Low-flow frequency analysis at ungauged sites based on regionally estimated streamflows

Estimation of low-flow quantiles or indices at ungauged sites is traditionally done through regional low-flow frequency analysis. However, traditional methods imply a prior aggregation of the regional information due to the usual focus on a given quantile. This leads to loss of information for estimating additional quantiles or performing additional applications. In the present study, the recently proposed regional streamflow based frequency analysis (RSBFA) approach is evaluated for the estimation of low-flow quantiles. The approach, originally applied to floods, is based on regionally estimating the daily streamflow series at the ungauged site to later obtain hydrological quantiles through a local frequency analysis. In the present study, the RSBFA approach is applied to low flows through a case study in the province of Quebec, Canada, considered in prior traditional regional low-flow studies. Although the RSBFA approach does not systematically lead to the best results, its relevance resides in the fact that the whole regionally estimated daily streamflow series is provided at the ungauged site. This allows an easy estimation of low-flow quantiles associated to particular durations, such as the 7-day (30-day) average low flows corresponding to a return period of 2 or 10 years (5 years). Furthermore, a large range of additional low-flow quantiles, such as q95% (streamflow expected to be exceeded 95% of the time divided by the catchment area), may be obtained from the regionally estimated streamflow series with a very good performance. Therefore, any specific or absolute, annual or seasonal low-flow quantile may be easily obtained by a local low-flow frequency analysis without repeating the regional procedure. The approach does not require complex models, and it may also allow combining regional and local information in a straight forward manner.