Editing IPCC:AR6/WGI/Chapter-2 (section)

===== 2.3.1.3.6 Streamflow =====

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The AR5 concluded that there was ''low confidence'' in a positive trend in global river discharge during the <sup></sup> 20th century. It noted that many of the largest rivers with long term streamflow records have been impacted by non-climatic human influences such as dam construction or land-use change.

River discharge is monitored widely, although gaps remain at a subcontinental scale over central Asia and Africa ( [[#Wei--2020|Wei et al., 2020]] ). Substantial recent efforts have been made to generate new global streamflow datasets, consolidating observations from many stream gauges to create streamflow indices ( [[#Do--2018|Do et al., 2018]] ; [[#Gudmundsson--2018|Gudmundsson et al., 2018]] ) and gridded products using neural networks ( [[#Barbarossa--2018|Barbarossa et al., 2018]] ) or combinations between observations and reanalyses ( [[#Suzuki--2018|Suzuki et al., 2018]] ; [[#Ghiggi--2019|Ghiggi et al., 2019]] ).

Human intervention on river discharge linked to increases in evapotranspiration and some reduction of intra-annual streamflow variability ( [[#Jaramillo--2015|Jaramillo and Destouni, 2015]] ; [[#Chai--2020|Chai et al., 2020]] ) might affect the detection of trends in extreme daily streamflow events ( [[#Do--2017|Do et al., 2017]] ; [[#Gudmundsson--2019|Gudmundsson et al., 2019]] ). However, these activities have a minor impact on annual streamflow compared to climate variations ( [[#Dai--2009|Dai et al., 2009]] ; [[#Alkama--2013|Alkama et al., 2013]] ). Available global studies post-1950 generally concur that there have been more rivers experiencing decreases than increases in runoff ( [[#Do--2017|Do et al., 2017]] ; [[#Su--2018|Su et al., 2018]] ; [[#Gudmundsson--2019|Gudmundsson et al., 2019]] ; X. [[#Shi--2019|]] [[#Shi--2019|Shi et al., 2019]] ). Most of the rivers have not experienced statistically significant changes in streamflow, and when globally aggregated there is no significant change ( [[#Dai--2017|Dai and Zhao, 2017]] ). Global streamflow variability is strongly modulated by ENSO and PDV, with below-normal global streamflow as a response to El Niño events and vice-versa during La Niña episodes ( [[#Dai--2016|Dai, 2016]] ; [[#Liang--2016|Liang et al., 2016]] ; [[#Kim--2019|Kim, 2019]] ). The response of streamflow to changes in precipitation associated with ENSO and PDV has heterogeneous regional patterns at subcontinental scales (Section 8.3.2.9.1). No significant trends are found for reanalysis-based discharge estimates over 1993 to 2015 ( [[#Chandanpurkar--2017|Chandanpurkar et al., 2017]] ). Uncertainties in global streamflow trends arise predominantly from changes in instrumentation, gauge restoration, recalibration of rating curves, flow regulation or channel engineering ( [[#Alkama--2011|Alkama et al., 2011]] ; [[#Gudmundsson--2018|Gudmundsson et al., 2018]] ; [[#Ghiggi--2019|Ghiggi et al., 2019]] ).

In summary, the sign of global streamflow trends remains uncertain, with slightly more globally gauged rivers experiencing significantly decreasing flows than significantly increasing flows since the 1950s ( ''low confidence'' ).

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