Vegetation.

It is true that climate determines vegetation but it is also true that vegetation, in turn, influences climate.  Swamps and luxuriant vegetation contribute to rainfall formation through evapo-transpiration.  A lot of moisture is released to the atmosphere which later condenses into rainfall.   The existence   of mangrove  vegetation  at  the  coast  and  papyrus swamps scattered forests in Central and Western Uganda have partially accounted for the  relatively high rainfall amounts in the respective places.