Comparison of downscaling techniques for high resolution soil moisture mapping

TL;DR

This paper compares various downscaling techniques for high-resolution soil moisture mapping using satellite data, addressing the challenge of achieving consistent, detailed soil moisture information from coarse-resolution satellite observations.

Contribution

It provides a comprehensive inter-comparison of optical, radiometer, and oversampling-based downscaling methods for soil moisture using SMAPEx-4 data.

Findings

Optical-based methods outperform radiometer-based approaches in certain conditions.

Oversampling techniques improve spatial resolution but have limitations in accuracy.

No single method is universally best; hybrid approaches may be needed.

Abstract

Soil moisture impacts exchanges of water, energy and carbon fluxes between the land surface and the atmosphere. Passive microwave remote sensing at L-band can capture spatial and temporal patterns of soil moisture in the landscape. Both ESA and NASA have launched L-band radiometers, in the form of the SMOS and SMAP satellites respectively, to monitor soil moisture globally, every 3-day at about 40 km resolution. However, their coarse scale restricts the range of applications. While SMAP included an L-band radar to downscale the radiometer soil moisture to 9 km, the radar failed after 3 months and this initial approach is not applicable to developing a consistent long term soil moisture product across the two missions anymore. Existing optical-, radiometer-, and oversampling-based downscaling methods could be an alternative to the radar-based approach for delivering such data. Nevertheless, retrieval of a consistent high resolution soil moisture product remains a challenge, and there has been no comprehensive inter-comparison of the alternate approaches. This research undertakes an assessment of the different downscaling approaches using the SMAPEx-4 field campaign data