Editing IPCC:AR6/WGI/Chapter-8 (section)

==== 8.3.1.5 Runoff, Streamflow and Flooding ====

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The AR5 reported ''low confidence'' in the assessment of trends in global river discharge during the 20th century. This is because many streamflow observations have been impacted by land use and dam construction, and the largest river basins worldwide differ in many characteristics, including geography and morphology. In regions with seasonal snow storage, AR5 WGII assessed that there is ''robust evidence'' and ''high agreement'' that warming has led to earlier spring discharge maxima and ''robust evidence'' of earlier breakup of Arctic river ice, as well as indications that warming has led to increased winter flows and decreased summer flows where streamflows are lower and that the observed increases in extreme precipitation led to greater probability of flooding at regional scales with ''medium confidence'' . The SROCC found ''robust evidence'' and ''high agreement'' that discharge due to melting glaciers has already reached its maximum point and has begun declining with smaller glaciers, but only ''low confidence'' that anthropogenic climate change has already affected the frequency and magnitude of floods at the global scale.

Significant trends in streamflow and continental runoff were observed in 55 out of 200 large river basins during 1948 – 2012, with an even distribution of increasing and decreasing trends ( [[IPCC:Wg1:Chapter:Chapter-2#2.3.1.3.6|Section 2.3.1.3.6]] ; [[#Dai--2016|Dai, 2016]] ). A global detection and attribution study shows that the simulation of spatially heterogeneous historical trends in streamflow is consistent with observed trends only if anthropogenic forcings are considered ( [[#Gudmundsson--2019|Gudmundsson et al., 2019]] ). [[IPCC:Wg1:Chapter:Chapter-3#3.3.2.4|Section 3.3.2.4]] assesses with ''medium confidence'' that anthropogenic climate change has altered regional and local streamflows, although a significant trend has not been observed in the global average (Sections 2.3.1.3.6 and 3.3.2.3). Multiple human-induced and natural drivers have been shown to play an important but variable role in observed regional trends of streamflow for several different areas (Fenta et al. , 2017; Ficklin et al. , 2018; Glas et al. , 2019; Vicente-Serrano et al. , 2019) . For instance, decreasing runoff during the dry season has been observed over the Peruvian Amazon since the 1980s ( [[#Lavado--2013|Lavado et al., 2013]] ; [[#Ronchail--2018|Ronchail et al., 2018]] ). Up to 30–50% of the recent multi-decadal decline in streamflow across the Colorado River Basin can be attributed to anthropogenic warming and its impacts on snow and evapotranspiration ( [[#Woodhouse--2016|Woodhouse et al., 2016]] ; [[#McCabe--2017|McCabe et al., 2017]] ; [[#Udall--2017|Udall and Overpeck, 2017]] ; [[#Xiao--2018|Xiao et al., 2018]] ; [[#Milly--2020|Milly and Dunne, 2020]] ). In the Upper Missouri River basin, [[#Martin--2020|Martin et al. (2020)]] found that warming temperatures have contributed to streamflow reductions since at least the late 20th century. Cold regions in the NH have experienced an earlier occurrence of snowmelt floods, an overall increase in water availability and streamflow during winter, and a decrease in water availability and streamflow during the warm season ( [[#Aygün--2019|Aygün et al., 2019]] ).

Some studies have suggested that dam construction and water withdrawals can be the dominant drivers in observed trends in streamflow amount ( [[#Wada--2013|Wada et al., 2013]] ). Regionally, land-use and land cover changes have been identified as important factors for streamflow (H. [[#Chen--2020|]] [[#Chen--2020|Chen et al., 2020]] ). The impact of surface dimming from aerosol emissions on evaporation was identified as a discernible influence in NH streamflows ( [[#Gedney--2014|Gedney et al., 2014]] ). While changes in annual mean streamflow present a complicated picture, recent studies of changes in the timing of streamflow in snow-influenced basins continue to support a prominent influence from warming ( [[#Kang--2016|Kang et al., 2016]] ; [[#Dudley--2017|Dudley et al., 2017]] ; [[#Kam--2018|Kam et al., 2018]] ). Global land runoff variations correlate significantly with ENSO variability ( [[#Miralles--2014b|Miralles et al., 2014b]] ; [[#Schubert--2016|Schubert et al., 2016]] ).

Observed changes in flooding are assessed in [[IPCC:Wg1:Chapter:Chapter-11#11.5.2|Section 11.5.2]] and are summarized as follows. For changes in the magnitude of peak flow, recent studies show strong spatial heterogeneity in the sign, size and significance of trends. For changes in timing of peak flows, recent studies further support observed changes in snowmelt-driven rivers. Observed changes in runoff and flood magnitude cannot be explained by precipitation changes alone given the possible season- and region-dependent decreases in antecedent soil moisture and snowmelt, which can partly offset the increase in precipitation intensity ( [[#Sharma--2018|Sharma et al., 2018]] ), or the expected effect of urbanization and deforestation which can, on the contrary, amplify the runoff response ( [[#Chen--2017|Chen et al., 2017]] ; [[#Abbott--2019|Abbott et al., 2019]] ; [[#Cavalcante--2019|Cavalcante et al., 2019]] ). Simulations of mean and extreme river flows are consistent with the observations only when anthropogenic radiative forcing is considered ( [[#Gudmundsson--2021|Gudmundsson et al., 2021]] ).

In summary, the assessment of observed trends in the magnitude of runoff, streamflow, and flooding remains challenging, due to the spatial heterogeneity of the signal and to multiple drivers. There is, however, ''high confidence'' that the amount and seasonality of peak flows have changed in snowmelt-driven rivers due to warming. There is also ''high confidence'' that land-use change, water management and water withdrawals have altered the amount, seasonality, and variability of river discharge, especially in small and human-dominated catchments.

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