More than 2 billion people in tropical and subtropical regions have limited access to clean water and sanitation services, and the fate and transport of pathogens in surface waters are known to be sensitive to climate change in these settings. This project addresses critical questions in waterborne disease transmission in a changing and variable climate: How can we make reliable predictions about the effects of future environmental change on water quality and specifically on waterborne pathogens? How can we direct social-economic decision-making to counteract potential increased water quality problems under future climate conditions? This project will develop and test methods for conceptualizing, quantifying and predicting the effects of multiple climatological drivers on water quantity and pathogen fate and transport using well studied test sites in northern Ecuador and western China. The team will collaborate with partners at the Universidad San Francisco de Quito and Sichuan University, leverage current NIH/NIAID projects in the regions, host workshops at each site, and facilitate interaction between US students and in-country collaborators. The objective of this project is to systematically investigate the impact of climate variability and change on water quantity and quality in tropical and subtropical zones. The central hypotheses are that 1) climate change can affect water quality by altering the dynamics of surface water flows and the fate and transport of pathogens in the water column, and that 2) both management decisions and forms of social organization can limit or exacerbate climate's influence. This project will develop and test methods for conceptualizing, quantifying and predicting the effects of multiple climatological drivers on water quantity and quality, using as test sites well-studied regions in northern Ecuador and western China where extensive data are available. The project activities will develop empirical and theoretical foundations for the modeling and prediction of coupled hydrological-microbiological systems, and have the potential to considerably improve understanding of how environmental change can alter microbiological water quality at multiple scales. The effects of changing climate on key pathways that determine water quality will be explored, including climate-mediated changes in hydrological dynamics; climate-mediated effects on conditions within the water column (e.g., micro-environmental temperature); and the effects of management decisions (e.g., agricultural practices, sanitation provision) and social processes (e.g., social stratification, migration), which can limit or exacerbate climate-related changes in water quality.