Use of L-moments in the Regional Frequency Analysis of Meteorological Drought Duration in the Transboundary Comoé River Basin

This study aims to estimate the duration of meteorological droughts in the Comoé River Basin. The sparse distribution of rainfall stations motivated the use of a regional frequency analysis based on L-moments. This method was applied to drought duration series derived from the 3-month Standardised Precipitation Index (SPI-3), using a drought threshold of -0.84. Classification and homogeneity tests identified four regions characterised by different drought patterns. Region R2 (Abidjan, Adiaké, Alépé, Bassam, Adzopé) represents the transitional equatorial climate in the southern part of the basin, while Region R4 (Tafiré, Tengréla, Ferkessédougou, Bobo-Dioulasso, Niangoloko, Kampti) is typical of the Sudanian climate in the north. The other two regions, R1 (Orodara, Ouangolo, Abengourou, Ouéllé, Dabakala, Bongouanou, Daoukro) and R3 (Bouna, Bondoukou, Agnibilekrou, M’Bahiakro, Aboisso), exhibit contrasting climatic characteristics because some rainfall stations that make up these groups are not located within the same climate zone. The Wakeby distribution was found to be the most suitable for modelling drought durations. Regional quantiles, estimated using the index flood approach, demonstrated greater accuracy than site-specific quantiles. These quantiles were subsequently interpolated spatially. The results indicate a relatively homogeneous pattern of drought durations across the basin, except in the far north. High probabilities of prolonged drought were observed in this northern area, which corresponds to a transitional tropical climate zone. For return periods (T) of 2, 5, and 10 years, the average drought duration was estimated at 2, 3, and 4 months across most of the basin, compared with 4, 6, and 8 months in the northern region. For T = 20 and 50 years, drought durations are expected to reach 7–8 months in the south, and up to 11 and 16 months, respectively, in the north.