*2.4. International Soil Moisture Network*

In this paper, the in situ SM observations from the ISMN sites [36] are used to validate the CYGNSS SM data predicted by the downscaling model. Globally, the ISMN has set up more than 50 SM monitoring networks that are either operational or experimental. These networks provide a unified in situ SM database on a global scale, with a standardized data format and pre-processing quality flags [37]. The majority of sites that offer time and space co-located with CYGNSS observables are located in North America. Consequently, we selected 78 available sites within the spatial coverage of CYGNSS for our study (Figure 2). These sites primarily belong to the Soil Climate Analysis Network (SCAN), the U.S. Climate Reference Network (USCRN), and the Snow Telemetry Network (SNOTEL). The hourly SM data from the ISMN was processed by filtering it with the provided quality mark (marked with a "G" for "good") and subsequently converting it into daily averages. The surface SM data utilized was at a depth of 5 cm, aligning with the penetration depth of L-band microwave signals. For a comprehensive overview of ISMN, readers can refer to [36,38]. The ISMN dataset can be accessed publicly (http://ismn.geo.tuwien.ac.at, accessed on 20 April 2023).

**Figure 2.** Geographic distribution of International Soil Moisture Network (ISMN) sites in the study area.
