Calculating groundwater stress and climate change-induced vulnerability of karst aquifers on a global scale

Approximately 10% of the global population rely on groundwater from karst aquifers. Due to complex karst structures, these aquifers have high infiltration capacities and hydraulic conductivities, which makes them vulnerable to pollution and, as prediction and management are complicated, overexploitation. As populations are growing and demand rises, we assess the current level of groundwater stress in karst aquifers with Mediterranean climates and their vulnerability (defined as the change in groundwater stress) to expected changes in temperature and precipitation on the global scale.

Our approach is based on a Groundwater Stress Index (GSI), which is calculated for 356 karst aquifers (as identified in the World Karst Aquifer Map) that have some of their area located in Mediterranean climate zones (Csa, Csb, and Csc after Köppen/Geiger). GSI are calculated from seven indicators: groundwater recharge, storage, and abstractions, surface runoff, climatic water balance, water-intensity of crops, and groundwater-dependent ecosystems. Each indicator is spatially and temporally averaged to describe a recent trend on aquifer level, resulting in one complex attribute table for the 356 aquifers. GSI is calculated as the average of the normalized indicators for each aquifer, ranging from 0 (no water stress) to 1 (extreme water stress).

Aquifers are then grouped based on similarities in two classification parameters – degree of karstification (P1) and land cover (P2). Comparison of aquifers with similar classification parameters allows to focus more directly on the relationship between groundwater stress and climate, disregarding relatively constant influences. For each group (e.g., well-developed karst, primarily agriculturally used), we plot calculated GSI values with current temperature and precipitation data. By investigating four Shared Socioeconomic Pathways (SSPs) until 2100, we identify aquifers that mimic future climate conditions for others with similar P1 and P2. We then measure the difference in groundwater stress accompanied by altered climatic factors. This change is interpreted as vulnerability to climate change.

This approach, which relies on present-day observed conditions, allows us to predict the effect of a changing climate without the need to develop a complex numerical model, which requires large amounts of data and functional understanding of aquifer behavior. While analysis is currently ongoing, we expect both groundwater stress and vulnerabilities to be high. Predicted climate zone shifts by Beck et al. (2018) indicate that, out of 356 karst aquifers with Mediterranean climates, 52 could move to more extreme arid climate zones by 2100.

Results will be visualized in the form of vulnerability maps that may serve as an “early-warning system”. For particularly threatened aquifers, we will derive recommendations for more sustainable management by suggesting strategies to lower groundwater stress. This is done by taking a closer look at the aquifer’s indicator values and identifying factors that currently contribute the most to groundwater stress.