Globally widespread and increasing violations of environmental flow envelopes

Abstract

Abstract. Human actions and climate change have drastically altered river flows across the world, resulting in adverse effects on riverine ecosystems. Environmental flows (EFs) have emerged as a prominent tool for safeguarding the riverine ecosystems, but at the global scale, the assessment of EFs is associated with high uncertainty related to the hydrological data and EF methods employed. Here, we present a novel, in-depth global EF assessment using environmental flow envelopes (EFEs). Sub-basin-specific EFEs are determined for approximately 4400 sub-basins at a monthly time resolution, and their derivation considers the methodological uncertainties related to global-scale EF studies. In addition to a lower bound of discharge based on existing EF methods, we introduce an upper bound of discharge in the EFE. This upper bound enables areas to be identified where streamflow has substantially increased above natural levels. Further, instead of only showing whether EFs are violated over a time period, we quantify, for the first time, the frequency, severity, and trends of EFE violations during the recent historical period. Discharge was derived from global hydrological model outputs from the ISIMIP 2b ensemble. We use pre-industrial (1801–1860) quasi-natural discharge together with a suite of hydrological EF methods to estimate the EFEs. We then compare the EFEs with recent historical (1976–2005) discharge to assess the violations of the EFE. These violations most commonly manifest as insufficient streamflow during the low-flow season, with fewer violations during the intermediate-flow season, and only a few violations during the high-flow season. The EFE violations are widespread and occur in half of the sub-basins of the world during more than 5 % of the months between 1976 and 2005, which is double compared with the pre-industrial period. The trends in EFE violations have mainly been increasing, which will likely continue in the future with the projected hydroclimatic changes and increases in anthropogenic water use. Indications of increased upper extreme streamflow through EFE upper bound violations are relatively scarce and dispersed. Although local fine-tuning is necessary for practical applications, and further research on the coupling between quantitative discharge and riverine ecosystem responses at the global scale is required, the EFEs provide a quick and globally robust way of determining environmental flow allocations at the sub-basin scale to inform global research and policies on water resources management.

Understanding the transgression of global and regional freshwater planetary boundaries

“…In the study, we show that EF deficit was below 5% of total discharge, about 2200 km 3 yr −1 , which is below the PB value of Steffen but relatively close. In terms of regional PB transgression we show similar patterns as in [ 11 , 18 ], except that in our study we also show the frequency of months with climate and anthropogenic deficit. In our study, we also provide the EF deficit globally and per river basin to represent water PB boundaries.…”

“…This is why we have zoomed to 23 major river basins. In order to look at other potential EF deficit and/or regional PB transgression, other studies have done similar analyses such as in [ 18 ].…”

“…To maintain rivers in ‘acceptable’ ecological conditions, environmental flow requirements (EFRs) have been defined as ‘the quantity, timing and quality of freshwater flows and levels necessary to sustain aquatic ecosystems which, in turn, support human cultures, economies, sustainable livelihoods and well-being’ [ 13 , 14 ]. Recent studies have assessed the transgressions of freshwater planetary boundaries [ 15 ,– 18 ], but still calculating the deficit over an averaged period without thorough evaluation. Indeed and to our knowledge, there is no study that looked at EF deficit (also defined here as water planetary boundary (PB) transgression) at such spatial (country, river basin and global) and time (monthly including the frequency (number of months and duration)) resolutions.…”

Global water cycle shifts far beyond pre-industrial conditions – planetary boundary for freshwater change transgressed

“…Various human pressuressuch as water and land use, climate change, and water infrastructurenow considerably modify the quantity and timing of freshwater flows, with crucial implications for Earth's climate and ecosystems (1). Remarkable signals of water cycle changes include widespread and severe river flow regime alterations (2), intensification (3) and homogenization (4) of the global water cycle, and increases in the severity, frequency, and duration of extreme events (5). Despite the alarming evidence of freshwater change, a comprehensive assessment of humanity's aggregate impact on the freshwater cycle and its life-supporting functions is still missing.…”

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

“…These competitions have led to an increasing trend for EF violation (insufficient streamflow compared to the recommended EF requirement; see Sect. 2.1 for more details) in the past decade in terms of both severity and frequency (Virkki et al, 2022). This wake-up call has led to several international and national efforts to legalize EF requirements through large-scale EF management schemes (Arthington and Pusey, 2003;Richter et al, 1997Richter et al, , 2003.…”