1 The Role of the Antecedent Soil Moisture Condition on the Distributed Hydrologic Modelling of the Toce Alpine Basin Floods Nicola Montaldo, Giovanni Ravazzani, Marco Mancini Dipartimento di Ingegneria Idraulica, Ambientale, Infrastrutture viarie, e del Rilevamento, Politecnico di Milano, Italy Brig, Switzerland, 21 May 2003

2 The role of soil moisture initial conditions in Toce flood simulation. Is it important in an Alpine basin? Can be improved the flood modeling using a continuous model instead of an event model? D efine an adequate complexity level of the LSM for the modeling of Alpine basin floods. Objectives of this study Comparison of three distributed hydrologic models for the Toce flood simulations: 1.FEST02. Event based model 2.TDHM. Topographically based distributed land surface continuous model (two soil layers) 3.SDHM. Simplified distributed land surface continuous model (one soil layers) Actions

3 Alpine glacial basin Maximum elevation: 4558 m Average elevation: 1641 m Steep hillslopes (about 90-95% of the total area) Mostly covered by trees on thin soil layers rested bedrock The case study Toce watershed at Candoglia section Total drainage area: 1534 km 2 The overland flow should be mainly Hortonian

4 The available data. Data collected during the EC RAPHAEL project Leaf Area Index Saturated Conductivity LitologyLitologyLitologyLitology

5 The available data Data collected during the EC RAPHAEL project The period: Rainfall (monthly aggregation) Atmospheric forcings (monthly averaged)

6 The distributed hydrological model structure Hillslopes – river network partition DEM Flood routing: Muskingum-Cunge Soil and vegetation parameter Land Use Atmospheric forcings Spatial interpolation Surface Runoff model Baseflow Snow Model

7 The TDHM model Topographically based distributed land surface continuous model (two soil layers) Famiglietti & Wood, 1994 Topographic Index Water Table Watershed Base Flow

8 The SDHM model Base Flow: lumped model (linear reservoir) Simplified Distributed Hydrologic continuous Model (single soil layer) G s = watershed total drainage  drainage

9 The FEST-02 (*) model Base Flow: lumped model (linear reservoir) FE S T Flash-flood Event-based Spatially-distributed rainfall- runoff Transformation I = watershed total infiltration  infiltration SCS-CN (*) Montaldo et al., Hydrol. Process., 2003

10 The continuous models calibration

11 Fluxes simulation ET Infiltration Soil moisture Drainage

12 The flood simulations Sim/Obs Qmax 0.84 Sim/Obs Vol 1 Nash Eff 0.83 RMSE 118 TDHM Sim/Obs Qmax 0.86 Sim/Obs Vol 1.12 Nash Eff 0.89 RMSE 96 SDHM Sim/Obs Qmax 1.28 Sim/Obs Vol 1.3 Nash Eff 0.7 RMSE 150 FEST-02

13 The flood simulations Sim/Obs Qmax 0.99 Sim/Obs Vol 0.9 Nash Eff 0.81 RMSE 101 TDHM Sim/Obs Qmax 1 Sim/Obs Vol 1.14 Nash Eff 0.91 RMSE 69 SDHM Sim/Obs Qmax 0.91 Sim/Obs Vol 0.99 Nash Eff 0.98 RMSE 28 FEST-02

14 Soil potential retention

15 Conclusions The proposed hydrologic continuous models (TDHM and SDHM) simulated quite well the total runoff at the basin outlet. The continuous water balance models can be used for flood simulations with better performance than the event based model FEST-02. (Note that the models need an extended spin-up period for soil moisture initial conditions). In Alpine basins (with steep slopes and thin soil layers) the simple hortonian continuous (one-layer) model simulate well the floods, and even better than the two-layers, topographically based model (more time consuming).