Description and validation of a streamflow assimilation system for a distributed hydrometeorological model over France. Impacts on the ensemble streamflow forecasts G. Thirel, E. Martin, J.-F. Mahfouf (CNRM/Météo-France), S. Massart, S. Ricci (CERFACS), F. Regimbeau (DCLIMHYDRO/Météo-France), F. Habets (UMR Sisyphe Mines ParisTech)

Introduction 2 ensemble streamflow prediction systems (ESPS) at a short- and mid-term range at Météo-France –Based on the distributed hydrometeorological model SIM –ECMWF EPS-based ESPS (10-day range, 1.5°, 51 members) –PEARP-based ESPS (60-h range, 0.25°, 11 members) Need to improve the initial states by an assimilation system –Use of observed discharges –Description –Validation over 18 months Impacts of the assimilation system on the ECMWF EPS-based ESPS

ISBA Physiographic data for soil and vegetation + MODCOU Qr Qi E H G Aquifer Daily Streamflow Surface scheme Snow SAFRAN Observations + NWP models PPrecipitation, temperature, humidity, wind, radiations Hydrological model Poor Weak to moderate Good Nash Habets et al. (2008) Meteorological analysis The SIM hydro-meteorological model

The SIM based ESPS Observations Meteor. models ANALYSIS RUN (daily) SAFRAN 10-year climatology Wind, Rad., Humidity SOIL WAT. TABLES RIVERS FINAL STATE ECMWF/PEARP Ensemble forecasts 51/11 members, 11/2-day forecasts ENSEMBLE FORECASTS T+ Precip Spatial DESAGGREGATION ISBA MODCOU ENSEMBLE FORECAST SOIL WAT. TABLES RIVERS FINAL STATES ISBA MODCOU SOIL WAT. TABLES RIVERS STATE Initial states of ESPS : need for improvement Adjusted by BLUE

Strategy 186 stations assimilated over France –Low human influence –Good quality of observations –Not too bad results given by SIM Aim : to use observed discharges in order to improve streamflow simulation, by adjusting the ISBA soil moisture

The BLUE equations Analysed state Background state Innovation vector Jacobian H : H determines the sensitivity of streamflows to soil moisture variations Hypothesis : linearity of the model -> H is computed with SIM runs initialized by perturbed soil moisture states (perturbation around 0.1%) Observed streamflows streamflows x : control variable

Jacobian matrix filling 3 gauging stations Q1, Q2 et Q3. w1, w2 et w3 moderated sums of soil moistures on the basins Jacobian matrix : basins stations 186 stations

Principle of the assimilation system

Experiments (10 March 2005 / 30 September 2006, 186 stations) 6 experiments : 3 variable states * 2 physics of the model Daily assimilation, daily observations

IS2 (and IS1) will be retained IS2 combines the best Nash and rmse scores, and the lowest increments The Doubs at Besançon Scores for a selection of 148 stations

An exemple of the impacts on the ESPS IS2 IS1 No assimilation

Some statistical scores Scores for a selection of 148 assimilated stations, for the 10-day SIM-ECMWF Spread of the ensemble

RMSE Scores computed against observed streamflows

Brier Skill Score day 1 and day 10 Day 1 Day 10

Conclusions and perspectives A streamflow assimilation system has been implemented and validated for the SIM suite –Better simulation of flows and initial states for the ESPSs (Thirel et al., submitted to the Journal of Hydrology) Significative improvement of ensemble streamflow forecasts when initialized by the assimilated SIM suite –Lower RMSE, better BSS and RPSS (due to the assimilation for the first days, then due to the exponential profile) –Few differences between SIM-PEARP and SIM-ECMWF, difficult to conclude –It is the first time that the ensemblist SIM is compared to observations, not a reference run Perspectives : –Optimizing computing costs and the quality of the assimilation system (R and B matrices to better estimate) –Using another operator (EnKF?) –Implementing the assimilation system into the SIM-ECMWF operational suite (2012?)

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