Fusion of SAR and Multi-spectral Time Series for Determination of Water Table Depth and Lake Area in Peatlands

Abstract

Peatlands as natural carbon sinks have a major impact on the climate balance and should therefore be monitored and protected. The hydrology of the peatland serves as an indicator of the carbon storage capacity. Hence, we investigate the question how suitable different remote sensing data are for monitoring the size of open water surface and the water table depth (WTD) of a peatland ecosystem. Furthermore, we examine the potential of combining remote sensing data for this purpose. We use C-band synthetic aperture radar (SAR) data from Sentinel-1 and multi-spectral data from Sentinel-2. The radar backscatter σ, the normalized difference water index (NDWI) and the modified normalized difference water index (MNDWI) are calculated and used for consideration of the WTD and the lake size. For the measurement of the lake size, we implement and investigate the methods: random forest, adaptive thresholding and an analysis according to the Dempster–Shafer theory. Correlations between WTD and the remote sensing data σ as well as NDWI are investigated. When looking at the individual data sets the results of our case study show that the VH polarized σ data produces the clearest delineation of the peatland lake. However the adaptive thresholding of the weighted fusion image of σ-VH, σ-VV and MNDWI, and the random forest algorithm with all three data sets as input proves to be the most suitable for determining the lake area. The correlation coefficients between σ/NDWI and WTD vary greatly and lie in ranges of low to moderate correlation.