As in many other cities, urbanization coupled with population growth worsens the water supply problem of Addis Ababa, Ethiopia, with a water supply deficit of 41% in 2016. To investigate the potential contribution of rooftop rainwater harvesting (RWH) from large public institutions, 320 such institutions were selected and grouped into 11 categories, from which 25-30% representative 588 rooftops were digitalized and the potential RWH volume computed based on a ten-year rainfall dataset. When comparing the resulting RWH potential with the water consumption, up to 2.3% of the annual, potable water supply can be provided. If reused only within one's own institution, the self-sufficiency varies from 0.9 to 649%. Non-uniform rainfall patterns add uncertainty to these numbers, since the size of the storage tank becomes critical for coverage in the dry season from October to May. Despite the low replacement potential at the city level, RWH from large institutions will enable a significant volume of potable water to be transferred to localities critically suffering from water shortage. Further, large institutions may demonstrate how RWH can be practiced, thus acting as a frontrunner for the dissemination of RWH to other types of rooftops. To narrow the water supply gap, considering rooftop RWH as an alternative water supply source is recommended. However, the present study assumed that financial constraints to install large sized storage tanks are considered as a possible challenge. Thus, future research is needed to investigate the cost-benefit balance along with the invention of a cheap storage tank as they may affect the potential contribution of RWH from rooftops.
Keywords: large public institutions; rooftop rainwater harvesting; water deficit.