Abstract

Vegetation optical depth (VOD) is a remotely sensed indicator characterizing the opacity of the vegetation layer. This study focuses on the behaviour of L-band VOD (L-VOD) retrieval algorithm over seasonally inundated areas, as previous observations have shown an unexpected decline in VOD during floods. The signal emitted by a mixed scene composed of soil and standing water was simulated, leading to an overestimation of the retrieved soil moisture (SM) and an underestimation of the retrieved L- VOD, typically by ~ 1 0% over flooded forests and up to 100% over flooded grasslands. We evaluated the induced underestimation of aboveground biomass (AGB) by 15/20 Mg ha-1 in the largest seasonal wetlands, which can represent more than 50% of the actual AGB of the savanna wetland, and up to higher values during exceptional years. Surface water seasonality needs to be taken into account in passive microwave retrieval algorithms to better estimate the global biomass.

Highlights

  • Large-scale monitoring of vegetation cover is crucial for under­ standing its behaviour and its links with climate evolution, extreme events, and land cover changes (Piao et al, 2019; Qin et al, 2019)

  • We highlighted the anomalous decrease of Vegetation optical depth (VOD) during flooding; and we showed with a modelling experiment that this phe­ nomenon was linked to the influence of standing water temporal vari­ ations

  • SMOS Level2, Level3, and SMAP operational algorithms take the major water bodies into account with a static map, but this study showed the importance of considering the temporal dynamics of water extent

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Summary

Introduction

Large-scale monitoring of vegetation cover is crucial for under­ standing its behaviour and its links with climate evolution, extreme events, and land cover changes (Piao et al, 2019; Qin et al, 2019). Visible frequencies have predominantly been used for these applica­ tions, thanks to the high spatial resolution of optical instruments. They are impaired by their inability to penetrate clouds and dense vegetation. The low frequency L-band (1.4 GHz) VOD (L-VOD) measured with SMOS satellite was proven to be highly sensitive to aboveground biomass (AGB) in Africa, with less saturation over dense forests than optical indices and than C- or X-VOD (Rodríguez-Fernandez et al, 2018). A strong correlation was found at the global scale between L-VOD and two AGB datasets (R = 0.91–0.94), but was shown to be highly dependent on the vegetation type

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