Estimating response times, flow velocities, and roughness coefficients of Canadian Prairie basins

Abstract

The hydrology and hydrography of the Canadian Prairies are complex and difficult to represent in hydrological models. Recent studies suggest that runoff velocities on the Canadian Prairies may be much smaller than generally assumed. Times to peak, basin-scale flow velocities and roughnesses were derived from hourly streamflow hydrographs from 23 basins in the central Alberta Prairies. The estimated velocities were much smaller than would be estimated from most commonly used empirical equations, suggesting that many existing methods are not suitable for estimating times to peak or lag times in these basins. Basin area was found to be a poor predictor of basin-scale rainfall-runoff flow velocity. Estimated velocities generally increased with basin scale, indicating that slow basin responses at small scales could be related to the predominance of overland and/or shallow sub-surface flow over the very level topography. Basin-scale values of Manning's roughness parameter were found to be orders of magnitude greater than values commonly used for streams in other parts of the world. The very large values of roughness call into question whether the Manning equation should be used to calculate runoff in the prairies. These results have important implications for calculating rainfall runoff in this region since using widely published values of roughness will result in poor model estimation of streamflow hydrographs. It is likely that the Darcy–Weisbach equation, which is applicable to all flow regimes, may perform better in high-resolution hydrological models of this region. Further modelling and field research will be required to determine the physical causes of these very small basin-scale velocities.