UAS-SfM-derived Elevation Models to Evaluate Changes in the Flow Accumulation and Wetness in Minerotrophic Peatland Restoration Monitoring

Abstract

Most northern peatlands are severely degraded by land use and drainage. Peatland restoration is an effective way to return the natural functions of peatlands in the catchment hydrology, discontinue the peat degradation and re-establish the long-term carbon sinks. The main aim of the rewetting is to direct the water flows back to the pristine routes and to increase the water-table levels. Conventional monitoring methods such as stand-pipe wells are typically limited to sparse locations and cannot give a spatially representative overview.We introduced a novel high-resolution approach to spatially evaluate the surface flow path and wetness changes after restoration. We applied a UAS SfM (Unmanned Aerial System Structure-from-Motion) method supported by ubiquitous LiDAR (Light Detection and Ranging) data to produce digital elevation models, flow accumulation maps and SWI (SAGA Wetness Index) models for two boreal, minerotrophic restoration sites and their pristine control sites. The pristine sites were to represent natural changes and technology-related uncertainty.According to our results, the hydrological restoration succeeded at the sites showing that the wetness increased by 2.9–6.9% and its deviation decreased by 13–15% 1–10 months after the restoration. Absolute changes derived with data from simultaneous control flights at the pristine sites were 0.4–2.4% for wetness and 3.1–3.6% for the deviation. Also, restoration increased the total length of the main flow routes by 25–37% while the controlling absolute change was 3.1–8.1%.The validity of the topography-derived wetness was tested with field-gathered soil moisture samples which showed a statistically significant correlation (R2 = 0.26–0.42) for the restoration sites but not for the control sites. We conclude the water accumulation modelling based on topographical data potential for assessing the changed surface flows in peatland restoration monitoring. However, the uncertainties related to the heterogenic soil properties and complex groundwater interactions require further method development.