We investigate water usage and movement throughout the major reservoirs of Biosphere 2, Oracle AZ, over a period of 2 months. The data analyzed show reservoir turnover times ranging from hours to several years and the existence of three major water sub-cycles. A ‘fast pool’, or 60% of the available water used daily in Biosphere 2, is recycled within a month through the air-handler’s condensation system, as a consequence of plant and soil evapotranspiration; a ‘medium pool’, or 30% of available water, moves through the soil profile as drainage and is recycled within a year; while a ‘slow pool,’ or the remaining 10%, moves through the Biosphere 2 ocean and takes several years to turnover. We develop a simple model of the Biosphere 2 water cycle and discuss its utility in estimating global and single-biome ecosystem processes within this facility.