Editing IPCC:AR6/WGI/Chapter-11 (section)

=== 11.5.1 Mechanisms and Drivers ===

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Since AR5, the number of studies on understanding how floods may have changed, and will change in the future, has substantially increased. Floods are a complex interplay of hydrology, climate, and human management, and the relative importance of these factors varies for different flood types and regions.

In addition to the amount and intensity of precipitation, the main factors for river floods include antecedent soil moisture ( [[#Paschalis--2014|Paschalis et al., 2014]] ; [[#Berghuijs--2016|Berghuijs et al., 2016]] ; [[#Grillakis--2016|Grillakis et al., 2016]] ; [[#Woldemeskel--2016|Woldemeskel and Sharma, 2016]] ) and snow water-equivalent in cold regions ( [[#Sikorska--2015|Sikorska et al., 2015]] ; [[#Berghuijs--2016|Berghuijs et al., 2016]] ). Other factors are also important, including stream morphology ( [[#Borga--2014|Borga et al., 2014]] ; [[#Slater--2015|Slater et al., 2015]] ), river and catchment engineering ( [[#Pisaniello--2012|Pisaniello et al., 2012]] ; [[#Nakayama--2013|Nakayama and Shankman, 2013]] ; [[#Kim--2016|Kim and Sanders, 2016]] ), land-use and land-cover characteristics ( [[#Aich--2016|Aich et al., 2016]] ; [[#Rogger--2017|Rogger et al., 2017]] ) and changes ( [[#Knighton--2019|Knighton et al., 2019]] ), and feedbacks between climate, soil, snow, vegetation, etc. ( [[#Hall--2014|Hall et al., 2014]] ; [[#Ortega--2014|Ortega et al., 2014]] ; [[#Berghuijs--2016|Berghuijs et al., 2016]] ; [[#Buttle--2016|Buttle et al., 2016]] ; [[#Teufel--2019|Teufel et al., 2019]] ). Water regulation and management have, in general, increased resilience to flooding ( [[#Formetta--2019|Formetta and Feyen, 2019]] ), masking effects of an increase in extreme precipitation on flood probability in some regions, even though they do not eliminate very extreme floods ( [[#Vicente-Serrano--2017|Vicente-Serrano et al., 2017]] ). This means that an increase in precipitation extremes may not always result in an increase in river floods ( [[#Sharma--2018|Sharma et al., 2018]] ; [[#Do--2020|Do et al., 2020]] ). Yet, as very extreme precipitation can become a dominant factor for river floods, there can be some correspondence in the changes in very extreme precipitation and river floods ( [[#Ivancic--2015|Ivancic and Shaw, 2015]] ; [[#Wasko--2017|Wasko and Sharma, 2017]] ; [[#Wasko--2019|Wasko and Nathan, 2019]] ). This has been observed in the western Mediterranean ( [[#Llasat--2016|Llasat et al., 2016]] ), in China (Q. [[#Zhang--2015a|]] [[#Zhang--2015|Zhang et al., 2015]] a ) and in the USA ( [[#Peterson--2013b|Peterson et al., 2013b]] ; [[#Berghuijs--2016|Berghuijs et al., 2016]] ; [[#Slater--2016|Slater and Villarini, 2016]] ).

In regions with a seasonal snow cover, snowmelt is the main cause of extreme river flooding over large areas ( [[#Pall--2019|Pall et al., 2019]] ). Extensive snowmelt combined with heavy and/or long-duration precipitation can cause significant floods (D. [[#Li--2019|]] [[#Li--2019|]] [[#Li--2019|Li et al., 2019]] ; [[#Krug--2020|Krug et al., 2020]] ). Changes in floods in these regions can be uncertain because of the compounding and competing effects of the responses of snow and rain to warming that affect snowpack size: warming results in an increase in precipitation, but also a reduction in the time period of snowfall accumulation ( [[#Teufel--2019|Teufel et al., 2019]] ). An increase in atmospheric CO <sub>2</sub> enhances water-use efficiency by plants ( [[#Roderick--2015|Roderick et al., 2015]] ; [[#Milly--2016|Milly and Dunne, 2016]] ; [[#Swann--2016|Swann et al., 2016]] ; [[#Swann--2018|Swann, 2018]] ); this could reduce evapotranspiration and contribute to the maintenance of soil moisture and streamflow levels under enhanced atmospheric CO <sub>2</sub> concentrations ( [[#Yang--2019|Yang et al., 2019]] ). This mechanism would suggest an increase in the magnitude of some floods in the future ( [[#Kooperman--2018|Kooperman et al., 2018]] ). But this effect is uncertain as an increase in leaf area index, and vegetation coverage could also result in overall larger water consumption ( [[#Mátyás--2014|Mátyás and Sun, 2014]] ; [[#Mankin--2019|Mankin et al., 2019]] ; [[#Teuling--2019|Teuling et al., 2019]] ), and there are also other CO <sub>2</sub> -related mechanisms that come into play (Cross-Chapter Box 5.1).

Various factors, such as extreme precipitation ( [[#Cho--2016|Cho et al., 2016]] ; [[#Archer--2018|Archer and Fowler, 2018]] ), glacier lake outbursts ( [[#Schneider--2014|Schneider et al., 2014]] ; [[#Schwanghart--2016|Schwanghart et al., 2016]] ), or dam breaks ( [[#Biscarini--2016|Biscarini et al., 2016]] ) can cause flash floods. Very intense rainfall, along with a high fraction of impervious surfaces can result in flash floods in urban areas ( [[#Hettiarachchi--2018|Hettiarachchi et al., 2018]] ). Because of this direct connection, changes in very intense precipitation can translate to changes in urban flood potential ( [[#Rosenzweig--2018|Rosenzweig et al., 2018]] ), though there can be a spectrum of urban flood responses to this flood potential ( [[#Smith--2013|Smith et al., 2013]] ), as many factors, such as the overland flow rate and the design of urban ( [[#Falconer--2009|Falconer et al., 2009]] ) and storm water drainage systems ( [[#Maksimović--2009|Maksimović et al., 2009]] ), can play an important role. Nevertheless, changes in extreme precipitation are the main proxy for inferring changes in some types of flash floods, (which are addressed in [[IPCC:Wg1:Chapter:Chapter-12#12.4|Section 12.4]] ), given the relation between extreme precipitation and pluvial floods, the very limited literature on urban and pluvial floods (e.g., [[#Skougaard%20Kaspersen--2017|Skougaard Kaspersen et al., 2017]] ), and limitations of existing methodologies for assessing changes in floods ( [[#Archer--2016|Archer et al., 2016]] ).

In summary, there is not always a one-to-one correspondence between an extreme precipitation event and a flood event, or between changes in extreme precipitation and changes in floods, because floods are affected by many factors in addition to heavy precipitation ( ''high confidence'' ). Changes in extreme precipitation may be used as a proxy to infer changes in some types of flash floods that are more directly related to extreme precipitation ( ''high'' ''confidence'' ).

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