Low flow estimates from short stream flow records-a comparison of methods

We compare a number of methods of adjusting Q95 estimates from short stream flow records for climate variability. Q95 is the discharge that is exceeded on 95% of all the time for one particular site. The climate adjustment methods consist of two steps, donor site selection and record augmentation, and use information from nearby sites with longer stream flow records. The accuracy of the methods is assessed by comparing the adjusted estimates from hypothetically shortened records with estimates from the full 20-year record at the same site. 132 catchments in Austria are used with catchment areas ranging from 9 to 479 km2. The results indicate that the downstream donor selection method performs best on all scores. The catchment similarity and correlation donor selection methods do not perform as well. The relative performance of the record augmentation methods depends on the donor selection method but, overall, the choice of record augmentation method is less important than the choice of the donor site. The value of the climate adjustment methods is very significant for record lengths shorter than 5 years. The coefficient of determination of q95 specific low flows increases from 63 to 89% for 1-year records, and from 86 to 93% for 3-year records when adjusting the estimates by the downstream site method. For 5 years or more, the value of the climate adjustment methods is much smaller. A method that uses spot gaugings of stream flow during a low flow period only performs slightly better than a simple regionalisation procedure in terms of predicting Q95 at an otherwise ungauged site. Comparisons with more sophisticated regionalisation procedures suggest that, on average over the study region, 1 year of continuous stream flow data clearly outperforms the more sophisticated regionalisation method while the spot gauging method provides less accurate low flow estimates than the sophisticated regionalisation method.