Satellite Determination of Peatland Water Table Temporal Dynamics by Localizing Representative Pixels of A SWIR-Based Moisture Index

Abstract

The OPtical TRApezoid Model (OPTRAM) is a physically-based approach for remote soil moisture estimation. OPTRAM is based on the response of short-wave infrared (SWIR) reflectance to vegetation water status, which in turn responds to changes of root-zone soil moisture. In peatlands, the latter is tightly coupled to water table depth (WTD). Therefore, in theory, the OPTRAM index might be a useful tool to monitor WTD dynamics in peatlands, although the sensitivity of OPTRAM index to WTD changes will likely depend on vegetation cover and related rooting depth. In this study, we aim at identifying those locations (further called ‘best pixels’) where the OPTRAM index is most representative of overall peatland WTD dynamics. In peatlands, the high saturated hydraulic conductivity of the upper layer largely synchronizes the temporal WTD fluctuations over several kilometers, i.e., even though the mean and amplitude of the WTD dynamics may vary in space. Therefore, it can be assumed that the WTD time series, either measured at a single location or simulated for a grid cell with the PEATland-specific adaptation of the NASA Catchment Land Surface Model (PEATCLSM), are representative of the overall peatland WTD dynamics. We took advantage of this concept to identify the ‘best pixel’ of all spatially distributed OPTRAM pixels within a peatland, as that pixel with the highest time series Pearson correlation (R) with WTD data accounting for temporal autocorrelation. The OPTRAM index was calculated based on various remotely sensed images, namely, Landsat, MODIS, and aggregated Landsat images at MODIS resolution for five northern peatlands with long-term WTD records, including both bogs and fens. The ‘best pixels’ were dominantly covered with mosses and graminoids with little or no shrub or trees. However, the performance of OPTRAM highly depended on the spatial resolution of the remotely sensed data. The Landsat-based OPTRAM index yielded the highest R values (mean of 0.7 across the ‘best pixels’ in five peatlands). Our study further indicates that, in the absence of historical in situ data, PEATCLSM can be used as an alternative to localize ‘best pixels’. This finding enables the future applicability of OPTRAM to monitor WTD changes in peatlands on a global scale.

Soil carbon balance of afforested peatlands in the maritime temperate climatic zone

“…Another limitation of the chamber method is that the proposed R H models require continuous measurements of soil T and WTD to simulate annual CO 2 fluxes accurately. While soil temperature is usually recorded at weather stations and it may be estimated accurately over large areas using remote sensing techniques (Xu et al, 2020), continuous WTD data on peatland forests is scarce and the use of remote sensing techniques to simulate WTD time series accurately needs further refinement (Bechtold et al, 2018; Burdun et al, 2020). Therefore, if continuous WTD is not available or if the uncertainty of simulating WTD gaps between consecutive measurements by linear interpolation is a concern, it is suggested that temperature exponential models are used, instead of combined model of soil temperature and WTD, to scale up R H effluxes at a regional level (Jovani‐Sancho et al, 2018).…”

“…Another limitation of the chamber method is that the proposed R H models require continuous measurements of soil T and WTD to simulate annual CO 2 fluxes accurately. While soil temperature is usually recorded at weather stations and it may be estimated accurately over large areas using remote sensing techniques (Xu et al, 2020), continuous WTD data on peatland forests is scarce and the use of remote sensing techniques to simulate WTD time series accurately needs further refinement (Bechtold et al, 2018;Burdun et al, 2020).…”

Remotely Sensed Land Surface Temperature Can Be Used to Estimate Ecosystem Respiration in Intact and Disturbed Northern Peatlands

“…However, only a small number of peatlands have in situ historical observations, which limit the future applicability of the provided model. Therefore, remotely sensed proxies of WTD must be used, such as radar data (Asmuß et al., 2019; Tampuu et al., 2020) and Optical Trapezoid Model (Burdun, Bechtold, Sagris, Lohila, et al., 2020). Furthermore, given the well‐established respiration dependency on LST in disturbed sites, future work could focus on the benefits of combining various remotely sensed data.…”

Drought and Waterlogging Stress Regimes in Northern Peatlands Detected Through Satellite Retrieved Solar‐Induced Chlorophyll Fluorescence

“…In peatlands, root‐zone soil moisture conditions are closely linked to WTD due to the generally shallow water table (Burdun et al., 2020). Therefore, WTD was taken as a proxy to describe plant water availability in a peatland.…”