Accuracy of ASCAT-DIREX Soil Moisture Mapping in a Small Alpine Catchment

Recent improvements in soil moisture mapping using satellites provide estimates at higher spatial and temporal resolutions. The accuracy in alpine regions is, however, still not well understood. The main objective of this study is to evaluate the accuracy of the experimental ASCAT-DIREX soil moisture product in a small alpine catchment and to identify factors that control the soil moisture agreement between the satellite estimates and in situ observations in open and forest sites. The analysis is carried out in the experimental mountain catchment of Jalovecký Creek, situated in the Western Tatra Mountains (Slovakia). The satellite soil moisture estimates are derived by merging the ASCAT and Sentinel-1 retrievals (the ASCAT-DIREX dataset), providing relative daily soil moisture estimates at 500 m spatial resolution in the period 2012–2019. The soil water estimates represent four characteristic timescales of 1, 2, 5, and 10 days, which are compared with in situ topsoil moisture observations. The results show that the correlation between satellite-derived and in situ soil moisture is larger at the open site and for larger characteristic timescales (10 days). The correlations have a strong seasonal pattern, showing low (negative) correlations in winter and spring and larger (more than 0.5) correlations in summer and autumn. The main reason for low correlations in winter and spring is insufficient masking of the snowpack. Using local snow data masks and soil moisture retrieval in the period December–March, improves the soil moisture agreement in April was improved from negative correlations to 0.68 at the open site and 0.92 at the forest site. Low soil moisture correlations in the summer months may also be due to small-scale precipitation variability and vegetation dynamics mapping, which result in satellite soil moisture overestimation.