Poor correlation between large-scale environmental flow violations and freshwater biodiversity: implications for water resource management and the freshwater planetary boundary

Abstract

Abstract. The freshwater ecosystems around the world are degrading, such that maintaining environmental flow1 (EF) in river networks is critical to their preservation. The relationship between streamflow alterations (subsequent EF violations2) and the freshwater biodiversity response 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 boundary3 for freshwater. Therefore, this study intends to conduct 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, probability of shifting to a violated state, and probability of staying 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. These findings imply the need for a holistic bio-geo-hydro-physical approach in determining the environmental flows. While our results thus suggest that streamflow and EF may not be the 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.

Review of Quantitative Applications of the Concept of the Water Planetary Boundary at Different Spatial Scales

“…Another critical issue is that few studies have tried to link the water PB across different scales; specifically, the following two issues are not well resolved: (a) the connections among local water PBs, as most studies used local data without considering its connectivity with other regions, and (b) the impact of local water-PB transgressions at large scales, as no previous studies have investigated how local water-PB transgressions could trigger a water-PB transgression at a larger scale or at the global scale. However, Mohan et al (2022) found poor relationship between EWF-biodiversity at large scales, even though it is well established at small scales, implying the need for a holistic approach for water PB research across different scales. Recently, Zipper et al (2020) proposed a novel framework to address the challenge associated with mismatched control variables between the global and local scales, but the above two issues are still not well-resolved in a quantitative way.…”

“…The seasonal variation of water flow rate (e.g., flood flow) and the mismatch between the water supply and human water demand can result in a portion of unused water resources. Second, the control‐response relationship between freshwater biodiversity and EWF has not been well represented in the determination of EWF (Mohan et al., 2022), which further challenges the quantification of water PB based on EWF. Third, there is a large uncertainty in runoff estimates derived through multiple global hydrological models (GHMs), which would propagate into the estimation of EWF.…”

“…However, Mohan et al. (2022) found poor relationship between EWF‐biodiversity at large scales, even though it is well established at small scales, implying the need for a holistic approach for water PB research across different scales. Recently, Zipper et al.…”

“…Third, there is a large uncertainty in runoff estimates derived through multiple global hydrological models (GHMs), which would propagate into the estimation of EWF. Finally, the fixed coefficients derived through the VMF method in water PB expressions are major source of uncertainties, as flow patterns among river basins and relationship between EWF-biodiversity are highly spatially heterogeneous (Mohan et al, 2022). Moreover, considering only a mini- 2022), deviations in the whole freshwater cycle beyond the pre-industrial variability range were assessed based on the blue-green water division, indicating a likely water PB transgression (Porkka et al, 2022).…”

Streamflow Depletion Caused by Groundwater Pumping: Fundamental Research Priorities for Management‐Relevant Science

“…Impacts of streamflow depletion on aquatic ecosystems may be through direct changes in streamflow or indirectly through changes in water quality or temperature caused by depletion, as in the case of salmonids which are highly dependent on cool groundwater inflows (Larsen & Woelfle-Erskine, 2018) and are a major driver of policy and management decisions in many settings (Lackey, 1999). While a variety of approaches to estimate environmental flow needs for aquatic ecosystems have been developed and proposed, these are not well-correlated with freshwater biodiversity (Mohan et al, 2022), suggesting that more information beyond just changes in water quantity is needed to link streamflow depletion to adverse ecological outcomes (Lapides et al, 2022;Yarnell et al, 2020). While recent studies have identified low-flow signatures as most sensitive to streamflow depletion across diverse hydrologic regions (Lapides et al, 2023), links between hydrologic change and ecological outcomes have predominantly been local (Liu et al, 2020;Perkin et al, 2017).…”