Proximal measurements of microwave reflectance using GNSS-IR track semi-arid grassland vegetation dynamics during greening and browning phases

Authors: Devine, Charles; Scott, Russell; Biederman, Joel; DU, JINYANG - University Of Montana; MOORE, DAVID - University Of Arizona; FELDMAN, ANDREW - Nasa Marshall Space Flight Center; GUO, JESSICA - University Of Arizona; ADAMS, DAVID - University Of Arizona; SMITH, WILLIAM - University Of Arizona

Submitted to: Agricultural and Forest Meteorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/16/2026
Publication Date: 3/21/2026
Citation: Devine, C.J., Scott, R.L., Biederman, J.A., Du, J., Moore, D., Feldman, A.F., Guo, J.S., Adams, D.K., Smith, W.K. 2026. Proximal measurements of microwave reflectance using GNSS-IR track semi-arid grassland vegetation dynamics during greening and browning phases. Agricultural and Forest Meteorology. 383; 111136. https://doi.org/10.1016/j.agrformet.2026.111136.
DOI: https://doi.org/10.1016/j.agrformet.2026.111136

Interpretive Summary: Microwaves that reflect off the Earth's surface are modulated by the amount of water stored in plants. While certain satellites can be used to estimate plant water over large regional areas, their sensing field is very large, making it hard to interpret and compare with ground-based measurements. However, measurements from ground-based global positioning systems (GPS) also receive reflected microwave signals transmitted by GPS satellites, and they can provide finer-scale observations. ARS researchers in Tucson, AZ and university collaborators determined how ground-based reflected microwave measurements compare with site-based measurements that are sensitive to plant water at a grassland site. They found that the GPS data was strongly related to plant productivity, evaporation, and canopy greenness, but the relationships were not as strong for the later half of the growing season when the plants were running out of water. They also found that nighttime plant water content estimated from GPS was higher at night than the subsequent day indicating the strength of daytime evaporation and productivity. Finally and contrary to the large mismatch between the size of the sensing areas, ground-based GPS measurements matched well with some satellite microwave measurements. We demonstrate the utility of GPS reflectance reflectometry for improved understanding of microwave sensitivity to vegetation dynamics.

Technical Abstract: The normalized microwave reflection index (NMRI) derived using Global Navigation Satellite Systems (GNSS) microwave interferometric reflectometry (GNSS-IR) has the potential to reveal high spatiotemporal insights into vegetation water status and growth. We explored diurnal to seasonal relationships of the NMRI with co-located soil water content (SWC), green chromatic coordinate (GCC), and eddy covariance gross primary productivity (GPP) and evapotranspiration (ET) at a semiarid grassland site in southeastern Arizona over the 2021 summer growing season. NMRI tracked the state (e.g., GCC) and function (e.g., GPP) of the vegetation more so than the state of the soil (e.g., SWC), but relationships were markedly different during vegetation growing season greening and browning phases. During the greening phase, NMRI showed strong positive correlation with next-day GCC (R2 = 0.94) and GPP (R2 = 0.95). During the browning phase, NMRI most closely tracked the relatively slow decline of GCC (R2 = 0.78) and less so the relatively rapid decline of GPP (R2 = 0.36). At the diurnal temporal scale, NMRI corresponded with expected patterns of vegetation water content (VWC) attributed to nighttime hydraulic recharge and daytime transpiration loss. We explored relationships between NMRI and satellite vegetation optical depth (VOD) estimates and found NMRI was strongly related to satellite X-band VOD during both phases (greening R2 = 0.94; browning R2 = 0.88) but weakly related to L-band VOD (greening R2 = 0.01; browning R2 = 0.34). Our findings indicate that NMRI: 1) tracks the co-development of semiarid grassland biomass and function during the greening phase and biomass decline during the browning phase; 2) can be scaled with satellite X-band VOD. Our first-time evaluation of the potential for coupled ground-based GNSS-IR NMRI and flux measurements highlights the potential of NMRI as an independent vegetation biomass and functional proxy that could complement other widely used optical remote sensing-based vegetation proxies.