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The hydrological simulations are generated by Physically Based Distributed hydrological model TOPKAPI-X (TOPographic Kinematic APproximation and Integration – extended). TOPKAPI-X has already accumulated a long history in the literature as well in operational experiences all over the world. Thanks to its computational efficiency, global hydrological simulations can be performed with hourly time-step and a 1×1 km2 spatial resolution.

TOPKAPI-X is a fully distributed and continuous hydrologic model, with a simple and parsimonious parameterization. The model is based on the idea of combining the kinematic approach and the topography of the basin. Spatial distribution of catchment parameters, precipitation input and hydrologic response is achieved horizontally by an orthogonal grid network and vertically by soil layers at each grid pixel. Four ‘structurally similar’ non-linear reservoir differential equations characterize the TOPKAPI-X approach and are used to describe subsurface flow (superficial and deep layers), overland flow and channel flow. Moreover, the TOPKAPI-X model includes components representing the processes of the hydrologic cycle: infiltration, percolation, evapotranspiration and snowmelt, plus a lake/reservoir component, a parabolic routing component and a groundwater component. Being a physically based model, the values of the model parameters can be easily derived from digital elevation maps, soil type and land use maps in terms of topology, slope, soil permeability, soil depth and superficial roughness. A calibration based on observed streamflow data is then necessary for ‘fine tuning’ the model to reproduce the behavior of the catchment. Thanks to its physically based parameters, the TOPKAPI-X model can be successfully implemented also in un-gauged catchments where the model cannot be calibrated using measured data. In this case the model parameters can be derived from thematic maps.