The developing world is rapidly urbanizing. Oneof the challenges associated with this growth will be to supply water to growing cities of the developing world. Traditional planning tools fare poorly over 30–50 year time horizons because these systems are changing so rapidly. Modelsthat hold land use, economic patterns, governance systems ortechnology static over a long planning horizon could result ininaccurate predictions leading to sub-optimal or paradoxicaloutcomes. Most models fail to account for adaptive responsesby humans that in turn influence water resource availability, resulting in coevolution of the human–water system. Is aparticular trajectory inevitable given a city’s natural resourceendowment, is the trajectory purely driven by policy or arethere tipping points in the evolution of a city’s growth thatshift it from one trajectory onto another?Socio-hydrology has been defined as a new science ofwater and people that will explicitly account for such bidirectional feedbacks. However, a particular challenge in incorporating such feedbacks is imagining technological, social and political futures that could fundamentally alter future water demand, allocation and use. This paper offers analternative approach – the use of counterfactual trajectories– that allows policy insights to be gleaned without having topredict social futures. The approach allows us to “reimaginethe past”; to observe how outcomes would differ if differentdecisions had been made.The paper presents a “socio-hydrological” model that simulates the feedbacks between the human, engineered and hydrological systems in Chennai, India over a 40-year period.The model offers several interesting insights. First, the studydemonstrates that urban household water security goes beyond piped water supply. When piped supply fails, users turnto their own wells. If the wells dry up, consumers purchaseexpensive tanker water or curtail water use and thus becomewater insecure. Second, unsurprisingly, different initial conditions result in different trajectories. But initial advantagesin piped infrastructure are eroded if the utility is unable toexpand the piped system to keep up with growth. Both infrastructure and sound management decisions are necessaryto ensure household water security although the impacts ofmismanagement may not manifest until much later whenthe population has grown and a multi-year drought strikes.Third, natural resource endowments can limit the benefitsof good policy and infrastructure. Cities can boost rechargethrough artificial recharge schemes. However, cities underlain by productive aquifers can better rely on groundwater asa buffer against drought, compared to cities with unproductive aquifers.