Evaluation of Satellite-Based Optical and Thermal Trapezoid Methods for Groundwater Table Depth Monitoring in Estonian Bogs

Groundwater table depth and peat moisture content are of crucial importance for many peatland processes, like for example their greenhouse gas budget. Thus, there is a strong need for remote sensing techniques that allow to spatially monitor these critical moisture conditions to quantify the hydrological responses to climate change and other anthropogenic disturbances. Previous studies have demonstrated the usefulness but also limitations of microwave observations for peatland moisture monitoring at the large scale. Here, we explore the potential of techniques based on optical and thermal imagery for smaller scale applications.

Satellite-derived land surface temperature (LST) as well as shortwave infrared transformed reflectance (STR) are sensitive to soil moisture conditions in mineral soils. Both data form, together with remotely sensed vegetation indices (VIs), trapezoids in the LST-VI and STR-VI space with the highest range of possible LST and STR for bare soil conditions. The lowest and highest LST and STR along the vegetation cover gradient define the wet and dry edge, respectively. In this study, we used Landsat 7 and Landsat 8 satellite data for the vegetation periods from 2008 through 2019 to calculate various VIs, LST and STR for hemiboreal raised bogs in Estonia. Two common approaches for the determination of wet and dry edges for the LST-based method were applied and compared. The first approach estimates the edges directly from the observed values of VIs and LST for each scene; while the second one relies on modelled theoretical edges for each scene. In contrast, the STR-VI trapezoid is derived from observed values from all scenes as proposed in literature. The trapezoids are used to calculate the dryness index of each Landsat pixel by linearly scaling between the wet and dry edge. These indices are evaluated with measured groundwater table depth time series. Preliminary results indicate that, for our study area, suitable LST-based trapezoids cannot be derived from satellite observations alone, indicated by the low dependency of the resulting dryness index on groundwater table depth. Evaluation of the theoretically-derived trapezoids and the STR-VI is ongoing and will be discussed.