Comparing Global Violations of Environmentally Critical Groundwater Discharge Thresholds

• Published in: Water resources research Vol. 60; no. 12
• Main Authors: Marinelli, B. P. P., Mohan, C., Gleeson, T., Ludwig, F., Graaf, I. E. M.
• Format: Journal Article
• Language: English
• Published: Washington John Wiley & Sons, Inc 01.12.2024; Wiley
• Subjects: Assessments; Critical flow; Environmental assessment; environmental flows; Environmental health; Environmental impact; Flow index; Frequency dependence; global groundwater; Groundwater; Groundwater discharge; groundwater‐surface water interactions; Humid climates; Methods; Population density; River basins; Stream discharge; Stream flow; Surface water; Surface-groundwater relations; Thresholds; Violations; Water bodies; Water discharge; Water use
• ISSN: 0043-1397; 1944-7973
• DOI: 10.1029/2024WR037519

Groundwater is a crucial resource to support surface water bodies via groundwater discharge. In this study, we applied two methods of estimating global environmentally critical groundwater discharge, defined as the flux of groundwater to streamflow necessary to maintain a healthy environment, from 1960 to 2010: the Presumptive Standard stipulates that a standard proportion of groundwater discharge should be maintained at all timesteps, while the Q* is a low‐flow index that focuses on critical periods. We calculated these critical flow thresholds using simulated natural groundwater discharge, and estimated violations of the thresholds when human‐impacted groundwater discharge dropped too low. Our global assessment of the frequency and severity of violations over all timesteps in our study period showed that the Presumptive Standard estimated more frequent and severe violations than the Q*, but that the spatial patterns were similar for both methods. During low‐flow periods, when the relative importance of groundwater to support streamflow is greatest, both methods estimated similar magnitudes of violation frequency and severity. We further compared our results to a method of estimating environmentally critical streamflow, Variable Monthly Flow, which does not explicitly consider groundwater. From the differences in violation frequency between these groundwater‐centric and surface water‐centric methods, we evaluated the influence of including groundwater contributions to streamflow in environmental flow assessments. Our results show that including groundwater in such assessments is particularly important for regions with high groundwater demands in the drier climates of the world, while it is less important for regions with low groundwater demands and more humid climates. Plain Language Summary We used two global methods of estimating the necessary flow of groundwater to surface water to protect environmental health. One method (Presumptive Standard) is designed to maintain environmental flows over the whole year, while the other method (Q*) focuses on low‐flow periods when groundwater plays a larger role in supporting surface water. We estimated historic violations of these environmentally critical flow thresholds and found that they estimated similar spatial patterns (although at different magnitudes). We then compared the violations to a method of estimating environmentally critical streamflow (Variable Monthly Flow), which does not directly consider groundwater. Here, we found that considering groundwater contributions to surface water affects the estimated environmental impacts of water use, particularly in river basins that are dry, have high amounts of agriculture, and are densely populated. From this study, we conclude that including groundwater in environmental flow assessments is important in regions with significant groundwater use, and that the choice of method should depend on the period of focus. Key Points First global comparison of methods to calculate the environmentally critical contributions of groundwater to streamflow The methods identified similar hotspots of historic violations of environmentally critical groundwater discharge The utility of the methods depends on whether an environmental flow assessment is important for all flow seasons or only low‐flow periods