including meteorological, hydrological, agricultural, and groundwater droughts and at the two drought characteristics duration and deficit. To do so, we used two different datasets: i) a dataset of 137 catchments for which observed discharge and precipitation time series were available and ii) a simulated dataset consisting of precipitation, discharge, soil moisture, and low-flow storage simulations for 307 medium-sized catchments obtained by using the hydrological model PREVAH. The use of the simulated in addition to the observed dataset allowed for obtaining a spatially comprehensive picture and for considering soil moisture and low-flow storage in addition to precipitation and discharge. The analysis first identified drought events using a threshold-level approach, second performed frequency analysis on the individual variables and on variable pairs, and third compared the severity of the 2018 event with respect to the different drought variables to the severity of the 2003 and 2015 events. The frequency analysis was performed both in a univariate and in a bivariate manner. The univariate analysis allowed for assessing event severity with respect to individual drought characteristics. The joint analysis of two variables under the consideration of their dependence allowed for the determination of severity with respect to two variables. We focused on specific variable pairs, which were of interest in practice, e.g. discharge deficit and soil moisture deficit, which are relevant for agriculture where water for irrigation might be extracted from rivers once no more water for plant growth is available in the soil.

 

Results

The water balance analysis showed that annual precipitation and discharge only lie 6% below the long-term average. The winter months preceding 2018 and December 2017 were rather wet while the summer months were exceptionally dry with low soil moisture and thus also partly limited evapotranspiration values. The storage change through glaciers was considerable. Eastern and northern Switzerland were found to be most affected by precipitation and discharge deficits. Glacier fed rivers, such as the Rhône showed slightly above average discharge values.

The drought frequency analysis showed that the 2018 event was especially severe in northeastern Switzerland in terms of soil moisture with return periods longer than 100 years and in terms of precipitation and discharge deficit. The joint consideration of soil moisture and discharge deficits resulted in even higher return period estimates. Return period estimates depended both on the variable considered and on the type of return period chosen for the analysis. A unique answer to the question of how extreme the 2018 event was can therefore not be provided but depends on the perspective of the person asking it.