1190 IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING, VOL. 49, NO. 4, APRIL 2011 Sensitivity of Passive Microwave Observations to Soil Moisture and Vegetation Water Content: L-Band to W-Band Jean-Christophe Calvet, Jean-Pierre Wigneron, Senior Member, IEEE, Jeffrey Walker, Fatima Karbou, André Chanzy, and Clément Albergel Abstract—Ground-based multifrequency (L-band to W-band, 1.41–90 GHz) and multiangular (20 ◦ –50 ◦ ) bipolarized (V and H) microwave radiometer observations, acquired over a dense wheat field, are analyzed in order to assess the sensitivity of bright- ness temperatures (T b ) to land surface properties: surface soil moisture (m v ) and vegetation water content (VWC). For each frequency, a combination of microwave T b observed at either two contrasting incidence angles or two polarizations is used to retrieve m v and VWC, through regressed empirical logarithmic equations. The retrieval performance of the regression is used as an indicator of the sensitivity of the microwave signal to either m v or VWC. In general, L-band measurements are shown to be sensitive to both m v and VWC, with lowest root mean square errors (0.04 m 3 · m −3 and 0.52 kg · m −2 , respectively) obtained at H polarization, 20 ◦ and 50 ◦ incidence angles. In spite of the dense vegetation, it is shown that m v influences the microwave observations from L-band to K-band (23.8 GHz). The highest sensitivity to soil moisture is observed at L-band in all configu- rations, while observations at higher frequencies, from C-band (5.05 GHz) to K-band, are only moderately influenced by m v at low incidence angles (e.g., 20 ◦ ). These frequencies are also shown to be very sensitive to VWC in all the configurations tested. The highest frequencies (Q- and W-bands) are shown to be moder- ately sensitive to VWC only. These results are used to analyze the response of W-band emissivities derived from the Advanced Microwave Sounding Unit instruments over northern France. Index Terms—Microwave radiometry, soil moisture, vegetation. I. I NTRODUCTION P ASSIVE microwave remote sensing techniques have ap- plications in monitoring the terrestrial surfaces or the Manuscript received January 18, 2010; revised April 13, 2010; accepted May 9, 2010. Date of publication June 28, 2010; date of current version March 25, 2011. The work of C. Albergel was supported in part by the Centre National d’Etudes Spatiales and in part by Météo-France. J.-C. Calvet, F. Karbou, and C. Albergel are with the Centre Na- tional de Recherches Météorologiques/Groupe d’étude de l’Atmosphère Météorologique, Météo-France/Centre National de la Recherche Scientifique, 31057 Toulouse, France. J.-P. Wigneron is with the Ecologie Fonctionnelle et Physique de l’Environnement, Institut National de la Recherche Agronomique, 33883 Villenave d’Ornon, France. J. Walker was with the Department of Civil and Environmental Engineering, The University of Melbourne, Melbourne, Vic. 3010, Australia. He is now with the Department of Civil Engineering, Monash University, Clayton, Vic. 3800, Australia. A. Chanzy is with the Environnement Méditerranéen et Modélisation des Agro-Hydrosystèmes, Institut National de la Recherche Agronomique/ Université d’Avignon et des Pays de Vaucluse, 84914 Avignon Cédex 9, France. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TGRS.2010.2050488 atmosphere, depending on frequency. However, it is not clear to what extent higher frequency observations can be used for soil moisture monitoring. The main motivation of this study is to consolidate previous investigations on the sensitivity of microwave emission at different frequencies, polarizations, or incident angles to soil moisture and vegetation water content (VWC). Clearly, low frequencies (L-band in particular) are sensitive to surface soil moisture m v [1] and have been selected as the frequency of choice for such measurements. The Soil Moisture and Ocean Salinity mission is a dedicated soil mois- ture mission using L-band radiometry. However, the extremely high frequencies (e.g., Q- and W-bands) that are routinely used to characterize the atmosphere may also contain useful infor- mation on soil moisture content that is hitherto underutilized. For example, over continental areas, brightness temperatures (T b ) at extremely high frequencies require correction for land emissivity in order to retrieve relevant atmospheric information [2], but this land information has not been assessed for its soil moisture information. For bare soil, microwave brightness temperatures are sen- sitive to soil moisture at frequencies ranging from L-band to extremely high frequencies, e.g., 90 GHz at W-band [3]. However, vegetation canopies tend to mask the soil microwave emission, with this effect increasing at higher frequencies. While many studies have shown that L-band radiometry is able to retrieve soil moisture over relatively dense canopies (up to 3–5 kg · m −2 ), it has also been shown that C- and X-band observations can be used over areas where vegetation is not too dense (for a review, see [4]). However, microwave brightness temperatures may become dominated by vegetation character- istics at higher frequencies. For example, Wigneron et al. [5] have shown that observations over pine forests at 90 GHz are related to the density of trees, and Prigent et al. [6] have sug- gested that passive microwave observations at K-band (19 GHz) and higher frequencies are sensitive to the vegetation alone, and not to the underlying soil moisture. The objective of this study is to assess the extent to which C-band and higher frequencies are sensitive to surface soil moisture under dense vegetation and to compare the soil mois- ture sensitivity with the sensitivity to the VWC. Since the findings of Prigent et al. [6], showing a lack of sensitivity to soil moisture for observations at K-band and higher frequencies, are based on the analysis of satellite data, the use of ground- based microwave observations in this paper, measured under controlled m v and VWC conditions, can contribute to consoli- date those results. The Portos-93 experiment [1] is used for this 0196-2892/$26.00 © 2010 IEEE