Validation of Satellite Soil Moisture Products by Sparsification of Ground Observations

Abstract

Dense soil moisture observation networks serve as the primary means to validate large-scale satellite soil moisture products. However, maintaining intensive observation demands substantial labor and financial resources. It is therefore crucial to address the issue of how to sparsify ground observations while still achieving comparable validation results. Accordingly, this study takes an agropastoral interconnected zone of the Shandian River basin as an example. Representative soil moisture sites and the minimal number of required sites (NRS) were investigated at different sampling depths (3 and 50 cm) and at multiple scales (3, 9, 36, and 100 km). The average soil moisture estimated from the representative sites was then applied to validate SMAP L2, L3, and L4 multiscale soil moisture products. The findings indicate that the spatial heterogeneity of soil moisture within the Shandian River basin remained relatively consistent across varying spatial scales. However, it exhibited a notable increase in heterogeneity when moving vertically into deeper soil layers. Representative sites can more accurately determine the average soil moisture in a region, showing that SMAP notably overestimated root zone soil moisture and underestimated surface soil moisture. This research can serve as a theoretical guide for watershed-scale soil moisture estimation as well as a solid scientific foundation for improving the architecture of the watershed soil moisture network.

Keywords

• Representativeness
• SMAP
• Shandian River basin soil moisture network (SMN-SDR)
• soil moisture
• temporal stability analysis
• validation