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Focus Area on Soil Moisture Product Validation

John Bolten, NASA Goddard Space Flight Center, USA

Alexander Gruber, Vienna University of Technology, Austria


Soil Moisture Definition

Soil moisture is expressed as soil water content (mass or volume of water in the soil) or soil water potential (soil water energy status). The volumetric soil water content is defined as the volume of the water divided by the total volume (volume of dry soil, air, and water) of a soil sample. Conversion is possible if soil properties are known. (WMO, 2008, updated 2012.)

Units: Volumetric soil water content is expressed in units of m3m-3. Additionally, degree of saturation and gravimetric mass fraction are used for some products, expressed as percent and g cm-3, respectively.

Highest Validation Stage Currently Reached for Satellite-Derived Soil Moisture Products

Validation stage 4 (LPV validation stage hierarchy) - Recent efforts from the Quality Assurance for Soil Moisture (QA4SM) platform provide the means for SM to reach Stage 4 validation. The QA4SM platform enables the systematic, automated, and traceable validation of soil moisture products using various reference datasets following the existing SM Validation Good Practice guidelines (see below). These reference data sets included an International Soil Moisture Network (ISMN)-based Fiducial Reference Measurement (FRM) subset. The Fiducial Reference Measurements for Soil moisture (FRM4SM) lays out guidelines to establish, or to identify existing, soil moisture FRM datasets.

Validation Good Practices

Satellite product validation plan documents that serve as the best references for validation of soil moisture:

SMAP mission launch

SMAP launch, January 31, 2015

  • Montzka, C., M. Cosh, B. Bayat, A. Al Bitar, A. Berg, R. Bindlish, H. R. Bogena,
    J. D. Bolten, F. Cabot, T. Caldwell, S. Chan, A. Colliander, W. Crow, N.
    Das, G. De Lannoy, W. Dorigo, S. R. Evett, A. Gruber, S. Hahn, T.
    Jagdhuber, S. Jones, Y. Kerr, S. Kim, C. Koyama, M. Kurum, E. Lopez-
    Baeza, F. Mattia, K. McColl, S. Mecklenburg, B. Mohanty, P. O«Neill, D.
    Or, T. Pellarin, G. P. Petropoulos, M. Piles, R. H. Reichle, N. Rodriguez-
    Fernandez, C. Rüdiger, T. Scanlon, R. C. Schwartz, D. Spengler, P.
    Srivastava, S. Suman, R. van der Schalie, W. Wagner, U. Wegmüller,
    J.-P. Wigneron, F. Camacho, and J. Nickeson (2020). Soil Moisture
    Product Validation Good Practices Protocol
    Version 1.0. In: C.
    Montzka, M. Cosh, J. Nickeson, F. Camacho (Eds.): Good Practices for
    Satellite Derived Land Product Validation
    (p. 123), Land Product
    Validation Subgroup (WGCV/CEOS),
    DOI: 10.5067/doc/ceoswgcv/lpv/sm.001
  • Gruber, A., G. De Lannoy, C. Albergel, A. Al-Yaari, L. Brocca, J.-C. Calvet, A.
    Colliander, M. Cosh, W. Crow, W. Dorigo, C. Draper, M. Hirschi, Y. Kerr, A. Konings, W. Lahoz, K. McColl,
    C. Montzka, J. Muñoz-Sabater, J. Peng, R. Reichle, P. Richaume, C. Rźdiger, T. Scanlon, R. van der
    Schalie, and W. Wagner (2020). Validation practices for satellite soil moisture retrievals: What are
    (the) errors?
    Remote Sensing of Environment, 244, DOI:10.1016/j.rse.2020.111806
  • SMAP - The SMAP Handbook includes a description of the Cal/Val plan.
  • ASCAT - Brocca, L., F. Melone, T. Moramarco, W. Wagner, C. Albergel (2014) Scaling and filtering approaches
    for the use of satellite observations
    , Chapter 17 in Remote Sensing of Engergy Fluxes and Soil Moisture
    , G.P. Petropoulus (Ed), CRC Press, Boca Raton London New York, 411-425.

Soil Moisture Validation Reference Data Sets

Best currently available in situ reference data for satellite-derived soil moisture product validation are provided by in situ operational networks, most of which can be found at the ISMN web site. A match-up database of reference data from soil moisture core validation sites (including other land surface parameters) with the SMAP EASE grid at 3, 9, and 36 km resolution can be found here at the NSIDC DAAC.

LPV Focus Areas



Land Product Validation and Evolution, June 12 -14, 2023 ESA/ESRIN, Italy.

IGARRS 2023, 16-21 July 2023, Pasadena, CA. Abstract submission due 18 Nov 2022.

EGU 2023, 23-28 April 2023, Vienna, Austria. Abstract submission due 10 Jan 2023.

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