Rainfall in the upstream drainage area of the Ecuadorian Andean Mountains (EAM) is an important source of water supply in populated areas. Managing water resource projects depend on rainfall-runoff variation. Even though, it is difficult to understand the mechanism that controls rainfall variation because of the influence of several global and local hydrological processes, this type of research is needed to improve the management of water resources. Understanding these processes is complex due to inaccessibility to these remote zones leading to inefficacy in the monitoring of these gauge stations. Furthermore, there are reports that exposed that climatic anomalies are affecting rainfall-runoff processes around the world. These climate changes cause two main problems in urban infrastructure. First, the occurrence of extreme precipitation events increasing the risk of flooding. Second, changes on annual rainfall variation that could lead to water scarcity in the management of water resource projects. This study focuses on improving the understanding of rainfall trends at EAM and its implications in the management of water resources. The results indicate that 71% of extreme precipitation events were registered in the second period of the last twenty years (1995 - 2015) with severe short rainfall events, during ENSO years in the EAM, threatening hydraulic facilities.