| Distributed hydrological models like SWAT (Soil and Water Assessment Tool)
are often highly over-parameterized, making parameter specification and
parameter estimation inevitable steps in model calibration. Manual
calibration is almost infeasible due to the complexity of large-scale models
with many objectives. Therefore we used a multi-site semi-automated inverse
modelling routine (SUFI-2) for calibration and uncertainty analysis.
Nevertheless, the question of when a model is sufficiently calibrated
remains open, and requires a project dependent definition. Due to the
non-uniqueness of effective parameter sets, parameter calibration and
prediction uncertainty of a model are intimately related.
We address some calibration and uncertainty issues using SWAT to model a
four million km2 area in West Africa, including mainly the basins of
the river Niger, Volta and Senegal. This model is a case study in a larger
project with the goal of quantifying the amount of global country-based
available freshwater. Annual and monthly simulations with the "calibrated"
model for West Africa show promising results in respect of the freshwater
quantification but also point out the importance of evaluating the
conceptual model uncertainty as well as the parameter uncertainty. |