Ground-penetrating radar measurement of near-surface hydrologic processes

Ground-penetrating radar (GPR) with a suspended 1-GHz horn antenna was deployed over bare and vegetated soil surfaces. Water contents inferred from radar surface reflectivity (SR) were in agreement with gravimetric measurements from the top 1 cm soil layer, whereas measurements of signal propagation time (PT) agreed with the water content of the profile and with time-domain reflectometry (TDR). Stationary and subsequent concurrent radar measurements during the summer and fall over soil textures ranging from coarse sand to a sand-bentonite mixture showed rapid drainage from sand and slower drainage from higher-surface-area textured soils. Soil texture and temperature affected diurnal variations in uncorrected measured water content (occurrence of minima and maxima) for both SR and 2-cm TDR water content data. Thermodielectric bound water and bulk soil dielectric mixing models were inverted to more accurately determine soil water contents from SR data. Soil textures were delineated using two sets of SR data acquired at 12-hour intervals.; This GPR setup was deployed for measurement of soil water contents and crop canopy properties over bare and electrically terminating surfaces. SR and PT were used to independently determine dielectric permittivity and water content of soil and canopy. Measured surface reflection coefficients progressively decreased with increasing canopy biomass according to Beer-Lambert-type relationships. In contrast, PT measurements remained unaffected by canopy, and hence provided an accurate account of soil water content dynamics. Immediately after canopy removal, SR-based soil water content measurements were in close agreement with PT values. Canopy dielectric properties epsilonc were inferred from canopy water contents (CWC) and canopy propagation times (CPT), with the (epsilonc,CWC-1):(epsilon c,CPT-1) ratios indicative of canopy type. Distinct canopy reflections were correlated with key canopy biophysical parameters.; GPR with a horn antenna provided insights into soil water content dynamics and vegetation canopy characteristics within a well-defined footprint, enabling verification of air- and spaceborne radar measurements at detailed temporal resolutions not available by other radar remote-sensing systems.