Estimation of soil organic carbon using hyperspectral remote sensing data and a large scale soil spectral library

from laboratory to space

authored by: Kathrin Jennifer Ward
supervised by: Sabine Chabrillat
Abstract: Our soils are the largest terrestrial carbon storage of the planet. With proceeding climate change and to retain food security for a growing human population, the pressure rises to increase the knowledge about the soil’s current status and especially its carbon content. Mapping the status of the soil carbon content presents a challenge, especially for larger areas, since conventional methods require extensive sample collection and expensive laboratory analyses. To fully utilise the soil’s capacity and preserve the its health in the future, it is required to monitor key soil parameters thereby adding a temporal dimension. Therefore, suitable approaches are needed to estimate the carbon content regularly and on larger scales. Soil spectroscopy has the potential to support this aim. At laboratory and airborne scale soil spectroscopy already proved to be an accurate method to estimate certain soil properties. Recently, advanced optical hyperspectral spaceborne imaging sensors have become available that have the potential to create quantitative soil carbon maps. These Earth observation (EO) sensors can also be a future monitoring tool to e.g. combat soil degradation. In this thesis, approaches were developed and applied to evaluate these new hyperspectral EO sensors to map the spectrally active soil organic carbon (SOC) content at different scales. Ground reference data represent a bottle neck but are essential for model generation and evaluation. A potential solution is the additional usage of existing large scale soil spectral libraries (SSLs) which contain information on physiochemical and spectral properties. Thereby, the challenge is the decreasing model accuracy with increasing size of the study site. Therefore, the first aim was to improve the prediction accuracy of SOC content using laboratory spectra and the European wide LUCAS SSL. Especially a memory-based learning approach (local partial least square regression: local PLSR) could improve the prediction accuracy. To apply this laboratory based approach to EO sensors, an approach using two steps was defined to bridge the gap between the dissimilar laboratory and EO image spectra. The local PLSR is based on spectral similarity and uses of the memorized connections contained in the LUCAS SSL. The advantage of this technique is that few soil samples from the study site are required as input and thereof solely laboratory spectra. Laboratory spectra are non-destructive, fast, and cheaper since no chemical analyses are required. SOC maps were successfully generated using airborne and spaceborne images. These SOC maps represent the status quo at the image acquisition date and can be updated with new image data. To further increase the spatial coverage of the SOC maps innovative multitemporal compositing workflows were evaluated. This mostly applies to agricultural areas which have the required but temporarily limited bare soils. Two different workflows were inspected that were either based on a synthetical bare soil composite and subsequent SOC modelling, or on separate SOC maps that were generated for each image and merged in the end. The second workflow led to higher model accuracies. The future availability of longer time-series of spaceborne imaging spectroscopy data can be a valuable source to help monitor SOC changes in the uppermost soil layer.
Organisation(s): Soil Science Section
Type: Doctoral thesis
No. of pages: 143
Publication date: 24.06.2024
Publication status: Published
Sustainable Development Goals: SDG 2 - Zero Hunger, SDG 3 - Good Health and Well-being, SDG 13 - Climate Action, SDG 15 - Life on Land
Electronic version(s): https://doi.org/10.15488/17781 (Access: Open)