Environmental flow envelopes: quantifying global, ecosystem–threatening streamflow alterations

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 riverine ecosystems. However, at the global scale, the assessment of EFs is associated with significant uncertainty. Here, we present a novel method to determine EFs by Environmental Flow Envelopes (EFE), which is an envelope of variability bounded by discharge limits within which riverine ecosystems are not seriously compromised. The EFE is defined globally in approximately 4,400 sub–basins at monthly time resolution, considering also the methodological uncertainties related with global EF studies. In addition to a lower bound of discharge, the EFE introduces an upper bound of discharge, identifying areas where streamflow has increased substantially. Further, instead of only showing whether EFs are violated, as commonly done, we quantify, for the first time, the frequency, severity, and trends of EFE violations, which can be considered as potential threats to riverine ecosystems. We use pre–industrial (1801–1860) quasi-natural discharge and a suite of hydrological EFR methods and global hydrological models to estimate EFE, applying data from the ISIMIP 2b ensemble. We then compare the EFEs to recent past (1976–2005) discharge to assess the violations of the EFE. We found that the EFE violations most commonly manifest themselves by insufficient streamflow during the low flow season, with less violations during intermediate flow season, and only few violations during high flow season. These violations are widespread: discharge in half of the sub–basins of the world has violated the EFE during more than 5 % of the months between 1976 and 2005. The trends in EFE violations have mainly been increasing during the past decades and will likely remain problematic with projected increases in anthropogenic water use and hydroclimatic changes. Indications of excessive streamflow through EFE upper bound violations are relatively scarce and spatially distributed, although signs of increasing trends can be identified and potentially attributed to climate change. While the EFE provides a quick and globally robust way of determining environmental flow allocations at the sub–basin scale, local fine–tuning is necessary for practical applications and further research on the coupling between quantitative discharge and riverine ecosystem responses is required.

Towards the harmonization of global environmental flow estimates: comparing the Global Environmental Flow Information System (GEFIS) with country data

“…To avoid this type of model-specific bias in future developments, we recommend implementation of an ensemble approach whereby the mean (or median) of MAR predictions from multiple global hydrological models is used in conjunction with estimates of prediction uncertainties due to inter-model differences (Sood and Smakhtin 2015;Döll et al 2016). This approach has already been implemented for assessing global e-flows by Hogeboom et al (2020) and Virkki et al (2021). Alternatively, spatially explicit assessments of model errors may identify which hydrological model performs best for each region and river type.…”

Mapping hydrologic alteration and ecological consequences in stream reaches of the conterminous United States

“…Much of the exploitation of lotic systems is directly related to the manipulation of natural streaflow variability. For instance, roughly 50% of rivers and streams across the world are hydrologically altered from their natural state 5,6 , with as much as 80% of streams in Canada 7 and 80% of streams in the conterminous US 8 displaying signs of alteration in streamflow regimes. By 2050, climate change is expected to have altered streamflow regimes in as much as 80% of the global terrestrial land surface aside from that already altered from human infrastructures 9,10 -only exacerbating decades of declines in freshwater fauna populations 11 .…”

Analysing and evaluating environmental flows through hydrological methods in the regulated Indus River Basin

“…A multitude of frameworks and approaches with varying levels of complexity, time requirements and data requirement have been established to investigate the EFRs of rivers. These approaches are discussed in detail and used by various researchers (Abdi & Yasi, 2015; Acreman & Dunbar, 2004; Linnansaari et al, 2013; Theodoropoulos et al, 2018; Virkki et al, 2021; Zeiringer et al, 2018). Among the options for assessing EFRs of rivers (Tharme, 2003; Zeiringer et al, 2018), more than 200 techniques are currently available worldwide, broadly grouped into hydrological methods, habitat simulation methods, hydraulic rating methods and holistic solutions.…”

“…For instance, a 1% shift denotes a flow that was equalled or exceeded 80% of the time in the reference FDC now being equalled or exceeded 70% of the time in class A, with corresponding changes in the flow at other probabilities.The last step involves the generation of flow time series for each EMC by spatially interpolating monthly EF time series using available data on the reference FDC, environmental FDC and reference monthly time series. Figure3displays the resulting FDCs for each EMC at the Tarbela Dam on the river Indus, offering a more comprehensive understanding of the shifting procedure.Environmental flow envelopes (EFEs) coupled with different EFR methodsIn this study, the method developed byVirkki et al (2021) was employed to determine the minimum and maximum flow requirements for a river or stream to maintain its ecological integrity and to explore the violation ratio in the IRB. Environmental flow envelopes (EFEs) are a boundary of discharge limits within which river ecosystems are not disturbed.…”