Municipal water rationing has become increasingly common in developing urban centres, leading to substantial variation in service levels among residential customers. This paper introduces an open-source tool called Improving Quality of Urban Water Service by Engaging SMS Technology (IQUEST), a geographic decision support system that harnesses crowdsourced water data to enable Ghana Water Company Limited (GWCL) to monitor residential water service quality in Accra, Ghana. This paper presents the conceptual model, general architecture, and user interface, and highlights the potential for other municipal water managers to implement a decision support tool in a resource-constrained setting.