Understanding Soil Modulation of Drought Persistence in CMIP6 Models

 Drought persistence is a critical factor in assessing water availability and its impacts on agriculture, ecosystems, and society. In this respect, poorly constrained soil properties in climate models such as field capacity – i.e. the maximum water a soil can retain after drainage of excess moisture – may strongly affect severity and persistence of simulated soil drought conditions. This study examines for the first time the regulating role of soil properties, particularly of field capacity, in shaping drought memory and its broader impacts. Using the CMIP6 multi-model ensemble and observations, we analyze drought dynamics across various phases of the hydrological cycle applying non-parametric standardized indices: Standardized Precipitation Index (precipitation deficits), Standardized Precipitation-Evapotranspiration Index (precipitation-evapotranspiration balance), Standardized Soil Moisture Index (soil moisture deficits), and Standardized Runoff Index (reduced runoff). Our analysis investigates the persistence between hydrological drought indicators, showing that soils with greater field capacity sustain drought conditions longer, emphasizing the importance of accurately modeling soil properties to capture drought persistence effectively. The historical CMIP6 simulations are compared with observational datasets, including GLEAM and CRU, to assess the deviation between model outputs and observed climate conditions. The future scenarios (SSP126, SSP245, SSP370, SSP585) are also examined, revealing significant regional differences in projected drought behavior depending on the degree of radiative-forcing increase during 21st century. High-emission scenarios project prolonged drought conditions due to increased temperatures and evapotranspiration feedback, while low-emission pathways are effective in preserving more stable hydrological dynamics. Our results show that, in water limited and transition areas such as the Euro-Mediterranean region, the persistence of droughts and its projected change considerably depend on the modeled field capacity. This study highlights the essential role of field capacity and other soil characteristics in regulating the variability and the persistence of drought events. By bridging historical validation with future projections, it provides a comprehensive understanding of drought dynamics and trends, also identifying observational constraints for the Earth System Models. These findings are crucial for refining predictions of agricultural and hydrological drought impacts and for guiding adaptation strategies in water-limited regions that are vulnerable to drought exacerbation under climate change.