The objective of this study is to characterize the conditional probability of single or consecutive dry and wet days in West Africa using the first-order Markov chain approach during the West African monsoon season (June to October). The results show that the probabilities of having a wet day ( ), a wet day preceded by another wet day ( ) and a wet day preceded by a dry day ( ) are stronger in regions (i.e., mountain regions) where the rainfall is maximum (up to 90%). In contrast, the probabilities of having a dry day ( ), a dry day preceded by a wet day ( ), and a dry day preceded by another dry day ( ) are lower in the regions with higher precipitation. The seasonal cycle of is consistent with that of in all considered sub-domains with maxima observed from July to September, except the Guinea Coast. Additionally, the seasonal cycle of is consistent with that of in all considered sub-regions. At the interannual timescale, MK test results show that and exhibit significant increase trends at the 95% confidence level in the Western (p-value =0.00005 and p-value =0.0007 , respectively) and Central Sahel (p-value =0.00008 and p-value =0.00000 , respectively), while and show significant decrease trends in the same areas. Besides, the shorter dry spells (3 days) show significant decrease trend only in the Western Sahel (p-value = 0.0231). The longer dry spells (5, 7 and 10 days) show statistically significant trends over the Western Sahel and the Central Sahel. In the Sudanian Zone, only the 10 consecutive dry days show statistically significant trend (p-value = 0.0490). In Guinea Coast, the probability of longer dry spells exhibits a non-significant increase trend. The wet spells probabilities show non-significant decrease trends in all West African selected areas, except the 10 days wet spell probability in the Western Sahel (p-value = 0.0192). Knowledge of the probabilities of wet and dry days will contribute to develop efficient strategies for water resources management and agricultural decision making in West African countries.

Keywords: Markov chain; rainfall occurrence; wet spell; dry spell; West Africa