Study of SPM variability as a proxy of sedimentation in tidal beds at the Tagus estuary (Portugal): a test using Landsat 8 and Sentinel 2 optical satellite imagery.
Supervisor: Cristina Ponte Lira (Universidade de Lisboa)
Suspended Particulate Matter (SPM) is a major constituent of highly turbid waters and plays animportant role in differences in optical properties of water bodies. In fact, SPM affects water clarity, whichdecreases with the increase of SPM/turbidity, leading to differences in surface reflectances. Thisphenomenon can be observed by ocean colour remote sensors, such as OLI from Landsat 8 (L8) andMSI from Sentinel 2 (S2), by capturing water-leaving signals and deriving its information. Thanks toremote sensing, the spatial and temporal variability of SPM is better monitored (than with in-situ data),which is crucial in coastal environments, such as estuaries, where a significant amount of SPM circulateswithin transitional waters and affects biological, chemical, and geomorphological processes as well asanthropogenic activities. This is the case for the Tagus estuary (Portugal) chosen as the study area forthe present project. The composition of SPM in the Tagus estuary being dominated by inorganicparticles (against organic particulates and organic dissolved matter), SPM contributes to variations insedimentation within the Tagus estuary (geomorphological processes). Therefore, SPM compositionand distribution can be investigated to determine whether SPM is a good proxy for sedimentation andsedimentation rates in the Tagus estuary thanks to high and medium spatial resolution imagery of S2and L8. However, highly turbid waters being mostly near the coasts, they are significantly affected byatmospheric contributions due to their proximity with land and anthropogenic activities. The challengelies in finding the most suitable atmospheric corrections (AC) and algorithm to derive SPM, and validatethem at the study area (Tagus estuary). Therefore, Nechadet al. (2010) SPM algorithm is applied toLevel 2 surface reflectance of S2 and L8, as well as ACOLITE AC and was supposed to be comparedto in-situ data for validation. Due to COVID-19 world pandemic, no field measurements were collectedand Aqua MODIS imagery from the MYD09GQ Version 6 product was used as an alternative validationmethod. MODIS was compared to in-situ datasets collected by the InstitutoHydrografico(IH) in 2009and 2010 to validate Nechadet al. (2010) SPM algorithm in transitional and inland waters of the Tagusestuary. Then MODIS derived SPM estimations were compared to L8 and S2 derived SPM. Additionally,another in-situ dataset provided by PLATAGUS Project researchers was used to validate and calibratethe algorithm for L8 images in the study area. Here, we show that Nechadet al. (2010) SPM algorithmwas successfully validated for L8 band 5 (near infrared) by the PLATAGUS in-situ data to retrieve SPMin highly turbid waters of the Tagus estuary, while MODIS images show no correlation with the IH fielddata, and could not be used as a validation of the algorithm in turbid transitional/ inland waters, asexpected from the literature (Nechadet al., 2010; Vanhellemont and Ruddick, 2015). These resultsreveal that the low spatial resolution of MODIS instrument (250 m) is not appropriated to capture SPMvariations in a spatially narrow study area of highly turbid and dynamic waters, such as the Tagusestuary. The study also validates that OLI is a powerful sensor to monitor the dynamics of coastalenvironments. Comparing L8 to S2, it highlighted overestimations of SPM values from S2, which areemphasized with the increase in SPM concentrations. The primary objective of determining if SPM canbe a good proxy for sedimentation in the Tagus estuary could not be answered due to the incapacity tocollected in-situ data. We are conscious that the work lacks of reliable results to firmly assert that Nechadet al. (2010) SPM algorithm is suitable for the Tagus estuary. However, we view these primary resultsas part of an ongoing project, which is currently being discussed to be linked with the PLATAGUS Project3(Sent et al., 2019). This collaboration would bring greater SPM monitoring possibilities in the Tagusestuary. Finally, once the current world pandemic situation improves and IH allows field surveys anew,this project intends to answer the initial question of this project: “Can SPM be used as a proxy forsedimentation in the case study of the Tagus estuary?”.Watch this space, it is worth keeping an eye.
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