Mexico City is situated on an elevated valley surrounded by a ring of mountains within an endhoreic basin. The urban area continues to steal land from an ancient lake and the surrounding foothills. Two of its most significant environmental issues, are the simultaneous lack and excess of water -the paradox of an increasingly thirsty metropolis, developing within a flood plain. Major infrastructure has been built over time to extract seasonal rainfall and sewage out of the valley, and even larger

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... infrastructure has been introduced to import water from adjacent basins to complete the insufficient and overexploited aquifer extraction. However, the city is still unable to satisfy the needs of its over 20 million population. Rainwater falling on the surrounding hills infiltrates the soil, to refill the aquifers, and runs through streams that go into the sewage system when entering the urban area. Informal-settlements grow on these foothills, occupying former co-opted lands. They have gradually consolidated, progressively achieving basic urban infrastructure and legal ownership of the land, but they still face acute water scarcity and have a negative influence on water running downstream. This project conducts a study of one of these communities. It analyzes its water balance and its physical and socio-economical conditions in order to produce alternative solutions. First, one where water needs can be provided through rainwater harvesting and water reuse: Water Zero. A second one, where the negative influence on water running out reverts to become a clean water source for downstream communities: Water +. A non-governmental organization (Isla Urbana) working within the community has developed and installed appropriate rainwater harvesting systems. Through the evaluation of its vision, methods and performance, this Urban Water 161 project aims to produce a community action project methodology that can provoke the community's evolution towards water sustainability.