Groundwater was the main source for public supply in the Algarve, in the south of Portugal, until the end of the 20th century, after which it was replaced by surface water supplied by large reservoirs. The large drought that hit the region in 2004 and 2005 revealed the problems related to a water supply strategy based on a single source and stressed the need of an integrated water resource management (IWRM) scheme. Following a qualitative and quantitative screening of groundwater resources for integration into the public water supply system of the region, current work aims to address the regional quantification of groundwater availability in those aquifers that are available for public water supply. For this purpose the strengths and flaws of simple analytical water balance formulae to calculate annual aquifer storage volumes will be discussed. These calculations are validated with the help of a numerical finite-element groundwater flow model, and it is shown that their application is suitable for calculating available storage volumes in regions where groundwater models do not exist, as long as a conceptual hydrogeological model is developed.