The transformation of natural savannah, forests and grasslands to densely populated urban centers radically transforms how rainwater is digested by the landscape. Native vegetation and undisturbed soils allow infiltration of nearly all precipitation; roads, roofs and other compacted surfaces respond in an opposite manner. Rainwater that no longer infiltrates into the ground must go somewhere, usually collecting in roads and low areas as it finds the most direct path to a river or ocean.

Most large cities around the globe have expensive underground storm drainage systems to accommodate this excess water. Rapidly urbanizing areas in central Africa typically do not. It is simply too expensive for local governments to afford at a meaningful scale. Unless wealthy countries are going to swoop in and solve this problem with billions of dollars of storm drainage catchments and pipelines, which they are not, the solution is to address runoff one roof at a time.