3 Calibration – Validation periods The methodology used to derive discharge from altimetry WSE will highly depend on ancillary data available and especially discharge time series used for calibration. The selection of the calibration and validation period for altimeter calibration curves, correlating discharge and water surface elevation, is critically important for result precision.
In our approach, we have chosen, in agreement with WP3.1 and WP3.1.2 teams, a method involving the use of the period from the earliest date where simultaneous in situ discharge and merged water surface elevation from the altimeter data (from WP3.1) are available to the latest date when these two data sets overlap (Figure 2). This period is divided into three sections. The first section, in chronological order, is allocated for the validation period, during which altimeter data is compared to reference data to assess model performance. The subsequent two sections are dedicated to the calibration period (Table 1).
Using data from these latter two-thirds enables us to take advantage of the most recent satellite constellations and the least biased a priori data, thus ensuring optimal altimeter calibration. This methodological choice is designed to secure the reliability and precision of altimeter measurements across diverse hydrological conditions. Figure 2: Available data for in situ discharge from various sources (blue) and merged water surface elevation from altimeter (green) from each station. The period where the two data sets overlap is represented in red with the number of overlap’s days. D.3 River Discharge ATBD. Reference: RD_CCI-0006-ATBD - Issue 2.0 – 31/07/2025 Open/Public © 2019 CLS.
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