Regional frequency analysis of maximal daily annual rainfalls in Cheliff catchment, AlgeriaRegional frequency analysis of maximal daily annual rainfalls in Cheliff catchment, Algeria
Local extreme rainfall estimates are often inaccurate because of the short duration of the observed series. Regional aggregation of data can greatly improve these estimates. This article shows a regional approach for the estimation of daily rainfall frequency rare, in the Cheliff basin (northwestern Algeria). The first step consists of defining and validating the homogeneous regions of the study area. The homogeneity test is based on the H-statistic, which compares the ratios of L-moments calculated locally at each station to their average over the region. The second step is to identify the regional distribution and estimate its parameters by analyzing the diagram of the L-moments and/or calculation of the Z-statistic which compares the ratios of regional L-moments with those of the candidate distribution. GEV ("General Extreme Value"), which has been used in several studies of regionalization of extreme rainfall, has been identified as a suitable regional distribution. The parameters of the GEV were calculated using the definition of regional L-CV, L-CS and L-CK. The third step is to determine locally the quantiles of rain associated with different return periods by multiplying the regional L-coefficient of variation L-CV by the mean annual maximum daily rainfall observed at the considered site. Rainfalls calculated by this method, which can be applied to any station in the study area, are significantly higher than those calculated by local fit. The values of the mean square error (REQM) between quantiles derived from regional or local methods are 10% for the decennial annual maximum daily rainfall, and 35% for the centennial annual maximum daily rainfall. This error decreases when the length of the local series increases, suggesting that the regional method leads indeed to more reliable assessments of the rain quantiles.