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The assessment of water resources in soil is important in understanding the water cycle in the natural environment and the processes of water exchange between the soil and the atmosphere. The main objective of the study was to assess water resources (in 2010–2013) in the topsoil from satellite (SMOS) and in situ (ground) measurements using the  SWEX_PD  approach (Soil Water EXtent at Penetration Depth). The  SWEX_PD  is a result of multiplying soil moisture (SM) and radiation penetration depth ( PD ) for each pixel derived from the SMOS satellite. The  PD , being a manifold of the wavelength  λ  0  equal to 21 cm, was determined from the weekly SMOS L2 measurement data based on the real and imaginary part of complex dielectric constant. The  SWEX_PD  data were compared with soil water resources ( WR ) calculated from the sum of components derived from multiplication of soil moisture ( SM ) and layer thickness in nine agrometeorological stations located along the eastern border of Poland. Each study site consisted of seven neighbouring Discrete Global Grid pixels (nodes spaced at 15 km) including the central ones with agrometeorological stations. The study area included different types of soils and land covers. The agreement between the water resources obtained from the  SWEX_PD  and ground measurements ( WR ) was quantified using classical statistics and Bland–Altman's plots. Calibrated Layer Thickness (CLT =  d   bias  ) from 8 to 28 cm was obtained with a low values of  bias  (close to zero), limits of agreements, and confidence intervals for all the  SWEX_PD , depending on the pixel location. The results revealed that the use of the  SWEX_PD  for assessing soil water resources is the most reliable approach in the study area. Additionally, the data from Bland–Altman plots and the equation proposed in these studies allowed calculation of the Equivalent Layer Thickness (ELT =    \documentclass[12pt]{minimal} 				\usepackage{amsmath} 				\usepackage{wasysym}  				\usepackage{amsfonts}  				\usepackage{amssymb}  				\usepackage{amsbsy} 				\usepackage{mathrsfs} 				\usepackage{upgreek} 				\setlength{\oddsidemargin}{-69pt} 				\begin{document}$$d_{ei}^{SWEX}$$\end{document}    d   ei    SWEX), which corresponds to the water resources derived from the SMOS satellite at the same time as ( SM ) measurements performed in the agrometeorological stations. The ranges of the mean, standard deviation, minimum, maximum, and coefficient of variation (CV) of ELT among all pixels and stations were 8.28–28.7 cm, 3.27–12.66 cm, 3.03–10.87 cm, 19.23–94.97 cm, and 24.72–98.79%, respectively. The ranges of the characteristics depended on environmental conditions and their means were close to the values of the calibrated layer thickness. The impacts of soil texture, organic matter, vegetation, and their interactive effects on the differentiation and agreement of soil water resources obtained from  SWEX_PD  vs. data from ground measurements in the study area are discussed. Further studies are required to address the impact of the environmental factors to improve the assessment of soil water resources based on satellite SM products (retrievals).

The SMOS-Derived Soil Water EXtent and equivalent layer thickness facilitate determination of soil water resources