In instances where there are fewer than 20 common dates between discharge and water surface elevation data, all available data will be utilized for the calibration process, and validation will be conducted by comparing it with alternative data sources, such as in-situ water surface elevation measurements or simulated discharge data generated from modelling. 4 Derive discharge from altimeters (RD-alti) 4.1 Context Just as in-situ stage measurements can be used to gauge river discharge, altimetry-derived water surface elevation (WSE) can serve as an alternative means of estimating river discharge when discharge time series data is available.
Before selecting the appropriate method, the input WSE data type must be identified: WSE-merged (multi-mission altimetry time series): In this case, only the calibration period is retained to compute the rating curve. WSE mono-mission (separate altimetry missions): Each mission is processed independently using all available data. Based on the data sample define by the WSE data type, two main approaches have been used for deriving discharge time series from altimetry observations and supplementary data, depending on the available temporal overlap between discharge and altimetry water surface elevation (WSE) time series. Method 1:
Temporal overlap data (preferred approach) - When sufficient overlapping observations exist, a rating curve is derived by applying the Bayesian method to the paired WSE and discharge data. Method 2: No temporal overlap data (alternative approach) - When overlap is insufficient or absent, a rating curve is derived by applying the Bayesian method to the quantiles of the full WSE and discharge datasets, assuming stability of the rating curve over time. To better reflect river-specific conditions and improve the applicability oof rating curves, three main hydrological cases have been distinguished, each associated with adapted versions of these methodologies: Case 1 - General cases:
where we can directly compute the rating curve between the available WSE and Q Case 2 – Rivers with intermittent ice cover: Filtering ice-affected data points by excluding observations during frozen periods based on temperature thresholds. Case 3 – Arctic rivers: Arctic and complex hydraulic conditions requiring multiple rating curves for different flow and ice-cover states 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. 12/31