Evaluation of surface soil moisture retrievals from microwave satellite sensors using ground observations.

The aim of this work is to evaluate the performance of surface soil moisture (SSM) measurements from microwave satellite sensors, specifically the Copernicus Sentinel-1 and SMAP (Soil Moisture Active Passive) missions, using ground-based observations. Soil moisture is a critical environmental variable that influences various hydrological and ecological processes. Accurate measurement of soil moisture is essential for applications such as agricultural monitoring, flood forecasting, and climate modeling. This study focuses on two distinct regions: the Twente region in the Netherlands, known for its comprehensive soil moisture monitoring network, and the Italian basins affected by the severe flood event in May 2023. By comparing satellite-derived soil moisture estimates with in-situ measurements, the research assesses the temporal and spatial correlations between these data sources. Key methodologies include the use of statistical metrics such as Root Mean Square Error (RMSE), the coefficient of determination (R²), and correlation analyses to evaluate the accuracy of satellite data. Additionally, the study investigates how well the Sentinel-1 SSM1km product captures variations in soil moisture before, during, and after the flood event. This assessment aims to determine the capability of the SSM1km product in reflecting significant changes in soil moisture associated with flooding events, which is crucial for effective flood management and mitigation strategies. The results indicate that while satellite-derived soil moisture data generally correlate well with ground-based measurements, there are notable discrepancies influenced by factors such as land cover and surface roughness. The study found that the RMSE values ranged from 0.1065 to 0.306, with correlation coefficients (R²) varying across different stations. These findings highlight the strengths and limitations of current microwave remote sensing techniques for soil moisture retrieval and underscore the importance of continuous validation against ground observations.