This dataset presents nationally consistent simulations of available precipitation (rainfall + snowmelt) for Great Britain. These include simulations driven by observational data covering January 1961 – December 2018 and simulations driven by Regional Climate Model (RCM) projections covering December 1980 - November 2080. Both are provided as gridded data at 1km resolution. The observed input data was HadUK-Grid 1km daily precipitation and temperature. The RCM input data was a 12-member perturbed parameter ensemble from the UK Climate Projections 2018 (UKCP18), provided at a ‘Regional’ 12km resolution aligned with the GB national grid. A simple snowmelt module was used to transform these time-series of precipitation and temperature into available precipitation. This data was developed for, and used within, the Enhanced Future Flows and Groundwater (eFLaG) project (Hannaford et al., 2022). The eFLaG project was established through a partnership project funded by the Met Office led component of the Strategic Priorities Fund Climate Resilience Programme under contract P107493 (CR19_4 UK Climate Resilience).