Open-source Sentinel-2 turbidity retrieval validated against in situ data in the Westerschelde estuary
Five deliberate deviations, each recorded rather than hidden: 1. Atmospheric-correction processor. The paper selected C2RCC for its turbidity result (the "cN783" chain, C2RCC + Nechad 2009). C2RCC exists only inside ESA SNAP and crashes natively in the container, so it is excluded; this replication runs Acolite instead, making the chain "aN783" (Acolite + Nechad 2009), an open-source analogue. Note the original study itself reports Acolite as its worst-performing atmospheric-correction processor, so a strong turbidity result obtained through Acolite here would be a notable outcome rather than an expected one. 2. Site. Validation is in the Westerschelde estuary, not the Sado, because the Sado in situ match-up data are available only on request (paper Data Availability Statement) and no open, contemporaneous, in-estuary substitute exists. The Westerschelde is a comparable mesotidal, well-mixed, turbid estuary. 3. Period. The record spans January 2016 to July 2026 (the full Sentinel-2 era), not the original's March 2018 to March 2020 field campaign, to gain a more complete set of match-ups. 4. In situ reference. Coincident turbidity comes from Rijkswaterstaat routine monitoring stations, not the original AQUASado sampling campaign. 5. Fully open-source chain. Every step (Sentinel-2 L1C, Acolite, the native Nechad 2009 turbidity product) is open and reproducible, whereas the original's selected chain depends on the proprietary SNAP/C2RCC processor.
The satellite half of the paper's turbidity pipeline is reproduced with a fully open-source chain (the "aN783" chain) and validated against an independent in situ reference. Sentinel-2 A/B/C Level-1C granules over the Westerschelde estuary are atmospherically corrected with Acolite to water-leaving reflectance, clipped to an estuary bounding box covering the along-axis stations. Turbidity is taken from Acolite's own native Nechad et al. (2009) turbidity product, the open-source analogue of the paper's selected turbidity chain; the product is computed by Acolite with its published relative-spectral-response-convolved Nechad coefficients rather than a hand-transcribed algorithm, and the near-infrared band nearest a 783 nm target is selected, matched by nearest wavelength to absorb the Sentinel-2 A/B/C band-centre drift. At each in situ station a 3x3 pixel window is cut on the native 10 m UTM grid, with per-pixel quality control (water only, no cloud/shadow/glint, atmospheric-correction success), and the valid, finite pixels are averaged. Satellite retrievals are paired with coincident in situ turbidity from Rijkswaterstaat monitoring stations on station and time within a plus-or-minus two-hour window. Agreement is scored with the paper's metric suite (coefficient of determination, slope, intercept, RMSE, bias, unbiased RMS, absolute and relative percentage difference); unlike the chlorophyll-a limb, the regression-family metrics for turbidity are computed on untransformed values in linear space, not in log10 space, since turbidity does not span the orders of magnitude that motivate the log transform for chlorophyll-a. Statistics are reported both over the full Sentinel-2 record and over the original study's field-campaign window, to separate the effect of changing the period from the effect of changing the site.
This study tests only the turbidity limb of the original claim: the assertion that Sentinel-2 MSI retrieval of turbidity is the strong side of a parameter- dependent accuracy — that turbidity is highly consistent with in situ observations, demonstrating the sensor's capability to monitor this water quality parameter. In scope is whether an independent, satellite-derived turbidity product agrees with coincident in situ turbidity measurements in a mesotidal, well-mixed, turbid estuary, and whether the original study's strong-turbidity finding still holds when the retrieval chain is fully open source and transferred to a different estuary and a longer period. Out of scope, and left to separate atomic chains, are the chlorophyll-a, suspended-particulate-matter and coloured-dissolved-organic-matter limbs of the paper's asymmetry, the seasonal time-series analysis, and any grid-sensitivity (HEALPix / DGGS) representation of the corrected fields.