European daily dataset of soil moisture from in situ atmospheric observations

The development and validation of an in situ dataset for daily soil moisture values based on observed atmospheric variables from the European Climate Assessment & Dataset (ECA&D) is introduced. These data are input to the SoilClim model which provides soil water content values for the upper 10 cm, the root zone and the deeper layer of the soil. Soil characteristics are taken from the widely used SoilGrids system. 
Model results are validated against observations of actual evapotranspiration (ETa) from the FluxNet experiment. Calculated soil water content are compared against a selection of the observations of the International Soil Moisture Network (ISMN). In the comparisons against observations, calculated values of ETa and soil water content (SWC) from the state-of-the-art soil dynamics model SWAP, which is more advanced than SoilClim, are taken-in as well. The comparisons show that SoilClim and SWAP show generally the same behaviour albeit that SoilClim saturates quicker than SWAP. On average, the similarity between the models and observation for ETa is higher (correlations ±0.78) than for the observed soil water content (correlations between 0.52 and 0.62).
The pan-European dataset for soil water content results in a set of 5758 stations for which daily soil water content values are produced, for the surface zone (upper 10 cm) and for the root zone (upper 40 cm), for the period for which all input data is available.
Based on the rootzone soil water content, four climate indices are calculated. These are the Soil Moisture Index (SMI) which is a metric to compare soil water content values across the diverse climatic zones which are present in Europe. This index relates to water content levels which are above or below the 50% level of the field capacity. In addition, an index is introduced which simply counts the number of days where the soil water content drops below the 50% level of the field capacity. In addition, two indices assess more extreme soil water content values: the number days where the soil water content is either nearly at field capacity or close to the wilting point. The 1991-2020 spring and summer climatology is provided for this dataset in terms of these indices, the relatively recent 2018 drought is put in a historical perspective by a comparison against the 1976 drought and a trend analysis of summer desiccation since 1979 is provided.
Because of its daily resolution and the climate indices, this dataset may serve as validation of other soil moisture dataset and the assessment of changes in the occurrence of extremes in soil moisture.