Hydrological process representation at the meso-scale: the potential of a distributed, conceptual catchment model

In order to achieve a process-oriented simulation of hydrological processes in a meso-scale basin (10 1–10 3 km 2), the spatially and temporally variable basin inputs (precipitation and energy) and runoff generation processes need to be adequately addressed by the model. The catchment model TAC D (tracer aided catchment model, distributed) is based on experimental results including tracer studies at the mountainous Brugga basin (40 km 2). This raster-based model (50×50 m 2) works on an hourly basis, thus capturing the spatially and temporally variable inputs and processes. The model contains a process-realistic description of the runoff generation mechanism, which is based on a spatial delineation of eight units with the same dominating runoff generation processes. This defines the model structure and enables efficient model parameterisation. The model uses linear and non-linear reservoir routines to conceptualise runoff generation processes, and includes a routing routine (kinematic wave approach) to simulate surface runoff. The model is successfully applied to a 1-year period following minimal calibration (model efficiency 0.94). In addition, the runoff from both an independent 3-year period for the Brugga basin and a sub-basin (15.2 km 2) is modelled well (model efficiencies 0.80 and 0.85, respectively) without re-calibration. The use of tracer data (i.e. dissolved silica) measured in outlet discharge demonstrates that the temporal mixing pattern of different runoff components is modelled correctly (multiple-response validation). The results show that a validated process-based model that correctly simulates the origin of runoff components and flow pathways must be the basis for integrating solute transport modelling of non-conservative species. Such a model can serve as tool to make predictions and test hypotheses about the first-order controls on hydrological responses.