Global Scale Mapping of Subsurface Scattering Signals Impacting ASCAT Soil Moisture Retrievals

Abstract

Soil moisture retrievals from the Advanced Scatterometer (ASCAT) have so far relied on the assumption that soil backscatter increases monotonically with soil moisture content. However, under dry soil conditions, discontinuities in the soil profile caused by the presence of stones, rocks or distinct soil layers may disturb this relation, causing backscatter to decrease with increasing soil wetness. As of yet, subsurface scattering is a poorly understood phenomenon and some of its manifestations on ASCAT soil moisture retrievals have in the past been wrongly attributed to topographic effects or changes in soil surface roughness and vegetation. Therefore, this study aims at mapping subsurface scattering effects on a global scale, explore their dependency on land surface characteristics, and describe the impacts on ASCAT soil moisture retrievals. The results obtained with one statistical and two physically based indicators show that subsurface scattering is not only widespread in desert regions, but also in more humid climates with a dry season. Along with the dryness of the soil, the presence of coarse fragments in the soil profile and sparse vegetation cover are important factors that favor its occurrence. The impact on ASCAT soil moisture retrievals is severe, making subsurface scattering the most significant source of unaccounted errors in the current version of the ASCAT soil moisture data as provided by the EUMETSAT Satellite Application Facility on Support to Operational Hydrology and Water Management. Users of the product are recommended to mask soil moisture data affected by subsurface scattering effects using the indicators and masks developed in this study.