Author(s): Wanda Berolo; Jean-Pierre Laborde
Linked Author(s):
Keywords: Extreme rainfall data; High risk season; Gumbel distribution; Basin gradex; Areal reduction factor; Validation test
Abstract: Basin rainfall gradex over 1 to 7 days in 67 sub-basins of the Rhone catchment are assessed from point daily gradex available at more than 500 raingauges. Estimates being used beyond the present study for evaluating peak discharges of the Rhone tributaries, their reliability has to be asserted. The analysis is composed of 2 steps based upon 2 types of datasets: daily rainfall time series at 243 stations over the 11 main sub-catchments and daily seasonal gradex at 325 (recording) raingauges over the whole Rhone catchment. From the first type of data series, daily rainfall is interpolated over each of the 11 main sub-catchments by a kriging system. Weighting coefficients which are attributed to raingauges take into account the average range over a basin of extreme daily rainfall exceeding a threshold on average three times a year. The basin gradex over the 7 durations are determined by fitting the Gumbel distribution to monthly basin rainfall maxima during the high risk season. Results point out the areal reduction factor applied to point gradex. Secondly, daily rainfall time series being not available at a sufficient number of raingauges for assessing gradex values over 1 to 7 days in the 67 Rhone sub-catchments, a relation between the gradex of the 11 main basins estimated over the 7 durations and the daily seasonal point gradex available for all sub-basins is carried out. Basin gradex are function of the weighted mean of point daily seasonal gradex and of the basin area according to rainfall duration. The validation of the model, performed with a test sample composed of 3 sub-catchments, shows the less accurate estimates are included within a ±20% interval with regard to ‘true’ gradex and concern sub-catchments characterized by heavy Mediterranean rainfall. The comparison between the weighted mean of daily point gradex and daily basin gradex for all sub-catchments displays an areal reduction factor which becomes more significant with high gradex values.
Year: 2005