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Uncertainties in water stages by Altimetry assessed by field measurements

Uncertainties in water stages by Altimetry assessed by field measurements. Stephane CALMANT, Frédérique SEYLER & Jean-Francois Cretaux IRD/LEGOS-LMTG/OMP Toulouse, France. Plan: 1st case study: Altimetry vs GPS in the Amazon basin:

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Uncertainties in water stages by Altimetry assessed by field measurements

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  1. Uncertainties in water stages by Altimetry assessed by field measurements Stephane CALMANT, Frédérique SEYLER & Jean-Francois Cretaux IRD/LEGOS-LMTG/OMP Toulouse, France

  2. Plan: • 1st case study: Altimetry vs GPS in the Amazon basin: - Performance of altimetry data over the CW has evolved since the pionner T/P GDRs… • High space and time variations of water stage sampled • in different ways by gauges and altimetry • in-situ gauges and altimetry DO NOT measure the same quantities • 2nd case study: Altimetry vs GPS on the Issykul Lake: • Data are ok if used with caution since • WF Trackers are biased • Geophysical corrections are a key source of uncertainty in altimetry measurements over CW

  3. 1 example: Computing river depths from altimetry (Upper Rio Negro, Amazon basin)

  4. Depth of the Rio Negro at San Felipe Stage time series (ENVISAT) Cross section (ADCP) Rating curve D = a(H-z)b

  5. 2nd example: Space & time variations of the Water stage (Solimoes, Amazon basin)

  6. What water level do we measure: Example of profiles across the Solimoes river >25 cm/km ICESat @ Nov ‘03 4.2m 3.2m 3.7m 3cm/km Height (m) GPS @ Nov ‘05 <0 Latitude

  7. 3rd Example: Lake Issyk-kul • Free surface slope within the radar • footprints • - Range inter-calibration • Geophysical corrections

  8. GPS vs Altimetry on Lake Issyk-kul (Kirghizstan) See the poster by Cretaux et al. for details)

  9. GPS surveys beneath altimetric tracks on Lake Issyk-kul

  10. ~50 cm

  11. GFO ranges from ICE-1 & ICE-2 50 cm

  12. Synthesis: Reliable hydrological products can be derived from radar altimetry although tracker biases & geophy. corr still to be improved Assessing data uncertainty from relative comparaison of time series requires that both measurements were actually collected at the same time (daily might be not sufficient) …. and location Satellite altimeters and in situ gauges do not always measure the same hydrological body (footprint size, tracker algo., averaging procedure…)

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