Editing IPCC:AR6/WGII/Chapter-4 (section)

=== 4.2.3 Observed Changes in Streamflow ===

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AR5 ( [[#Jiménez%20Cisneros--2014|Jiménez Cisneros et al., 2014]] ) concluded with ''medium evidence'' and ''high agreement'' that trends in annual streamflow have generally followed observed changes in regional precipitation and temperature since the 1950s. AR6 WGI ( [[#Eyring--2021|Eyring et al., 2021]] ; [[#Gulev--2021|Gulev et al., 2021]] ) (12.4.5) conclude with ''medium confidence'' that anthropogenic climate change has altered local and regional streamflow in various parts of the world, but with no clear signal in the global mean.

Between the 1950s and 2010s, stream flows showed decreasing trends in parts of western and central Africa, eastern Asia, southern Europe, western North America and eastern Australia, and increasing trends in northern Asia, northern Europe, and northern and eastern North America ( [[#Dai--2016|Dai, 2016]] ; [[#Gudmundsson--2017|Gudmundsson et al., 2017]] ; [[#Gudmundsson--2019|Gudmundsson et al., 2019]] ; [[#Li--2020b|Li et al., 2020b]] ; [[#Masseroni--2020|Masseroni et al., 2020]] ). Significant spatial heterogeneity is also found in streamflow changes at the regional scale. Significant declines occurred at 11% of stations and significant increases at 4% of stations, with most decreases occurring in southern Canada ( [[#Bonsal--2019|Bonsal et al., 2019]] ). An increasing trend (1950–2010) is found in the northern region, mainly due to climate warming. Mixed trends are found in other regions.

The spatial differences in annual mean streamflow trends around the world are influenced by climatic factors, particularly changes in precipitation and evaporation ( [[#Zang--2013|Zang and Liu, 2013]] ; [[#Greve--2014|Greve et al., 2014]] ; [[#Hannaford--2015|Hannaford, 2015]] ; [[#Ficklin--2018|Ficklin et al., 2018]] ), as well as by anthropogenic forcing ( [[#Gudmundsson--2016|Gudmundsson et al., 2016]] ; 2017; 2021).. Other factors (e.g., land use change and CO 2 effects on vegetation) dominate in some areas, especially dryland regions ( [[#Berghuijs--2017b|Berghuijs et al., 2017b]] ). Human activities can reduce runoff through water withdrawal and land use changes ( [[#Zaherpour--2018|Zaherpour et al., 2018]] ; [[#Sun--2019a|Sun et al., 2019a]] ; [[#Vicente-Serrano--2019|Vicente-Serrano et al., 2019]] ), and human regulation of streamflows via impounding reservoirs can also play a major role ( [[#Hodgkins--2019|Hodgkins et al., 2019]] ).

Streamflow trends are attributed to varying combinations of climate change and direct human influence through water and land use in different basins worldwide, with conclusions on the relative contribution of climatic and anthropogenic factors sometimes depending on the methodology ( [[#Dey--2017|Dey and Mishra, 2017]] ). Precipitation explains over 80% of the changes in discharge of large rivers from 1950 to 2010 in northern Asia and northern Europe, where the impact of human activities is relatively limited ( [[#Li--2020b|Li et al., 2020b]] ). In northwest Europe, precipitation and evaporation changes explain many observed trends in streamflow ( [[#Vicente-Serrano--2019|Vicente-Serrano et al., 2019]] ). In several polar areas in northern Europe (e.g., Finland), North America (e.g., British Columbia in Canada) and Siberia, many studies reported increased winter streamflow primarily due to climate warming, for instance, more rainfall instead of snowfall and more glacier runoff in the winter period (e.g., [[#Bonsal--2020|Bonsal et al., 2020]] ) ( [[#4.2.2|Section 4.2.2]] ). A similar phenomenon of the earlier snowmelt runoff is also found in North America during 1960–2014 ( [[#Dudley--2017|Dudley et al., 2017]] ). Thus, climate drivers largely explain changes in the average and maximum runoff of predominantly snow-fed rivers ( [[#Yang--2015|Yang et al., 2015]] a; [[#Bring--2016|Bring et al., 2016]] ; [[#Tananaev--2016|Tananaev et al., 2016]] ; [[#Frolova--2017b|Frolova et al., 2017b]] ; [[#Ficklin--2018|Ficklin et al., 2018]] ; [[#Magritsky--2018|Magritsky et al., 2018]] ; [[#Rets--2018|Rets et al., 2018]] ).

In contrast, in southwestern Europe, land cover changes and increased water demands by irrigation are the main drivers of streamflow reduction ( [[#Vicente-Serrano--2019|Vicente-Serrano et al., 2019]] ) ( [[#4.3.1|Section 4.3.1]] ). In addition, the human intervention also contributed to the increase of the winter streamflow due to the release of water in the winter season for hydropower generation in large rivers in the northern regions ( [[#Rawlins--2021|Rawlins et al., 2021]] ). In some regions, the impact of human activities on runoff and streamflow outplays the climate factors, for example, in some typical catchments with area near to or less than 15000 km 2 in China ( [[#Zhai--2017|Zhai and Tao, 2017]] ).

[[#Shi--2019|Shi et al. (2019)]] found that in 40 major basins worldwide, both climatic and direct human impact contribute to observed flow changes to varying degrees. Climate change or variability is the main contributor to changes in basin-scale trends for 75% of rivers, while direct human effects on streamflow dominate for 25%. However, this does not consider attribution of the climate drivers to anthropogenic forcing. Using time series of low, mean and high river flows from 7250 observatories around the world (1971–2010) and global hydrological models (GHMs) driven by Earth System Model (ESM) simulations with and without anthropogenic forcing of climate change, [[#Gudmundsson--2021|Gudmundsson et al. (2021)]] also found direct human influence to have a relatively small impact on global patterns of streamflow trends. [[#Gudmundsson--2021|Gudmundsson et al. (2021)]] further identified anthropogenic climate change as a causal driver of the global pattern of recent trends in mean and extreme river flow (Figure 4.7). Overall, the sign of observed trends and simulations accounting for human influence on the climate system was found to be consistent for decreased mean flows in western and eastern North America, southern Europe, northeast South America and the Indian sub-continent, and increased flows in northern Europe. Similar conclusions were drawn for low and high flows, except for the Indian sub-continent. However, in some regions, the observed trend was opposite to that simulated with anthropogenic climate forcing. Thus, human water and land use alone did not explain the observed pattern of trends.

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[[File:985c07655e1d8baf026a9380d6ef56d8 IPCC_AR6_WGII_Figure_4_007.png]]

'''Figure 4.7 |''' '''Observed changes in river flows and attribution to externally forced climate change.'''

'''(a)''' Percentage changes in flow in individual rivers 1971 to 2010. Black box outlines show climatic regions with at least 80 gauging stations with almost complete daily observations over 1971–2010, using the SREX ( [[#Seneviratne--2012|Seneviratne et al., 2012]] ) regions.

'''(b)''' Left column: observed regional median trends from 1971 to 2010 in SREX regions with at least 80 gauging stations with almost complete daily observations over that period. Middle column: trends simulated by eight global hydrological models driven by four CMIP5 Earth System Models, with human water and land use from 1971 to 2020 and the pre-industrial control climate state. Right column: same as the middle column but with ESM-simulated climates from 1971 to 2010 with both anthropogenic forcings (greenhouse gases, aerosols and land use) and natural external forcings (solar variability and volcanic eruptions). Top row: low flows (annual 10th percentile). Middle row: mean flows. Bottom row: high flows (annual 90th percentile). Reproduced from [[#Gudmundsson--2021|Gudmundsson et al. (2021)]] .

Although there are different observational and simulated runoff and streamflow data sets (e.g., Global Runoff Data Centre, GRDC), it is still challenging to obtain and update long-term river discharge records in several regions, particularly Africa, South and East Asia ( [[#Dai--2016|Dai, 2016]] ). When observed data are scarce, hydrological models are used to detect trends in runoff and streamflow. However, simulations of streamflow can differ between models depending on their structures and parametrisations, contributing to uncertainties for trend detection, especially when considering human intervention (e.g., [[#Caillouet--2017|Caillouet et al., 2017]] ; [[#Hattermann--2017|Hattermann et al., 2017]] ; [[#Smith--2019b|Smith et al., 2019b]] ; [[#Telteu--2021|Telteu et al., 2021]] ).

In summary, both climate change and human activities influence the magnitude and direction of change in runoff and streamflow. There are no clear trends of changing streamflow on the global level. However, trends emerge on a regional level (a general increasing trend in the northern higher latitude region and mixed trend in the rest of the word) ( ''high confidence'' ). Climatic factors contribute to these trends in most basins ( ''high confidence'' ). They are more important than direct human influence in a larger share of major global basins ( ''medium confidence'' ), although direct human influence dominates in some ( ''medium confidence'' ). Overall, anthropogenic climate change is attributed as a driver to the global pattern of change in streamflow ( ''medium confidence'' ).

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