Analysis of soil moisture trends in Europe using rank-based and empirical decomposition approaches

Abstract

The impact of climate change on soil moisture (SM) dynamics is uncertain. Changes in the Earth's SM during recent decades have been studied globally and in different regions, but little attention has been given to Europe. In addition, most previous works have just relied on a monotonic behavior of SM changes, which is a strong assumption and not always valid. We argue that this fact, together with the use of large temporal scales, has prevented the observation of clear patterns of SM trends over the continent. In this work, we study European SM trends for a 30-year period, from 1991 to 2020, using two complementary databases, one from reanalysis project ERA5-Land and the other from the model Lisflood. Both rank-based and empirical decomposition approaches have been considered and applied to monthly and annual series of SM anomalies. The Köppen-Geiger classification allowed us to analyze the distribution of SM anomaly trends in the separate European climates. The results obtained with both databases, methods and temporal scales were consistent, with the empirical decomposition method generally detecting more significant trends. Our results show a general decreasing trend of SM, regardless of climate type but more intense in Eastern and Central Europe. In addition, the rank-based method detected fewer positive trends, suggesting a non-monotonic behavior in changes to wetter conditions. The most notable differences were obtained with the empirical decomposition method when comparing the different temporal scales. Hence, an intramonthly analysis was conducted to provide insight into the different patterns. An increase in significant trends was observed in April and the autumn (September–October-November). Furthermore, we conducted a similar analysis to study trends in extreme drought characteristics (annual duration, intensity and onset) and we obtained consistent results, with the empirical decomposition method detecting more significant trends. Our investigations show a general increase in the duration and intensity of extreme droughts over the European continent, tending to be delayed a few days per year in arid and temperate regions.