Spatial variability of seasonal rainfall onset, cessation, length and rainy days in Rwanda

This study investigates the spatial patterns and variabilities of Seasonal Rainfall Onset Day (OD), Cessation Day (CD), Seasonal Length (SL), and Number of Rainy Days (RD) in Rwanda for the long rain season (LR) of March–April-May (MAM) and the short rain season (SR) of September–October-November–December (SOND). Data used, provided by the Rwanda Meteorology Agency, consisted of a time series of gridded rainfall and temperature from 1983 to 2021. The northern, western, and southwestern regions experience earlier OD than the remaining parts of the country, [mid-February, early March] for LR and [early September, mid-October] for SR. The entire eastern region experiences later OD ([mid-March, end March]) during LR. During SR, the central east and the southeastern regions experience later OD ([mid-October, end November]). During LR and SR, the mean SL and mean RD are highest in the northwestern and southwestern regions and lowest in the central-eastern and southeastern regions. In those regions, the mean SL and mean RD are higher during SR ([81, 116], [49, 74] days) than during LR ([77, 99], [46, 68] days). In the remaining parts of the country, they are lower during SR ([46, 81], [24, 49] days) than during LR ([55, 77], [24, 46] days). The temporal variability (coefficient of variation) is relatively high in different places. During LR, for OD ([21.5, 34] %) over the northwest, central plateau, and eastern regions, for SL ([22.5, 35] %) over the northern and eastern regions, and for RD ([24.5, 32] over the eastern region. During SR, for SL ([23, 31] %) over the southcentral and the central plateau regions, and for RD ([25.5, 38] %) over the northern, western, southern, and central plateau regions. The seasonal length and the number of rainy days are strongly dependent to rainfall intensity, but more dependent in short rains seasons. An investigation done El Nino and the Indian Ocean Dipole indicates that they may have an influence on the studied rainfall characteristics in Rwanda. Results from this study are important, as the country’s economy remains dependent on rain-fed agriculture. They will help farmers, policy and decision-making for appropriate adaptation and mitigation strategies and policies.