## 2 Study Area  
Two different study sites, the Rur catchment in western Germany and the Apulian Tavoliere site in southern Italy, were selected for evaluation. The Rur catchment is located in the federal state of North-Rhine Westphalia in the West of Germany, while small parts of it are also situated in Belgium and the Netherlands. It has a total area of 2354 km\({}^{2}\), with an elevation of 680 m above sea level (asl) in the south and 30 m asl in the north (Figure 1) [32]. The northern, flat parts of the catchment are dominated by crop cultivation, with mainly sugar beet, maize, and winter cereals [33]. In the southern region, located in the low mountain range Eifel, the main landcovers are pastures and forest of deciduous and coniferous trees [32]. Looking at the soil types within the Rur catchment, at the most northern parts, Fimic Anthrosols and Dystric Cambisols soils with a loamy sand texture are present, while the other northern parts are characterized mostly by Cumulic Anthrosols and Haplic Luvisols with both silt loam textures. In the southern regions, Fluvisols, Gleysols, Eutric Cambisols, and Stagnic Gleysols with a silt loam texture are present [33]. While the mean field capacity in the northern part is greater than 200 mm, the soils in the southern part are less productive with field capacities ranging between 50 and 150 mm [32]. The mean annual precipitation ranges between 650 mm to 850 mm in the northern part. As the Rur catchment is located in the west wind zone, precipitation shadowing effects caused by the High Fen Mountains can be observed in the southeastern part of the catchment, causing lower annual precipitation of around 700 mm compared to the 1200 mm present in western luv site of the catchment [32,34]. In the southern Eifel region, most of the rainfall occurs in winter when evapotranspiration rates are at their lowest, which results in a large proportion of precipitation rapidly becoming runoff. Contrary to this, the northern part of the Rur catchment receives the highest rainfall during the summer, when potential evapotranspiration rates are high, which can effectively buffer precipitation events [33]. Previous studies revealed lower mean soil moisture values in the northern part of the Rur catchment, which could be attributed to the lower precipitation rates. The observed spatial variability in this region, being dominated by agricultural practices/farming and a very flat terrain, were found to be driven by vegetation, due to the temporal differences of evapotranspiration rates. Furthermore, varying management practices (e.g., planting and harvesting dates), and soil parameters (e.g., porosity and field capacity) were seen as the main reasons. In the southern region, higher mean soil moisture values were observed. Here, the highest variation occurs within sub-catchments dominated by forest stands, due to the heterogeneous topography and higher precipitation levels. For grassland areas, which are also characteristic for the southern part, also high mean soil moisture values, but with a much lower standard deviation could be observed [33]. During the observation period from 2018 to 2020, a long persisting drought period was present over central Europe, also leading to soil moisture deficits in the Rur catchment. For further information, we refer to the European Drought Observatory reports as well as the monthly drought monitoring from Helmholtz Centre for Environmental Research [35,36]. The Rur catchment also includes the Selhausen test site, which is due to the very good availability of in-situ data used as a validation site for several air- and space-borne microwave soil moisture products and listed as a super-site by the Committee on Earth Observation Satellites Land Product Validation Subgroup (CEOS LPV) [37, 38, 39, 40, 41, 42].