GPR measurement of crop canopies and soil water dynamics- implications for radar remote sensing

GPR with a suspended 1 GHz horn antenna was deployed in the greenhouse and field for measurements of wheait and corn canopy properties and soil water content dynamics over bare and electrically terminating surfaces. Dielectric permittivity and water content of soil and canopy were independently determined from surface reflection (SR) and from signal propagation time (PT) to a reflective layer (aluminum foil) underlying plant canopy or a soil layer. SR valu6:s progressively decreased with increasing canopy biomass accoi-ding to Beer-Lambert type relationships. More attenuation was measured for the denser greenhouse dwarf whealt than in the field study. In contrast to SR measurements, PT values remained unaffected by canopy thereby provided an accuirate account of soil water content dynamics. Canopy remand restored SR-based soil water content measurements to PT values as confirmed gravimetrically. Similarly, canopy dielectric properties were inferred from canopy propagation times (CPT) and related to canopy water contents (CWC) inferred from phonological measurements (height-mass relatiionships) and leaf type (narrow or broad). Distinct reflections were correlated with key canopy biophysical pammeters. Results demonstrate the usefulness of horn antenna GPR for characterization of vegetation canopy effects, and for subcanopy water content measurements within a welldefined footprint, thereby offering a means for calibration and verifiication of radar data collected from air- and spaceborne platfiorms.