Seres River Discharge ATBD - ESA Climate Change Initiative (EN)

ESA Climate Change Initiative River Discharge Precursor (RD_cci+) D.3 River Discharge (RD) from altimeters, river- width, multispectral images and data combination Algorithm Theoretical Basis Document (ATBD) Contract number: 4000139952/22/I-NB Reference: RD_CCI-0006-ATBD Issue 2.0 31/07/2025 Tel +33 (0)5 61 39 11 rue Hermиs, Parc Technologique du Canal — 31520 Ramonville Saint-Agne, France Tel +33 (0)5 61 39 47 00 Fax +33(0)5 61 75 10 14 www.cls.fr

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CHRONOLOGY ISSUES D.3 River Discharge ATBD. Reference: RD_CCI-0006-ATBD - Issue 2.0 – 31/07/2025 Open/Public © 2019 CLS. All rights reserved. Proprietary and Confidential. 2/31 Issue Date Object Written by 1.0 18/12/23 Initial version Laetitia Gal and Angelica Tarpanelli 1.1 12/01/24 New version replying to ESA comments Laetitia Gal and Angelica Tarpanelli 1.2 26/09/24 Updated version with last RD- multispec and RD-merged products Laetitia Gal and Angelica Tarpanelli 2.0 31/07/25 Updated version with river-width based RD, complement on RD-alti, RD-multispec and RD-merged Laetitia Gal, Angelica Tarpanelli, Mohammad Tourian, and Paolo Filippucci Checked by S.

Biancamaria - LEGOS 22/09/2025 Approved by Philippe Mourot - CLS 24/09/2025 Authorized by Clément Albergel - ESA 01/10/2025

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DISTRIBUTION D.3 River Discharge ATBD. Reference: RD_CCI-0006-ATBD - Issue 2.0 – 31/07/2025 Open/Public © 2019 CLS. All rights reserved. Proprietary and Confidential.

3/31 Company Names Email ESA Clément Albergel clement.albergel@esa.int CLS Philippe Mourot pmourot@groupcls.com CLS Daya Ceccone dceccone@groupcls.com CLS Maxime Vayre mvayre@groupcls.com CLS Nicolas Taburet ntaburet@groupcls.com CLS Gabriel Calassou gcalassou@groupcls.com CNRM Simon Munier simon.munier@meteo.fr EOLA Elena Zakharova zavocado@gmail.com Hydromatters Malik Boussaroque malik.boussaroque@hydro-matters.fr Hydromatters Laetitia Gal laetitia.gal@hydro-matters.fr Hydromatters Adrien Paris adrien.paris@hydro-matters.fr IRPI Silvia Barbetta silvia.barbetta@irpi.cnr.it IRPI Angelica Tarpanelli angelica.tarpanelli@irpi.cnr.it IRPI Paolo Filippucci paolo.filippucci@irpi.cnr.it LEGOS-CNRS Sylvain Biancamaria sylvain.biancamaria@univ-tlse3.fr LEGOS-CNES Julien Lefebve julien.lefebve@cnes.fr LEGOS Benjamin Kitambo benjamin.kitambo@univ-tlse3.fr GEOS-IRD Fabrice Papa fabrice.papa@ird.fr Magellium Gilles Larnicol gilles.larnicol@magellium.fr Magellium Vanessa Pedinotti vanessa.pedinotti@magellium.fr Magellium Malak Sadki malak.sadki@magellium.fr Univ.

Stuttgart Mohammad Tourian tourian@gis.uni-stuttgart.de LEGOS-IRD Nino Fernando fernando.nino@ird.fr LEGOS Fabien Blarel fabien.blarel@univ-tlse3.fr PML Stefan Simis stsi@pml.ac.uk Univ. Exeter - PML Jareno Nuria Bachiller nb748@exeter.ac.uk

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LIST OF CONTENTS/SOMMAIRE 1 Introduction ............................................................................................................................................... 7 2 Available ancillary data ............................................................................................................................ 7 3 Calibration – Validation periods ............................................................................................................ 10 4 Derive discharge from altimeters (RD-alti) ............................................................................................

12 4.1 Context ........................................................................................................................................... 12 4.2 Bayesian-based Rating Curve (RC) estimation ............................................................................ 13 4.2.1 Bayesian approach ................................................................................................................... 13 4.2.2 WSE data type ........................................................................................................................... 15 4.2.3 Method 1 – Bayesian estimation using overlapping dataset – preferred approach ...........

16 4.2.4 Method 2 – Bayesian estimation using quantile-based matching – alternative approach . 18 4.3 Uncertainties ................................................................................................................................. 19 5 Derive discharge from river-width (RD-width) ....................................................................................... 20 5.1 River width estimation using optical satellite imagery ............................................................... 20 5.2 River discharge estimation using river width ..............................................................................

21 6 Derive discharge from multispectral images (RD-multispec) .............................................................. 23 6.1 Context ........................................................................................................................................... 23 6.2 Reflectance indices definition ...................................................................................................... 23 6.3 Multi-mission reflectance time series .......................................................................................... 24 6.4 River discharge estimation from reflectance indices .................................................................

27 6.4.1 Calibrated Approach ................................................................................................................. 27 6.4.2 Uncalibrated Approach ............................................................................................................. 28 7 Derive discharge from merged products............................................................................................... 29 8 References .............................................................................................................................................. 29 D.3 River Discharge ATBD. Reference: RD_CCI-0006-ATBD - Issue 2.0 – 31/07/2025 Open/Public © 2019 CLS.

All rights reserved. Proprietary and Confidential. 4/31

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