Modelling hydrologic and hydrodynamic processes in basins with large semi-arid wetlands

Abstract Hydrologicaland hydrodynamic models are core tools for simulation of large basins and complex river systems associated to wetlands. Recent studies have pointed towards the importance of online coupling strategies, representing feedbacks between floodplaininundation and vertical hydrology. Especially across semi- aridregions, soil- floodplaininteractions can be strong. In this study, we included a two-way coupling scheme in a large scale hydrological-hydrodynamic model (MGB) and tested different model structures, in order to assess which processes are important to be simulated in large semi- aridwetlands and how these processes interact with water budget components. To demonstrate benefits from this coupling over a validation case, the model was applied to the Upper Niger River basin encompassing the Niger Inner Delta, a vast semi- aridwetland in the Sahel Desert. Simulation was carried out from 1999 to 2014 with daily TMPA 3B42 precipitation as forcing, using both in-situ and remotely sensed data for calibration and validation. Model outputs were in good agreement with discharge and water levels at stations both upstream and downstream of the Inner Delta (Nash-Sutcliffe Efficiency (NSE) >0.6 for most gauges), as well as for flooded areas within the Delta region (NSE = 0.6; r = 0.85). Model estimates of annual water losses across the Delta varied between 20.1 and 30.6 km 3 /yr, while annual evapotranspirationranged between 760 mm/yr and 1130 mm/yr. Evaluation of model structure indicated that representation of both floodplainchannels hydrodynamics (storage, bifurcations, lateral connections) and vertical hydrologicalprocesses ( floodplainwater infiltration into soil column; evapotranspirationfrom soil and vegetation and evaporation of open water) are necessary to correctly simulate flood wave attenuation and evapotranspirationalong the basin. Two-way coupled models are necessary to better understand processes in large semi- aridwetlands. Finally, such coupled hydrologicand hydrodynamic modelling proves to be an important tool for integrated evaluation of hydrologicalprocesses in such poorly gauged, large scale basins. We hope that this model application provides new ways forward for large scale model development in such systems, involving semi- aridregions and complex floodplains.