Global Assessment of Current and Future Groundwater Stress With a Focus on Transboundary Aquifers

We quantified groundwater stress worldwide by applying the global water resources and water use model WaterGAP 2.2b (Water - Global Assessment and Prognosis) for current conditions (1981-2010) as well as for the 2050s under the worst-case greenhouse gas emissions scenario RCP8.5. To improve global-scale groundwater stress assessments, we suggest three new water quantity-related groundwater stress indicators as well as a new way for communicating projected future groundwater stress at the grid-cell level (similar to 55 x 55 km) and for larger spatial units such as transboundary aquifers (>20,000 km(2)). The new indicators encompass the ratio of net abstractions from groundwater to groundwater recharge, human-induced changes in groundwater discharge, and human-induced groundwater depletion. We compare them to four conventional indicators used in the Transboundary Waters Assessment Programme and show how they can add value to global-scale studies or are even more suitable for indicating groundwater stress. We assess potentials and limitations of all indicators by addressing their level of process representation, data requirements, uncertainty, and the underlying different concepts of sustainable groundwater use. To support adaptation to climate change, we recommend showing both the ensemble mean and the worst-case scenario of future groundwater stress that we derived from five climate and two irrigation scenarios. For characterizing groundwater stress in spatial units such as transboundary aquifers, areal fractions where a selected indicator threshold is exceeded should be considered. Finally, hot spots of future groundwater stress should be identified by combining relative changes from current conditions with absolute values of future groundwater stress.