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 X 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. (C) 2004 Elsevier B.V. All rights reserved.
