Title:

Evaluation of C-Band Freeze-Thaw Retrievals: A Field-Scale Case Study in New Hampshire, USA

Soil freeze/thaw (FT) has important implications for Earth’s energy, carbon, methane, nitrogen, and water cycles. FT dynamics over agricultural lands are of particular interest, given that this process greatly impacts nutrient and water storage conditions in spring. Those areas are particularly sensitive to nutrient and water availability. Our understanding of freeze/thaw processes at field-scales are complicated by spatial heterogeneity, stemming from differences in soil properties, land cover types, vegetation density, and topographic variations. This study employs microwave radar data from Sentinel-1 satellites over an intensively studied cold-season testbed area (Thompson Farm, Durham, NH) to examine C-band radar capabilities in accurately detecting soil FT states over a heterogeneous, low biomass area, similar to agricultural field conditions, that is also prone to multiple mid-winter thawing and re-freezing events. Three distinct approaches were tested at VV and VH polarizations, evaluated against on-site weather station data and other ancillary data (e.g., subpixel topography, land cover, insolation/shading): (1) a general threshold approach, (2) the seasonal threshold approach, and (3) an interferometric coherence approach. Results showed that approach (1) and (2) were comparable, yielding near 80% accuracy with slightly improved (but less robust) results for VH polarization whereas approach (3) performed poorly/ was difficult to interpret.

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