Differences and similarities between the 2018 and 2003 droughts for the Rhine basin studied in terms of evaporative sources

Droughts can be studied from an atmospheric perspective by analysing the large-scale circulation resulting in a lack of water, or from a hydrological perspective by understanding the interaction of precipitation, evaporation, soil moisture and temperature at the land surface. Here, both perspectives are captured as we study the evaporative sources resulting in precipitation over the Rhine basin. These evaporative sources, being continental or oceanic, can give an indication of the vulnerability of a basin to ongoing and future land-use changes. We focus on the anomalous evaporative sources of the Rhine basin during the dry summers of 2018 and 2003, to understand what the contribution is of local recycling of precipitation versus advection of moisture into the basin to the total amount of precipitation over the Rhine basin. We do so by using ERA5 re-analysis data from 1979 to 2018 and the Eulerian moisture tracking model WAM-2layers.

During an average summer, the evaporative sources of the Rhine basin are mostly located over the Atlantic ocean. In addition there is a substantial contribution of continental evaporation, mostly from land regions west of the Rhine basin. During the summer of 2018 a persistent high pressure system (blocking) prevented moisture input from the Atlantic ocean and therefore relative more recycling of moisture over land took place (both continental areas outside the basin and within the Rhine basin). Due to the anti-cyclonic movement around the high pressure area, we also found a larger contribution of evaporative sources from continental regions east of the Rhine basin.

The amount of local recycling can be expressed in the precipitation recycling ratio, the local generated precipitation divided by the total precipitation in a region. We found higher than average recycling ratios during the dry summer months of 2018. Thus, due to the blocking more local evaporation resulted in precipitation over the Rhine basin, indicating the increased dependence on local land-surface processes. In general, we found a clear correlation between higher than normal recycling ratios and lower than normal precipitation in summer. An exception is the end of the dry summer of 2003, when low recycling ratios are found, probably indicating drying out of the soils and therefore lower evaporation rates.

To conclude, although the summer of 2003 and 2018 were both very dry, their characteristics in terms of moisture sources and thereby their dependence on the land surface were found to be rather different. In 2018, local recycling was important, contrasting to 2003 when the drying out of the soils made local recycling less important. These differences between two dry years over the same region highlight the important role of the land surface in precipitation feedbacks.