Étalonnage de sondes FDR (« frequency domain reflectometry ») sur les cinq premiers centimètres des sols et des couverts de bryophytes de deux tourbières minérotrophes du milieu boréal Québécois (Canada)

Abstract

To map the water content of Quebec’s northern boreal soils using remote sensing, it is necessary to calibrate the radar signal for fens (minerotrophic peatlands), which represent a high percentage of the land cover. Frequency domain reflectometry (FDR) probes were inserted in mosses and upper horizons (not presented here) of two representative minerotrophic peatlands, the “L’Abeille” (54°06′51.81″N, 72°30′03.73″O) and “La Chenille” (54°07′00.34″N, 72°30′15.43″O) fens, near Hydro-Quebec’s “La Forge” dam. The FDR probes were installed in the main environments located on both peatlands: plateaus (ridges, lawn plateaus), hollows (wet hollows, lower lawns, shallow pool margins), and hummocks (sphagnum hummocks and Pleurozium schreberi hummocks). However, the default calibrations of the FDR probes, performed for mineral soils, are not adapted to the materials analysed in this study. The main objective of this article was to establish specific relationships between apparent dielectric constants (Ka) and volumetric moisture contents (θv) for the organic soils and vegetative covers found in the studied peatlands. Hence, microenvironments (blanket mosses and upper peat horizons) were sampled and brought back into the laboratory. Several successive stable periods of pressure head (h) were applied to determine equivalent θv and Ka values. For all samples, a general equation Ka–θv was determined. The θv values measured in the laboratory were compared with values of volumetric moisture contents obtained with the default calibration (θFDR). The comparison confirmed that it is required to make specific FDR probe calibrations for organic soils and moss blankets, because default calibrations consistently underestimated actual volumetric moisture contents.