Regional dimensionless hydrograph for Alberta foothills

The majority of hydrologic engineering applications deal with ungauged watersheds. Various empirical methods are available for synthesizing unit hydrographs for ungauged watersheds from information obtained from maps or field inspection of the watershed. An alternative to the employment of generalized synthetic unit hydrographs is to develop a regional dimensionless hydrograph to characterize the local rainfall-runoff processes better. The derivation of such a dimensionless hydrograph is described for Alberta foothills, based on the analysis of 31 basins and 61 rainfall-runoff events. The analysis shows that it is possible to derive a representative dimensionless hydrograph for the region with reasonable accuracy by averaging the observed direct runoff hydrographs converted into a dimensionless form. However, considerable uncertainty is associated with the estimation of the excess rainfall duration and the lag time of the events analysed. The lag time is the key parameter needed to convert the regional dimensionless hydrograph into an ungauged watershed unit hydrograph. Possible reasons for unexplained lag time variations are discussed. The regional dimensionless hydrograph developed and lag time curve were used to regenerate the original 61 hydrographs. Results were compared with the generalized Soil Conservation Service dimensionless unit hydrograph which tended to produce larger errors in predicted peak flows. Error analysis indicates the limits of accuracy that may be expected from the method. Copyright (C) 2003 John Wiley Sons, Ltd.