A review of groundwater stress indices for a quantitative assessment of the resource

Groundwater represents about 30% of the global fresh water and is the primary source of drinking water for 3 billion people. In the context of climate change, reliance on this resource is increasing, but the altered precipitation patterns, and consequently aquifer recharge, increase groundwater supply vulnerability. In this framework, monitoring and quantifying groundwater stress is an urgent need. This review aims to provide a critical overview of the approaches proposed in the literature to assess the sustainability of groundwater exploitation from a quantitative perspective. Three main approaches are identified: those based on water flux balances, which define overexploitation as an excess of consumption with respect to the available renewable resource; those based on the evaluation of stored volumes, which identify groundwater stress as a progressive depletion; and hybrid approaches, which combine the previous two while incorporating economic, social and regulatory constraints. The analysis highlights a substantial lack of standardization in the definition of core variables (e.g., recharge) and a limited comparability among the results of stress indices, inhibiting their application in the regulatory framework. By identifying these limitations, this review aims to promote the development of more transparent interdisciplinary indicators capable of informing groundwater management in a context of growing stress and uncertainty.