Global groundwater environmental flow violations

Groundwater serves as a vital resource to meet growing freshwater demands, particularly for the irrigation sector, driven by population growth and socioeconomic development. Apart from this application, however, groundwater also supports surface water bodies through groundwater discharge. While irrigation demands are important, environmentally safe levels of groundwater discharge must also be maintained.

We applied two methods of calculating groundwater environmental flows using modelled groundwater discharge. The first, Presumptive Standard¹, stipulates that 90% of naturally occurring groundwater discharge should be maintained. The second, Q₉₀², considers the 90^th percent exceedance from a 60-month moving window as the environmental flow. We then calculated violations when the human-impacted groundwater discharge, accounting for sectoral water use, dropped below the environmental flows.

At the river basin scale, we assessed violation frequency and severity. Notably, despite Presumptive Standard violations occurring more frequently than Q₉₀ violations, both methods identified the same spatial trends: basins in intensively irrigated regions experienced the most frequent and severe violations.

Similarly, we assessed the frequency and severity of violations during low-flow periods, isolated using the Q₉₀ as a low-flow threshold, when the role of groundwater to support surface water bodies increases. During these critical instances, the Presumptive Standard and Q₉₀ estimated nearly identical violation schemes.

The groundwater environmental flow violations were further compared to surface water environmental flow violations, following the methodology of Variable Monthly Flow³. This comparison highlighted the importance of including groundwater in environmental flow assessments, as many regions experience high levels of groundwater violations compared to surface water violations.

In addition to frequency and severity, we assessed the timing of violations in select basins with high levels irrigated agriculture. Timing refers to the specific instances when violations occurred. This analysis showed the progression of violation trends over time and emphasized the driving force of groundwater abstractions on environmental flow violations.

Our study shows that including groundwater in assessments of environmental flows is vital, as groundwater is a finite source which plays a crucial role in supporting surface water bodies. When conducting such an assessment, however, the selected groundwater environmental flow threshold may have an effect. If all timesteps are to be considered, the choice of methodology between the Presumptive Standard and Q₉₀ will make a difference. If the focus is on low-flows, however, the choice of methodology will not greatly impact the assessment.

1. Gleeson, T. & Richter, B. How much groundwater can we pump and protect environmental flows through time? Presumptive standards for conjunctive management of aquifers and rivers. River Res Appl 34, 83-92 (2018).

2. de Graaf, I. E. M., Gleeson, T., (Rens) van Beek, L. P. H., Sutanudjaja, E. H. & Bierkens, M. F. P. Environmental flow limits to global groundwater pumping. Nature 574, 90-94 (2019).

3. Pastor, A. V., Ludwig, F., Biemans, H., Hoff, H. & Kabat, P. Accounting for environmental flow requirements in global water assessments. Hydrol Earth Syst Sci 18, 5041-5059 (2014).