Water dynamics of short-statured vegetation inferred from field versus satellite-scale microwave remote sensing

Monitoring plant water stress requires plant hydraulic measurements, such as measurements of water potential. However, such measurements are challenging to make at scales beyond single plants and over extended time periods. Observing plant water conditions across broad spatiotemporal scales is now enabled by passive microwave remote sensing. Specifically, vegetation optical depth (VOD) retrieved from satellite radiometers (SMAP, AMSR) provides a measure of vegetation water volume in the canopy at tens of kilometers. While satellite-based VOD has been used for a range of applications, rigorous validations of satellite VOD have not been carried out due to a need for labor intensive, widespread in-situ biomass and plant water potential measurements. A new method has enabled direct measurements of in-situ VOD, from Global Navigation Satellite Systems (GNSS). However, they have been less commonly used to evaluate shorter statured vegetation, which dominates most ecosystems. Here, we explore how satellite-based VOD from SMAP and AMSR-2 compare with field-based microwave observations from 272 GNSS-based interferometric reflectometry (GNSS-IR) sites located throughout the Western U.S as a part of the Plate Boundary Observatory (PBO) H20 network. These sensors use GNSS signals to estimate a normalized microwave reflectance index (NMRI), a proxy for VOD at a scale of tens of meters. We find that satellite VOD generally positively correlates with GNSS NMRI with correlations between 0.2 to 0.6 across sites, which is encouraging considering the vast differences in spatial scale (10s of meters for field sensors versus 10s of kilometers for the satellites). These correlations increase to 0.3 to 0.7 when evaluating sites in regions with low spatial vegetation type heterogeneity, low tree cover, and large seasonal vegetation dynamics. The correlations are higher for X-band VOD, likely related to our finding that both X-band VOD and NMRI are both more sensitive to seasonal vegetation variations relative to daily-scale responses than C-band and L-band VOD products are. These findings suggest that satellite VOD is capturing field-based GNSS signals, and therefore that these sensors are a critical (and arguably the only feasible) resource for calibrating and validating satellite VOD across spatial scales.