Reply on RC1

Vili Virkki

doi:10.5194/hess-2021-260-ac1

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.

Highlights

The human exploitation of rivers is a sensitive balance between benefits gained from water use and adverse Earth system 50 responses
Our findings show that Environmental Flow Envelopes (EFE) violations are widespread around the world, concentrating on lower bound violations in the arid 240 and dry temperate climate zones (Fig. 2)
The median discharge over General Circulation Models (GCMs) violates the EFE in 49.8% of the total 3,860 sub–basins during more than 5.0% of the total 1,440 months of record across all

Summary

Introduction

The human exploitation of rivers is a sensitive balance between benefits gained from water use and adverse Earth system 50 responses. Regardless and because of their importance, rivers are under increasing anthropogenic pressure due to direct human actions, such as flow regulation and damming, excessive water withdrawals, pollution, and land use change (Best, 2019; Kummu et al, 2016). One of the root causes behind this degradation is the anthropogenic alteration of the natural flow regime of a river, i.e. the 65 magnitude, frequency, duration, timing, and rate of change in flow (Poff et al, 1997). These natural streamflow dynamics have already changed in major rivers across the globe (Grill et al, 2015). Due to its profound effect on the physical habitat of the river, the flow regime is one of the key factors in defining the well–being of riverine ecosystems and maintaining their ecological status (Bunn and Arthington, 2002; Poff and Zimmerman, 2010)

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