Metrology for Multi-Scale Soil Moisture Monitoring (SoMMet) and High-Resolution Earth Observation Validation

Surface soil moisture (SSM) products derived from microwave remote sensing technology are currently operational at coarse resolutions (10-40 km) and global scale. Despite the utility of existing Earth Observation (EO) SSM products, there is significant scientific interest in enhancing the ability to resolve fine-scale surface heterogeneity. High spatial resolution soil moisture patterns (e.g., 0.1-1 km) can improve our quantitative understanding of the soil-vegetationatmosphere system and enhance applications such as mapping the impact of irrigation on local water budgets, assessing the effects of local soil moisture variability on atmospheric instability, and improving numerical weather prediction (NWP) and hydrological modeling at regional scales. Additionally, these high-resolution data are crucial for hydrometeorological research focusing on extreme weather events in the context of climate change. The European Copernicus program, with its sustained observation strategy using Synthetic Aperture Radar (SAR) sensors, including the European Radar Observatory Sentinel-1 (S-1), the S-1 Next Generation satellites, and the forthcoming EU L-band Radar Observation System for Europe (ROSE-L), motivates and stimulates the development of operational land surface monitoring at high spatial resolution. From the EO SSM validation perspective, significant efforts have been made to define protocols, identify reference measurements (RMs), and address the spatial mismatch between EO SSM products and RMs, which are typically point-scale measurements from hydrologic networks. However, this process is still ongoing, particularly for high-resolution SSM products, and requires a collaborative effort among different scientific communities to achieve metrologically traceable EO SSM. This paper presents the European project “Metrology for Multi-Scale Monitoring of Soil Moisture” (SoMMet), which aims to establish a metrological basis and harmonization in soil moisture measurements across scales, from point scale to remote sensing, through cosmic ray neutron sensors (CRNS). These sensors are characterized by different measurement supports in the horizontal, vertical, and temporal dimensions. A key aspect of the project is to conduct field campaigns at three high-level field sites across Europe: Marquardt in Northern Germany, Bondeno in Northern Italy, and the Apulian Tavoliere in Southern Italy. The comparison of soil moisture data from point scale, CRNS, and S-1 SSM at these experimental sites is discussed, and recommendations on EO SSM validation practices are provided.