Human and natural elements of the hydro-terrestrial system operate across spatial and temporal scales. Integrated human-Earth systems models, such as GCAM, can project future water demand at a coarse, regionally-relevant scale by modeling long-term interactions between multiple sectors under a variety of scenarios, while gridded hydrology models simulate physical processes at a much finer spatial and temporal resolution. Tethys is a spatiotemporal downscaling model built to maintain global dynamics in water use at coarser scales but provide gridded products for use by models with finer resolution requirements. It disaggregates regional and basin scale water demand and consumption to a 1/8th degree gridded resolution at a global scale with monthly timesteps using scenario-consistent proxies, such as crop maps from Demeter (a land-use downscaling model) and population data products consistent with the Shared Socioeconomic Pathways. Tethys is open-source and designed to function as both part of the Global Change Intersectoral Modeling System (GCIMS) and as a standalone tool, enabling workflows that combine global human systems with locally-relevant hydrologic considerations in order to answer important questions about the impacts of future water availability.