The freshwater ecosystems around the world are degrading, such that maintaining environmental flow (EF) in river networks is critical to their preservation. The relationship between streamflow alterations and, respectively, EF violations, and freshwater biodiversity is well established at the scale of stream reaches or small basins (~<100 km²). However, it is unclear if this relationship is robust at larger scales even though there are large-scale initiatives to legalize the EF requirement. Moreover, EFs have been used in assessing a planetary boundary for freshwater. Therefore, this study intends to carry out an exploratory evaluation of the relationship between EF violation and freshwater biodiversity at globally aggregated scales and for freshwater ecoregions. Four EF violation indices (severity, frequency, the probability to shift to violated state, and probability to stay violated) and seven independent freshwater biodiversity indicators (calculated from observed biota data) were used for correlation analysis. No statistically significant negative relationship between EF violation and freshwater biodiversity was found at global or ecoregion scales. While our results thus suggest that streamflow and EF may not be an only determinant of freshwater biodiversity at large scales, they do not preclude the existence of relationships at smaller scales or with more holistic EF methods (e.g., including water temperature, water quality, intermittency, connectivity etc.) or with other biodiversity data or metrics.

Increasing recognition of the importance of ecosystem services in water resources management has accelerated the development and applications of environmental flows requirements for lotic ecosystems which are often dependent on groundwater. However, most environmental flows management focuses on water infrastructure, like dams or diversions, without explicitly taking groundwater into account and ignoring the importance of groundwaters’ contribution to environmental flows. Here, we introduce two methods for estimating groundwater contribution to environmental flows: 1) a groundwater-centric method, which proposes that high levels of ecological protection are maintained if 90% of groundwater discharge is preserved and 2) a surface water-centric method, which quantifies groundwater’s contribution to environmental flows from streamflow using region-specific streamflow sensitivity metrics and local environmental flows policies. The two methods are tested in British Columbia, Canada, which has a diverse, complex, and highly coupled groundwater-surface water systems. The two methods gave comparable results in different hydrogeoclimatic settings. Though the two methods are demonstrated using British Columbia as a case study, this framework can be implemented across different spatial and temporal scales for different regions and globally in data-scarce, hydrologically complex landscapes. Application of these methods can aid in a robust and holistic assessment of environmental flows, taking into account the often missing groundwater component. Keywords: Groundwater, Environmental flows, British Columbia, Surface water centric method, Groundwater centric method