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

==== 2.2.5.4 Impacts of precipitation extremes on different land cover types ====

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More intense rainfall leads to water redistribution between surface and ground water in catchments as water storage in the soil decreases (green water) and runoff and reservoir inflow increases (blue water) (Liu and Yang 2010 <sup>[[#fn:r384|384]]</sup> ; Eekhout et al. 2018 <sup>[[#fn:r385|385]]</sup> ). This results in increased surface flooding and soil erosion, increased plant water stress and reduced water security, which in terms of agriculture means an increased dependency on irrigation and reservoir storage (Nainggolan et al. 2012 <sup>[[#fn:r386|386]]</sup> ; Favis-Mortlock and Mullen 2011 <sup>[[#fn:r387|387]]</sup> ; García- Ruiz et al. 2011 <sup>[[#fn:r388|388]]</sup> ; Li and Fang 2016 <sup>[[#fn:r389|389]]</sup> ; Chagas and Chaffe 2018 <sup>[[#fn:r390|390]]</sup> ). As there is high confidence of a positive correlation between global warming and future flood risk, land cover and processes are likely to be negatively impacted, particularly near rivers and in floodplains (Kundzewicz et al. 2014 <sup>[[#fn:r391|391]]</sup> ; Alfieri et al. 2016 <sup>[[#fn:r392|392]]</sup> ; Winsemius et al. 2016 <sup>[[#fn:r393|393]]</sup> ; Arnell and Gosling 2016 <sup>[[#fn:r394|394]]</sup> ; Alfieri et al. 2017 <sup>[[#fn:r395|395]]</sup> ; Wobus et al. 2017 <sup>[[#fn:r396|396]]</sup> ).

In agricultural systems, heavy precipitation and inundation can delay planting, increase soil compaction and cause crop losses through anoxia and root diseases (Posthumus et al. 2009 <sup>[[#fn:r397|397]]</sup> ). In tropical regions, flooding associated with tropical cyclones can lead to crop failure from both rainfall and storm surge. In some cases, flooding can affect yield more than drought, particularly in tropical regions (e.g., India) and in some mid/high latitude regions such as China and central and northern Europe (Zampieri et al. 2017 <sup>[[#fn:r398|398]]</sup> ). Waterlogging of croplands and soil erosion also negatively affect farm operations and block important transport routes (Vogel and Meyer 2018 <sup>[[#fn:r399|399]]</sup> ; Kundzewicz and Germany 2012 <sup>[[#fn:r400|400]]</sup> ). Flooding can be beneficial in drylands if the floodwaters infiltrate and recharge alluvial aquifers along ephemeral river pathways, extending water availability into dry seasons and drought years, and supporting riparian systems and human communities (Kundzewicz and Germany 2012; Guan et al. 2015 <sup>[[#fn:r401|401]]</sup> ). Globally, the impact of rainfall extremes on agriculture is less than that of temperature extremes and drought, although in some regions and for some crops, extreme precipitation explains a greater component of yield variability, for example, of maize in the Midwestern USA and southern Africa (Ray et al. 2015 <sup>[[#fn:r402|402]]</sup> ; Lesk et al. 2016 <sup>[[#fn:r403|403]]</sup> ; Vogel et al. 2019 <sup>[[#fn:r404|404]]</sup> ).

Although many soils on floodplains regularly suffer from inundation, the increases in the magnitude of flood events mean that new areas with no recent history of flooding are now becoming severely affected (Yellen et al. 2014 <sup>[[#fn:r405|405]]</sup> ). Surface flooding and associated soil saturation often results in decreased soil quality through nutrient loss, reduced plant productivity, stimulated microbial growth and microbial community composition, negatively impacted soil redox and increased GHG emissions (Bossio and Scow 1998 <sup>[[#fn:r406|406]]</sup> ; Niu et al. 2014 <sup>[[#fn:r407|407]]</sup> ; Barnes et al. 2018 <sup>[[#fn:r408|408]]</sup> ; Sánchez-Rodríguez et al. 2019 <sup>[[#fn:r409|409]]</sup> ). The impact of flooding on soil quality is influenced by management systems that may mitigate or exacerbate the impact. Although soils tend to recover quickly after floodwater removal, the impact of repeated extreme flood events over longer timescales on soil quality and function is unclear (Sánchez-Rodríguez et al. 2017 <sup>[[#fn:r410|410]]</sup> ).

Flooding in ecosystems may be detrimental through erosion or permanent habitat loss, or beneficial, as a flood pulse brings nutrients to downstream regions (Kundzewicz et al. 2014 <sup>[[#fn:r411|411]]</sup> ). Riparian forests can be damaged through flooding; however, increased flooding may also be of benefit to forests where upstream water demand has lowered stream flow, but this is difficult to assess and the effect of flooding on forests is not well studied (Kramer et al. 2008 <sup>[[#fn:r412|412]]</sup> ; Pawson et al. 2013 <sup>[[#fn:r413|413]]</sup> ). Forests may mitigate flooding, however flood mitigation potential is limited by soil saturation and rainfall intensity (Pilaš et al. 2011 <sup>[[#fn:r414|414]]</sup> ; Ellison et al. 2017 <sup>[[#fn:r415|415]]</sup> ). Some grassland species under heavy rainfall and soil saturated conditions responded negatively with decreased reproductive biomass and germination rates (Gellesch et al. 2017 <sup>[[#fn:r416|416]]</sup> ), however overall productivity in grasslands remains constant in response to heavy rainfall (Grant et al. 2014 <sup>[[#fn:r417|417]]</sup> ).

Extreme rainfall alters responses of soil CO <sub>2</sub> fluxes and CO <sub>2</sub> uptake by plants within ecosystems, and therefore result in changes in ecosystem carbon cycling (Fay et al. 2008 <sup>[[#fn:r418|418]]</sup> ; Frank et al. 2015a <sup>[[#fn:r419|419]]</sup> ). Extreme rainfall and flooding limits oxygen in soil which may suppress the activities of soil microbes and plant roots and lower soil respiration, therefore lowering carbon cycling (Knapp et al. 2008 <sup>[[#fn:r420|420]]</sup> ; Rich and Watt 2013 <sup>[[#fn:r421|421]]</sup> ; Philben et al. 2015 <sup>[[#fn:r422|422]]</sup> ). However, the impact of extreme rainfall on carbon fluxes in different biomes differs. For example, extreme rainfall in mesic biomes reduces soil CO <sub>2</sub> flux to the atmosphere and GPP whereas in xeric biomes the opposite is true, largely as a result of increased soil water availability (Knapp and Smith 2001 <sup>[[#fn:r423|423]]</sup> ; Heisler and Knapp 2008 <sup>[[#fn:r424|424]]</sup> ; Heisler-White et al. 2009 <sup>[[#fn:r425|425]]</sup> ; Zeppel et al. 2014 <sup>[[#fn:r426|426]]</sup> ; Xu and Wang 2016 <sup>[[#fn:r427|427]]</sup> ; Liu et al. 2017b <sup>[[#fn:r428|428]]</sup> ; Connor and Hawkes 2018 <sup>[[#fn:r429|429]]</sup> ).

As shown above GHG fluxes between the land and atmosphere are affected by climate. The next section assesses these fluxes in greater detail and the potential for land as a carbon sink.

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