Abstract
For soils with shallow groundwater and high organic carbon content, water table depth (WTD) is a key parameter to describe their hydrologic state and to estimate greenhouse gas emissions (GHG). Since the microwave backscatter coefficient (σ0) is sensitive to soil moisture, the application of Sentinel-1 satellite data might support the monitoring of these climate-relevant soils at high spatial resolution (~100 m) by detecting spatial and temporal changes in local field and water management. Despite the low penetration depth of the C-band, σ0 is influenced by shallow WTD fluctuations via the soil hydraulic connection between the water table and surface soil. Here, we analyzed σ0 at 60 monitoring wells in a drained temperate peatland with degraded organic soils used as extensive grassland. We evaluated temporal Spearman correlation coefficients between σ0 and WTD considering the soil and vegetation information. To account for the effects of seasonal vegetation changes, we used the cross-over (incidence) angle method. Climatologies of the slope of the incidence angle dependency derived from two years of Sentinel-1 data and their application to the cross-over angle method did improve correlations, though the effect was minor. Overall, averaged over all sites, a temporal Spearman correlation coefficient of 0.45 (±0.17) was obtained. The loss of correlation during summer (higher vegetation, deeper WTD) and the effects of cuts and grazing are discussed. The site-specific general wetness level, described by the mean WTD of each site was shown to be a major factor controlling the strength of the correlation. Mean WTD deeper than about −0.60 m lowered the correlations across sites, which might indicate an important limit of the application.
Highlights
Water table depth (WTD) is one of the major factors controlling biogeochemical processes in peatlands and other organic soils
Partial correlation analysis indicates a slight decrease in the correlations between VV polarized σ0θ and water table depth (WTD)
Our study demonstrated a dependency of Sentinel-1 σ0 on WTD in a drained temperate peatland with degraded organic soils used as extensive grassland (Figure 5), as previously shown by Kim et al [28] in a forested peatland for Radarsat-1 data and by Bechtold et al [29] for ENVISAT ASAR data across several study sites in Germany
Summary
Water table depth (WTD) is one of the major factors controlling biogeochemical processes in peatlands and other organic soils. Pristine peatlands are characterized by shallow water table depth, which protects the remains of peat forming plants (e.g., Sphagnum spp., Carex spp.) from complete decomposition. Peat soils differ greatly from mineral soils due to their high content of soil organic matter (30–100% according to the German soil classification [2]), low bulk density (from 0.09 to 0.68 g cm−3 for peat soils under agriculture [3]), high porosity (up to 98% [4]), and potentially, very high volumetric water content. E.g., from agriculture or forestry increases the thickness of the aerated soil layer and accelerates the decomposition rates of organic carbon.
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