An evaluation of SMOS L-band vegetation optical depth (L-VOD) data sets: high sensitivity of L-VOD to above-ground biomass in Africa

Abstract

Abstract. The vegetation optical depth (VOD) measured at microwave frequencies is related to the vegetation water content and provides information complementary to visible/infrared vegetation indices. This study is devoted to the characterization of a new VOD data set obtained from SMOS (Soil Moisture and Ocean Salinity) satellite observations at L-band (1.4 GHz). Three different SMOS L-band VOD (L-VOD) data sets (SMOS level 2, level 3 and SMOS-IC) were compared with data sets on tree height, visible/infrared indexes (NDVI, EVI), mean annual precipitation and above-ground biomass (AGB) for the African continent. For all relationships, SMOS-IC showed the lowest dispersion and highest correlation. Overall, we found a strong (R > 0.85) correlation with no clear sign of saturation between L-VOD and four AGB data sets. The relationships between L-VOD and the AGB data sets were linear per land cover class but with a changing slope depending on the class type, which makes it a global non-linear relationship. In contrast, the relationship linking L-VOD to tree height (R = 0.87) was close to linear. For vegetation classes other than evergreen broadleaf forest, the annual mean of L-VOD spans a range from 0 to 0.7 and it is linearly correlated with the average annual precipitation. SMOS L-VOD showed higher sensitivity to AGB compared to NDVI and K/X/C-VOD (VOD measured at 19, 10.7 and 6.9 GHz). The results showed that, although the spatial resolution of L-VOD is coarse ( ∼ 40 km), the high temporal frequency and sensitivity to AGB makes SMOS L-VOD a very promising indicator for large-scale monitoring of the vegetation status, in particular biomass.

Satellite and In Situ Observations for Advancing Global Earth Surface Modelling: A Review

“…C or X-band do not penetrate much into the canopy and are more sensitive to the top part of the canopy. Figure 15 shows that L band is sensitive up to very dense canopies (300 Mg/hectare) enabling the probing of almost all vegetation canopies [182]. A further advantage of the VOD is that data can be acquired even in the presence of clouds.…”

“…The authors wish to make the following corrections to this paper [2]: Correction to the legend of Figure 15 to mention this is adapted from Rodriguez-Fernandez et al, 2018 ([2] and referenced as [182] in [1]).…”

Mean European Carbon Sink Over 2010–2015 Estimated by Simultaneous Assimilation of Atmospheric CO2, Soil Moisture, and Vegetation Optical Depth

“…Apart from those, only Liu et al (2015) report a larger European sink strength, based on trends in C-VOD data from remotely sensed passive microwave observations as a proxy for changes in AGB. However, the relationship between AGB and C-VOD saturates and C-VOD becomes only weakly sensitive to AGB changes in the range ≈50 to ≈250 Mg/ha, which is a typical range for European forest ecosystems (Brandt et al, 2018;Rodríguez-Fernández et al, 2018).…”

“…It therefore remains to be seen whether future studies inferring the European strength from biomass changes based on longer-wavelength L-band data will also estimate a moderate sink strength, similar to the present study, also using L-band microwave data. This is relevant, because L-band microwaves penetrate considerably deeper into forest canopies than C-band, and therefore are less prone to signal saturation (Brandt et al, 2018;Rodríguez-Fernández et al, 2018).…”

Correction: Balsamo, G., et al. Satellite and In Situ Observations for Advancing Global Earth Surface Modelling: A Review. Remote Sensing 2018, 10, 2038

“…The authors would like to apologize for any inconvenience caused to the readers by these changes. The authors wish to make the following corrections to this paper [2]: Correction to the legend of Figure 15 to mention this is adapted from Rodriguez-Fernandez et al, 2018 ([2] and referenced as [182] in [1]).…”