Scale effects in conceptual hydrological modeling

We simulate the water balance dynamics of 269 catchments in Austria ranging in size from 10 to 130,000 km(2) using a semidistributed conceptual model with 11 parameters based on a daily time step. The simulation results suggest that the Nash-Sutcliffe model efficiencies increase over the scale range of 10 and 10,000 km(2). The scatter of the model performances decreases with catchment scale, particularly the volume errors. This implies that the model simulates the long-term water balance more reliably as one goes up in scale. Most calibrated parameters do not change with catchment scale, but there is a trend with catchment area of the upper and lower envelope curves of some parameters. We also examine time scale effects. Calibration efficiencies decrease and verification efficiencies increase with the number of years available for calibration. The change in efficiencies is largest between 1 and 5 years used for calibration. This suggests that a calibration period of 5 years captures most of the temporal hydrological variability, so this would be the minimum for achieving a reasonable predictive model performance. The correlation of model parameters between different calibration periods, as a measure of the degree to which parameters can be identified, increases with increasing length of the calibration period. For some parameters, the correlation increases beyond 5 years of calibration. This suggest that although runoff may be simulated well using 5 years of calibration, some parameters may not be well constrained and hence internal state variables and fluxes may still be associated with larger uncertainties than with a larger calibration period.