Apport altimétrie pour modélisation hydrologique des grands fleuves

Transcription

1 Apport altimétrie pour modélisation hydrologique des grands fleuves C. Emery, V. Pedinotti, S. Biancamaria, A. Boone, A. Paris, S. Calmant, S. Ricci, P.-A. Garambois, B. Decharme, Workshop Niger, September

2 Global Land Surface Model Study continental part of energy and water cycle At large scale, multiple sources of errors: simplified physics, uncertainties on atmospheric forcing, uncertainties on initial conditions, uncertainties on input parameters Purpose: improve water fluxes estimation on the continental surfaces at interannual and interseasonal scale for big river basins Use satellite products for sparsely observed basins Workshop Niger, September

3 Global Model used ISBA-CTRIP model (0.5 x0.5 ~50km x 50km): Evap Precipitation Surface runoff Incoming water from upstream cell Floodplains River Infiltration ISBA Gravitational drainage Outgoing water to downstream cell CTRIP Estimation of: - River water level - River discharge - Floodplain water storage Groundwaters Initially designed to convert surface runoff into discharge over a global river network, Equivalent river with rectangular section + groundwater + floodplain reservoirs, Variant flow velocity calculated with the Manning equation Workshop Niger, September

4 Framework: Data assimilation CTRIP Friction Pseudo-SWOT elev. Alti-derived disch. Parameter estimation State estimation River water stock - OSSE (twin experiments) - Correcting friction param. - Using: Extended Kalman Filter (EKF) over Niger (Pedinotti et al., 2014) Asynchronous Ens. Kalman Filter (AEnKF) over Amazon (Emery et al., in prep) Niger bassin AEnKF Amazon bassin EnKF Amazon bassin - ENVISAT-derived disch. - Correcting CTRIP river storage - Using Ensemble Kalman Filter (EnKF) over Amazon (Emery et al., submitted) Workshop Niger, September

5 CORRECTING MODEL PARAMETER Workshop Niger, September

6 Water elev [m] Satellite observations used Pseudo-SWOT obs.: twin experiments Simplified SWOT simulator ISBA-CTRIP reference simulation SWOT simulator mask SWOT swath for the 21-day repeat period at ISBA-CTRIP resolution (0.5 x 0.5 ) above the Amazon river. Extraction Day 06 Day 11 Add uncertainties (white noise, std=10cm) SWOT repeatitivity cycles from Jan 1st, 2008 Day 16 Day 21 SWOT pseudo-observations at Saõ Francisco Reference run water elevation SWOT pseudo-observed water elevation Workshop Niger, September

7 n true n assim (%) Results: Niger (friction param.) ISBA/CTRIP + aquifer (2002/2007), validated against in situ + sat. -> Issue on Niger Inland Delta (Pedinotti et al., 2012) Assim. (Extended Kalman Filter, EKF) pseudo-swot -> correct friction param (Pedinotti et al., 2014) Manning coefficient distribution truth no assim Uncertainty on friction coefficient after assim Time (days) assim (green) assim (blue) Relative difference on water level improved by 30% over the river But important non linearity (issue for EKF) + need to take other unertainties (precip ) Workshop Niger, September

8 Assimilation experiments Use AEnKF instead of EKF (linearity issue) Correcting a multiplification factor of the Manning distribution over 9 zones 3 experiments to tests different observations: Name Observation type Start of experiment Time period EP1F Water Elevations (W Elev) 01/01/ year EP3-1 W Elev Anomalies 04/03/2008 (high flows) 2 months EP3-2 W Elev Anomalies 09/09/2008 (low flows) 2 months Workshop Niger, September

9 Control variables Control variables Results: Amazon (friction param.) AEnKF assimilation platform to correct CTRIP parameters (Manning coeff only): OSSE: twin experiment with SWOT obs.= 10 cm white noise on water depth Model error = 0.25 white noise on Manning multiplicative coefficient 100 members and 21 days assimilation window Results with pseudo-swot water elevation anomalies: High-flow season (March-April) Truth No assim Mean analysis Low-flow season (September-October) Truth No assim Mean analysis Hydrogeomorph zones Hydrogeomorph zones Workshop Niger, September

10 Results: Amazon (friction param.) The assimilation platform is able to correct friction coeff. distribution (even with water elevation anomalies): No assim Assim W Elev Assim W Elev anom high flows Assim W Elev anom low flows Manning 33% 12.2% 11.3% 18% Elevation 26% 4% 4.5% 8.5% Correction of other parameters not tested (potential equifinality issue) Workshop Niger, September

11 CORRECTING MODEL STATE (AMAZON BASIN) Workshop Niger, September

12 MGB discharge (m3/s) Satellite observations used Sat. water elev. + rating curves (Paris et al., 2016) ENVISAT water elevation ( ) Satellite altitude Alitmeter distance Geoid Alitmeter water elev. In situ + MGB model discharges: ENVISAT water elev. (m) >750 «ENVISAT» discharge time series over the Amazon Workshop Niger, September

13 Assimilation experiments Using ENVISAT-derived discharge: Real data (not twin experiment) Test benefits of a satellite discharge product (-> SWOT discharge product) Obs error: white noise (20% of instantaneous discharge) EnKF assimilation platform to correct CTRIP model states: Precipitation errors (~30%) + error on initial storage (depending of the zone) 100 members Test different mode state corrections: Name Localization Control variables EE1-direct No River storage EE1-local Yes River storage Treatment of error covariance matrices to avoid spurious elements Workshop Niger, September

14 Discharge (m 3 /s) Discharge (m 3 /s) Results at Obidos: Legend: Insitu Observations with spread Free run Background ensemble spread Analysis ensemble mean Results # Days from Sept 25th, 2002 # x10 5 EE1-direct, RMSE=13,3% x10 5 Days since Sept 25th, 2002 EE1-local, RMSE=12,6% Days from Sept 25th, 2002 Days since Sept 25th, 2002 Workshop Niger, September

15 Results CTRIP discharge is improved with ENVISAT-derived discharge: RMSE insitu No assim EE1-direct (no local.) EE1-local Global 74% 56% 57% Óbidos 29% 13.3% 12.6% Results limited by ENVISAT low observation frequency (1 observation every 35 days) Localization is needed Workshop Niger, September

16 Conclusions Twin experiments with virtual SWOT water elevations/anomalies observations to correct CTRIP river Manning coefficients Platform able to correct a priori Manning distribution for all configurations Feasibility of water elevation anomalies assimilation Real-data experiments with ENVISAT discharges observations to correct CTRIP river storage/discharge Satellite-derived discharge assimilation improve CTRIP discharge for all configurations Localization is necessary Scale issue: how to couple 50kmx50km global model outputs with local information from satellite or in situ measurements? Workshop Niger, September

17 Perspectives Methodologies can be applied to Niger, but issue with NID Parameter estimation perspectives Need more realistic OSSE experiments Correct other parameters (slope, width, groundwater time constant) separately or simultaneously (equifinality issues) and according the hydrological season State estimation perspectives Improve localization mask (and ensemble generation) Complete ENVISAT data with other discharge products from altimetry (JASON-2 and future missions) Assimilating other satellite data? ISBA? CTRIP? -> more work needed! Of course, satellite data are complementary to in situ data Workshop Niger, September

18 THANK YOU!!! Funding from CNES and Région Occitanie is acknowledged Workshop Niger, September