Soil moisture retrieval in the boreal zone with a high resolution airborne radiometer

Jaakko Seppänen

Research output: ThesisDoctoral ThesisCollection of Articles

Abstract

Moisture, stored in the Earth's surface layer, is a vital constituent for life on land and the central parameter in many science disciplines. As the moisture in soil affects the thermal radiation, emitted by the soil, it can be indirectly measured by a radiometer. This phenomenon has been utilised in spaceborne microwave radiometers, such as SMOS and SMAP, which are currently the most important measurement systems for global soil moisture assessment. Despite improvements in soil moisture retrieval, brought by satellite radiometers, there are still areas where soil moisture estimates need substantial improvement. One of the central problems is caused by the rather large resolution cell of a spaceborne instrument which usually includes emission from various vegetation and land use types. As vegetation, especially tall vegetation, has a significant effect on microwave emission, the soil moisture estimates over the regions, where forested and cultivated areas alternate, have still large errors.This thesis concentrates on studying and improving soil moisture estimation algorithms over boreal zone conditions and terrain types, forest, marsh, and agricultural fields. All studies have been made by utilising high resolution L-band airborne radiometer, which enables isolation of the signal precisely from the desired terrain type. In this work, airborne measurements have been used to study the factors affecting the emissivity of agricultural fields, forests, and peatlands to explain their contribution to total radiation, separately and together. It was discovered in this study, that the humus and litter layer of forest affects the measurement result more significantly than was expected. It appears that radiation of the layer can mask the radiation from the underlying soil completely or partially depending on thickness. It was demonstrated that this can have a significant effect on derived soil moisture estimates. Another result showed that emissivity of peatlands varies greatly not only due to varying moisture but also due to surface vegetation type. This work also proposes for the first time a method to approximate the bog emissivity at L-band, by using LAI measurements, the daily average temperature, and soil moisture. The work also shows that L-band brightness temperature is very sensitive to freezing and thawing of the soil as the as freezing increases soil emissivity. The results of this study help to understand how the complex boreal environment and vegetation affect microwave emission from terrain and paves the road to more accurate soil moisture models.
Translated title of the contributionMaankosteuden mittaaminen boreaalisella vyöhykkeellä käyttäen korkearesoluutioista lentokäyttöistä radiometriä
Original languageEnglish
QualificationDoctor's degree
Awarding Institution
  • Aalto University
Supervisors/Advisors
  • Praks, Jaan, Supervising Professor
Publisher
Print ISBNs978-952-60-7198-5
Electronic ISBNs978-952-60-7197-8
Publication statusPublished - 2017
MoE publication typeG5 Doctoral dissertation (article)

Keywords

  • soil moisture
  • soil water content
  • SMOS
  • SMAP
  • remote sensing
  • airborne Radiometer
  • estimation algorithms
  • boreal zone
  • emission

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