Climate change and demographic pressures are affecting both the urban water balance and microclimate, thus amplifying urban flooding and the urban heat island phenomena. These issues need to be addressed when engaging in urban planning activities. Local authorities and stakeholders have therefore opted for more nature-based adaptation strategies, which are especially suitable in influencing hydrological and energy processes. Assessing the multiple benefits of such strategies on the urban microclimate requires high-performance numerical tools. This paper presents recent developments dedicated to the water budget in the Town Energy Balance for vegetated surfaces (TEB-Veg) model (surface externalisée; SURFEX v7.3), thus providing a more complete representation of the hydrological processes taking place in the urban subsoil. This new hydrological module is called TEB-Hydro. Its inherent features include the introduction of subsoil beneath built surfaces, the horizontal rebalancing of intra-mesh soil moisture, soil water drainage via the sewer network and the limitation of deep drainage. A sensitivity analysis is then performed in order to identify the hydrological parameters required for model calibration. This new TEB-Hydro model is evaluated on two small residential catchments in Nantes (France), over two distinct periods, by comparing simulated sewer discharge with observed findings. In both cases, the model tends to overestimate total sewer discharge and performs better under wet weather conditions, with a Kling–Gupta efficiency (KGE) statistical criterion greater than 0.80 vs. approximately 0.60 under drier conditions. These results are encouraging since the same set of model parameters is identified for both catchments, irrespective of meteorological and local physical conditions. This approach offers opportunities to apply the TEB-Hydro model at the city scale alongside projections of climate and demographic changes.
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