Plot-scale assessment of soil freeze/thaw detection and variability with impedance probes: implications for remote sensing validation networks

Several large in-situ soil moisture-monitoring networks currently exist over seasonally frozen regions that may have use for the validation of remote sensing soil freeze/thaw (F/T) products. However, further understanding of how the existing network instrumentation responds to changes in near surface soil F/T is recommended. This case study describes the results of a small plot-scale (7 x 7 m) study from November 2013 through April 2014 instrumented with 36 impedance probes. Soil temperature and real dielectric permittivity (epsilon'(r)) were measured every 15 minutes during F/T transition periods at shallow soil depths (0-10 cm). Categorical soil temperature and real dielectric permittivity techniques were used to define the soil F/T state during these periods. Results demonstrate that both methods for detecting soil F/T have strong agreement (84.7-95.6%) during the fall freeze but weak agreement (53.3-60.9%) during the spring thaw. Bootstrapping results demonstrated both techniques showed a mean difference within +/- 1.0 degrees C and +/- 1.4 epsilon'(r) between the standard 5 cm below surface measurement depth and probes at 2, 10 and integrated 0-5.7 cm depths installed within the same study plot. Overall this study demonstrates that the Hydra Probe offers promise for near surface soil F/T detection using existing soil moisture monitoring networks particularly for the fall freeze.