Soil surface roughness observed during SMAPVEX16-IA and its potential consequences for SMOS and SMAP

Abstract

The European Space Agency's SMOS and NASA's SMAP soil moisture remote sensing satellite missions do not perform well in several agricultural regions. For example, we have found that SMOS and SMAP soil moisture retrievals are 0.09 to 0.07 m3 m−3 lower, respectively, than an in-situ soil moisture network in one such agricultural region in Central Iowa, USA. We hypothesize that this dry bias is in part caused by an incorrect parameterization of the effect of soil surface roughness, the mm-scale variations in the height of the soil surface. A large field campaign called SMAPVEX16-IA was held in this region in 2016. As part of the campaign, we measured soil surface roughness at nearly 20 sites within the domain of a SMOS/SMAP satellite pixel. We found that our observed soil surface roughness was higher than what is used by the SMOS soil moisture retrieval algorithm. If the SMOS and SMAP retrieval algorithms were adjusted to account for a higher soil surface roughness, this would increase retrieved soil moisture and decrease the soil moisture bias. We also found that our measurements of soil surface roughness are consistent with changes in the τ parameter, which is sensitive to soil surface roughness, retrieved by both the SMOS and SMAP missions.