Role of precipitation deficits versus increased evapotranspiration for dry soils in Europe

Soil moisture dynamics are governed by the balance between water supply from precipitation and atmospheric demand driving evapotranspiration. Thereby, the relative roles of precipitation (P) deficits and enhanced evapotranspiration (ET) for inducing dry soils are unclear, including their variation across regions and drought phases. However, this is crucial because heat-driven drying and rainfall deficits imply distinct drought evolution patterns, different drought responses to global change, and require different water management strategies. 

In this study, we identify anomalously low surface soil moisture events and compare concurrent precipitation deficits with actual ET anomalies in a consistent framework. More specifically, we separate each dry event into two development and two recovery phases, and classify each phase into P-dominated, ET-dominated, Compound-dominated (P deficit with increased ET), or non-dominant regimes. We use gridded observation-based datasets over Europe at a daily resolution covering the study period 2001–2021.

Across Europe, the drought development phase is mostly characterized as Compound-dominated in humid to transitional climate regions in central and northern Europe. By contrast, in more arid Mediterranean regions, we find P-dominated regimes toward which become more frequent as drought development progresses. The weaker role of ET in southern Europe has to do with less amount of vegetation and more vegetation water limitation which constrains transpiration as a main contributor of ET, while atmospheric water demand is actually high in these regions. 

For the drought recovery phase we find mostly compound-dominated regimes. This indicates that rainfall events contribute to overcoming the peak dry soil moisture anomalies while this is supported by reduced ET. The latter may be relatively cloudy and colder-than-usual weather associated with precipitation as well as drought legacy effects limiting vegetation functioning and hence transpiration beyond the actual water deficit period.

While the overall results are robust, regional patterns depend on the choice of datasets and thresholds used in the identification of dry events. Overall, our analysis provides a physically interpretable typology of soil drought evolution that can support drought diagnosis and early-warning systems.