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Communication Dans Un Congrès Année : 2019

Global Surface Water Product Reliability for Amazon Floodplain Hydrology

Auréa Pottier
Marie-Paule Bonnet
Laurent Durieux
  • Fonction : Auteur
Laura Hess
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Frédéric Frappart
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  • IdHAL : ffrappart

Résumé

Freshwater wetlands ensure fundamental functions such as flood mitigation, groundwater recharge, water purification, and nutrient and sediment retention, as well as supporting high levels of biodiversity. Amazonian floodplains support one of Earth´s largest reservoirs of biodiversity, yet are increasingly threatened by land cover and land use changes induced by large-scale agriculture expansion, waterway network development, and hydroelectric dam construction. These pressures conjointly with climate change may have dramatic impacts on floodplain biodiversity and endemic plant and animal species. Because flooding dynamics is an important driver of floodplain biodiversity and productivity, characterizing and monitoring floodplain hydrology are important to supporting biodiversity conservation. Several global- or regional-scale wetland or flood extent maps have been produced. Until recently, these maps were of coarse spatial resolution, inadequate to support wetlands biodiversity conservation. Recently, based on Landsat imagery, Pekel et al. (2016) produced a global surface water (GSW) map at 30 m and analyzed changes in minimum/maximum flood extent and flood duration over the past three decades. However, in tropical regions, clouds and vegetation may significantly impact the accuracy of surface water mapping based on optical data. On the other hand, SAR sensors acquire data regardless of weather conditions and SAR imagery has been widely used over the past decades to monitor and map wetland inundation and vegetation worldwide, including in the Amazon region. In this study, we use Sentinel-1 Synthetic Aperture Radar (SAR) time series (12-day repeat cycle at this latitude, 10 m resolution) to monitor the flood dynamics of a segment of the Solimões/Amazon river encompassing the Curuai floodplain (eastward) and the Janauacá floodplain (westward) for the year 2017 (covering 6 S1 tiles). The Curuai floodplain (4000 km2 , including the local watershed) forms a vast complex system of temporally connected lakes, flooded forest and fringing wetlands along the Amazon river right margin. Several perennial or intermittent channels of various size link the floodplain lakes system with the Amazon River. The Janauacá floodplain is a medium size system (786 km2 , including the local watershed), composed by a lake and associated flooded forest and other wetlands, linked to the Solimões River by a single channel. Images from the S1 time series were stacked (around 30 images per tile), a mean image was calculated and a thresholding classification was applied on the basis of optical data (Sentinel 2). In this mean classification, areas always flooded will appear in black, areas never flooded in light grey and areas occasionally flooded in shades of grey on flood duration. For each date of the stack, the same thresholding classification is performed and compared to the mean classification in order to produce 4 land cover classes: open water, potentially flooded vegetation, low vegetation and forest. Classification results are then refined applying posttreatments: we used the HAND index combined with spatial and temporal rules to avoid overestimation of water in areas that are not compatible with the hydrodynamics of the area. We compare our results with the GSW products in terms of maximum water extent and inundation duration in order to assess the reliability of GSW for large Amazonian floodplains. Maximum open water extent Both studies are in good agreement, with an estimated open water maximal extent of 27 000 km2 (our study) and 30 000 km2 (Pekel et al., 2016). Most of the discrepancies are observed along floodplain and mainstream margins, and differences are greater at Curuai than at Janauacá. The lengths of the time series used to construct our product and GSW products are very different. Pekel et al. (2016) used a Landsat chronology over the 32 last years, while we used only the year 2017. Consequently, the maximum water level recorded at Óbidos gauge for our study was 760 cm while it was 860 cm over the last 32 years included in Pekel et al.’s study. As reported in Sippel et al. (1998), flood extent and main stream water level are directly related. According to their relationship between water level and flood extent, a 1 m water level variation induces an increase of roughly 11% of the flood extent, comparable with the expected flood extent increase for the same water level variation in the Curuai floodplain (Bonnet et al, 2008). Applying this percentage to our results leads to a maximal flood extent of circa 30 000 km2 , similar to the extension found by Pekel et al. (2016). Flood duration Discrepancies between the GSW and Sentinel-based products are larger in the case of flood duration. These differences are not related to upstream or downstream position but to lateral flow propagation across the floodplain. Throughout the entire study area, strong heterogeneities are observed with variations between both results of several months. At the level of the floodplains, we evidence smaller water residence duration in the main lake of the Janauacá floodplain (between 0 and 2 months). In Curuai, we observe longer water residence duration throughout the floodplain (up to 8 months). Part of the discrepancies might be explained by water level differences between the time series used to build the product (2014-2015 for GSW vs 2017 in this study). Thanks to the repetitivity of cloud-free Sentinel 1, we provide a finer quantification of the temporal dynamics of floods in the floodplains and explain the over-estimation and under-estimation of flood duration in Janauacá and Curuai respectively by the GSW product. Therefore, compared with the GSW product, the flood duration dynamics of our product correspond more closely with hydrodynamic modelling results obtained by Bonnet et al. (2017) for the Janauacá floodplain and Rudorff et al. (2014) and Bonnet et al. (2008) for the Curuai floodplain. We conclude that GSW provides realistic maximum open water extents even at local scale and with accuracies suitable for supporting hydrologic applications, for example model calibration or validation. On the other hand, GSW should be used cautiously when looking at flood duration and subsequent hydrological connectivity analysis, which are fundamental properties to support biodiversity conservation. The Sentinel 1 and Sentinel 2 constellation should provide improved mapping of flood duration at global scale.
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Dates et versions

hal-04399025 , version 1 (17-01-2024)

Identifiants

  • HAL Id : hal-04399025 , version 1

Citer

Thibault Catry, Auréa Pottier, Marie-Paule Bonnet, Laurent Durieux, F. Seyler, et al.. Global Surface Water Product Reliability for Amazon Floodplain Hydrology. ESA Living Planet Symposium, May 2019, Milan (Italie), Italy. ⟨hal-04399025⟩
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