Amplified signals of soil moisture and evaporative stresses across Poland in the twenty-first century

In Poland, recent summer droughts have had devastating environmental, social, and economic consequences, but the trend of growing season dryness remains unclear. This study focuses on the soil moisture and evaporative stress conditions, analyzed in a multiyear period between 1981 and 2019. Country scale trends in growing season drought severities are assessed using indices derived from the model-based estimates of soil moisture and evapotranspiration. These are compared with indices derived from meteorological variables. Soil droughts are assessed by the Standardized Soil Index (SSI), while the ecological droughts are related to evapotranspiration by the Standardized Evaporative Stress Ratio (SESR). Moreover, the Standardized Precipitation Index (SPI), Standardized Precipitation Evapotranspiration Index (SPEI), and self-calibrating Palmer Drought Severity Index (sc-PDSI) are used for comparison. A Drought-Prone Area (DPA) is delineated based on a criterion defined as simultaneous occurrence of statistically significant drying trends in surface and root zone soil moisture and evaporative stress. It was found that soil and ecological drought severities have remarkably increased in the growing season. This confirms the hypothesis that intensified soil drying is accompanied by intensified water stress imprinted in evapotranspiration. The most severe drought sequence has occurred in recent years, amplified by exceptionally high air temperature, low precipitation, and high deficit in the climatic water balance. The highest correlation is observed between annual growing season drought severities derived from the SSI and SESR; only SPEI approximates an increasing trend, while the SPI and sc-PDSI do not follow such a trajectory. The study shows an almost contiguous spatial pattern of DPA, which takes 42% of the country. One important implication of this study is that soil moisture and evapotranspiration should be considered in assessing drought severity in addition to traditionally used meteorological variables. (c) 2021 The Author. Published by Elsevier B.V. This is an open access article under the CC BY license (http:// creativecommons.org/licenses/by/4.0/).

Automated summary

In Poland, the severity of summer droughts and their impacts on the environment, society, and economy are investigated in this study. The results show that both soil and ecological drought severities have significantly increased during the growing season, confirming the hypothesis that intensified soil drying leads to intensified water stress in evapotranspiration. The study suggests that soil moisture and evapotranspiration should be considered together with meteorological variables in assessing drought severity.