Editing IPCC:AR6/WGI/Chapter-12 (section)

=== 12.3.2 Wet and Dry ===

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==== 12.3.2.1 Mean Precipitation ====

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Changes in mean precipitation alter total water resources and long-term surface, snowpack and groundwater reservoirs ( [[#Schewe--2014|Schewe et al., 2014]] ). Annual and seasonal wet trends can alter the suitable geographic range of species, with implications for biodiversity and vector-borne diseases ( [[#Knouft--2017|Knouft and Ficklin, 2017]] ; [[#Smith--2020|Smith et al., 2020]] ). The rate at which higher total streamflow increases river erosion and changes sediment loading is relevant for fish breeding ( [[#Scheurer--2009|Scheurer et al., 2009]] ), the location of riverine salt fronts that affect coastal agriculture and ecosystems ( [[#Chun--2018|Chun et al., 2018]] ; [[#Vu--2018|Vu et al., 2018]] ), coastal freshwater stratification ( [[#Baker-Austin--2013|Baker-Austin et al., 2013]] ; [[#Bell--2013|Bell et al., 2013]] ), and the accretion of sediment in estuaries and beaches ( [[#Syvitski--2007|Syvitski and Milliman, 2007]] ). Wetter conditions may shift tourist appeal ( [[#Kovács--2017|Kovács et al., 2017]] ) and alter the pace of degradation for paved and especially unpaved roads ( [[#Chinowsky--2012|Chinowsky and Arndt, 2012]] ).

Many agricultural systems require minimum rainfall totals or rely upon irrigation ( [[#Mbow--2019|Mbow et al., 2019]] ). The length of the wet season helps determine the potential for multiple cropping seasons, but inconsistency of wet season arrival times poses challenges for farm management ( [[#Waha--2020|Waha et al., 2020]] ). Wetter growing season conditions increase the chance of waterlogging, which can delay planting or damage planted seeds ( [[#Rosenzweig--2002|Rosenzweig et al., 2002]] ; [[#Ben-Ari--2018|Ben-Ari et al., 2018]] ; [[#Mäkinen--2018|Mäkinen et al., 2018]] ; [[#Wolfe--2018|Wolfe et al., 2018]] ; [[#Kolberg--2019|Kolberg et al., 2019]] ; [[#Grotjahn--2021|Grotjahn, 2021]] ). [[#Tomasek--2017|Tomasek et al. (2017)]] calculated ‘workable days’ for agricultural machinery around planting and harvest time set in part by limits in soil moisture saturation below which farmers can utilize critical machinery with less rutting or soil compaction. Wetter conditions may also increase canopy moisture that is conducive to crop pathogens ( [[#Garrett--2006|Garrett et al., 2006]] ; [[#Kilroy--2015|Kilroy, 2015]] ; [[#Grotjahn--2021|Grotjahn, 2021]] ).

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==== 12.3.2.2 River Flood ====

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A large variety of climate indices and models are utilized to understand how river flooding affects both natural or built environments with highly variable hazard thresholds, given unique local topography and engineered defences such as dams and polders ( [[#Arnell--2016|Arnell and Gosling, 2016]] ; [[#Ekström--2018|Ekström et al., 2018]] ). Key transportation routes, built infrastructure and agricultural lands are threatened when floods exceed design standards commonly based around flood magnitudes of a given historic return period (e.g., 1-in-100-year flood event), an annual exceedance probability or precipitation intensity-duration-frequency relationships with key indices (e.g., 10-day cumulative precipitation) related to catchment size and properties ( [[#Hirabayashi--2013|Hirabayashi et al., 2013]] ; [[#Arnell--2014|Arnell and Lloyd-Hughes, 2014]] ; [[#Kundzewicz--2014|Kundzewicz et al., 2014]] ; [[#Arnell--2016|Arnell and Gosling, 2016]] ; [[#Dikanski--2016|Dikanski et al., 2016]] ; [[#Gosling--2016|Gosling and Arnell, 2016]] ; [[#Forzieri--2017|Forzieri et al., 2017]] ; [[#Fluixá-Sanmartín--2018|Fluixá-Sanmartín et al., 2018]] ; [[#Koks--2019|Koks et al., 2019]] ). Floods and high-flow events can scour river beds and elevate silt loads, reducing water quality and accelerating deposition in estuaries and reservoirs ( [[#Khan--2018|Khan et al., 2018]] ; [[#Parasiewicz--2019|Parasiewicz et al., 2019]] ). Floods can knock down, drown or wash away crops and livestock, and partially submerged plants can have yield reduction depending on water turbidity and their development stage ( [[#Ruane--2013|Ruane et al., 2013]] ; [[#Shrestha--2019|Shrestha et al., 2019]] ). Basin snowpack properties may also be important during heavy rain events, as rain-on-snow events can lead to rapid acceleration of flood stages that threaten wildlife and society ( [[#Hansen--2014|Hansen et al., 2014]] ).

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==== 12.3.2.3 Heavy Precipitation and Pluvial Flood ====

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Heavy downpours can lead to pluvial flooding in cities, roadways, farmland, subway tunnels and buildings (particularly those with basements; [[#Grahn--2017|Grahn and Nyberg, 2017]] ; [[#Palko--2017|Palko, 2017]] ; [[#Pregnolato--2017|Pregnolato et al., 2017]] ; [[#Orr--2018|Orr et al., 2018]] ). Heavy precipitation may overwhelm city transportation and storm water drainage systems, which are typically designed using intensity-duration-frequency information such as the return periods for 1-, 6- or 24-hour rainfall totals ( [[#Kermanshah--2017|Kermanshah et al., 2017]] ; [[#Depietri--2018|Depietri and McPhearson, 2018]] ; [[#Rosenzweig--2018|Rosenzweig et al., 2018]] ; [[#Courty--2019|Courty et al., 2019]] ). Heavy rain events can directly cause leaf loss and damage, or knock over crops, also driving pollutant entrainment and erosion hazards in terrestrial ecosystems and farmland, with downstream ramifications for water quality ( [[#Hatfield--2014|Hatfield et al., 2014]] ; [[#Segura--2014|Segura et al., 2014]] ; [[#Li--2016|Li and Fang, 2016]] ; [[#Chhetri--2019|Chhetri et al., 2019]] ). The proportion of total precipitation that falls in heavy events also affects the percentage that is retained in the soil column, altering groundwater recharge and deep soil moisture content for agricultural use ( [[#Fishman--2016|Fishman, 2016]] ; [[#Lesk--2020|Lesk et al., 2020]] ).

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==== 12.3.2.4 Landslide ====

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Landslides, mudslides, rockfalls and other mass movements can lead to fatalities, destroy infrastructure and housing stock, and block critical transportation routes. Climate models cannot resolve these complex slope failure processes (nor triggering mechanisms such as earthquakes), so most studies rely on proxies or conditions conducive to slope failure ( [[#Gariano--2016|Gariano and Guzzetti, 2016]] ; [[#Ho--2017|Ho et al., 2017]] ). Common indices include precipitation intensity-duration thresholds ( [[#Brunetti--2010|Brunetti et al., 2010]] ; [[#Khan--2012|Khan et al., 2012]] ; [[#Melchiorre--2012|Melchiorre and Frattini, 2012]] ) and thresholds related to antecedent wet periods and extreme rainfall intensities ( [[#Alvioli--2018|Alvioli et al., 2018]] ; [[#Monsieurs--2019|Monsieurs et al., 2019]] ). Landslides and rockfalls may also be exacerbated by permafrost thaw and receding glaciers in polar and mountain areas ( [[#Cook--2016|Cook et al., 2016]] ; [[#Haeberli--2017|Haeberli et al., 2017]] ; [[#Patton--2019|Patton et al., 2019]] ).

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==== 12.3.2.5 Aridity ====

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Aridity indices may track long-term changes in precipitation, evapotranspiration demand, surface water, groundwater or soil moisture ( [[#Sherwood--2014|Sherwood and Fu, 2014]] ; [[#Herrera-Pantoja--2015|Herrera-Pantoja and Hiscock, 2015]] ; B.I. [[#Cook--2020|]] [[#Cook--2020|Cook et al., 2020]] ). Changes in soil moisture and surface water can shift the rate of carbon uptake by ecosystems ( [[#Humphrey--2018|Humphrey et al., 2018]] ) and alter suitable climate zones for wild species and agricultural cultivation ( [[#Feng--2013|Feng and Fu, 2013]] ; [[#Garcia--2014|Garcia et al., 2014]] ; [[#Huang--2016a|Huang et al., 2016a]] ; [[#Schlaepfer--2017|Schlaepfer et al., 2017]] ; [[#Fatemi--2018|Fatemi et al., 2018]] ; [[#IPCC--2019c|IPCC, 2019c]] ) as well as the prevalence of related pests and pathogen-carrying vectors ( [[#Paritsis--2011|Paritsis and Veblen, 2011]] ; [[#Smith--2020|Smith et al., 2020]] ). Water table depth, in relation to rooting depth, is also important for farms and forests under dry conditions ( [[#Feng--2006|Feng et al., 2006]] ). A reduction in water availability (via aridity or hydrological drought) challenges water supplies needed for for municipal, industrial, agriculture and hydropower use ( [[#Schaeffer--2012|Schaeffer et al., 2012]] ; [[#Arnell--2014|Arnell and Lloyd-Hughes, 2014]] ; [[#Schewe--2014|Schewe et al., 2014]] ; [[#Gosling--2016|Gosling and Arnell, 2016]] ; [[#van%20Vliet--2016|van Vliet et al., 2016]] ).

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==== 12.3.2.6 Hydrological Drought ====

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Water managers often utilize a variety of hydrological drought indices and hydrological models to characterize water resources, low flow conditions and the potential for irrigation ( [[#Wanders--2015|Wanders and Wada, 2015]] ; [[#Mukherjee--2018|Mukherjee et al., 2018]] ). Low flow volume and intermittency thresholds can indicate reductions in dissolved oxygen, more concentrated pollutants, and higher stream temperatures relevant for ecosystems, water resource quality and thermal power plant cooling ( [[#Feeley--2008|Feeley et al., 2008]] ; [[#Döll--2012|Döll and Schmied, 2012]] ; [[#Schaeffer--2012|Schaeffer et al., 2012]] ; [[#Prudhomme--2014|Prudhomme et al., 2014]] ; [[#van%20Vliet--2016|van Vliet et al., 2016]] ). Low water levels may also restrict waterway navigation for commerce and recreation ( [[#Forzieri--2018|Forzieri et al., 2018]] ).

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==== 12.3.2.7 Agricultural and Ecological Drought ====

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Agricultural and ecological drought indices relate to the ability of plants to meet growth and transpiration needs (Table 11.3; [[#Zargar--2011|Zargar et al., 2011]] ; [[#Lobell--2015|Lobell et al., 2015]] ; [[#Pedro-Monzonís--2015|Pedro-Monzonís et al., 2015]] ; [[#Bachmair--2016|Bachmair et al., 2016]] ; [[#Wehner--2017|Wehner et al., 2017]] ; [[#Naumann--2018|Naumann et al., 2018]] ) and the timing and duration of droughts can lead to substantially different impacts ( [[#Peña-Gallardo--2019|Peña-Gallardo et al., 2019]] ). Drought stress for agriculture and ecosystems is difficult to directly observe, and therefore scientists use a variety of drought indices (Table 11.3), proxy information about changes in precipitation supply and reference evapotranspiration demand, the ratio of actual/potential evapotranspiration or a deficit in available soil water content, particularly at rooting level ( [[#Park%20Williams--2013|Park Williams et al., 2013]] ; [[#Trnka--2014|Trnka et al., 2014]] ; C.D. [[#Allen--2015|]] [[#Allen--2015|Allen et al., 2015]] ; [[#Svoboda--2017|Svoboda and Fuchs, 2017]] ; [[#Mäkinen--2018|Mäkinen et al., 2018]] ; [[#Otkin--2018|Otkin et al., 2018]] ). Severe water stress can lead to crop failure, in particular when droughts persist for an extended period or occur during key plant developmental stages ( [[#Hatfield--2014|Hatfield et al., 2014]] ; [[#Jolly--2015|Jolly et al., 2015]] ; [[#Leng--2019|Leng and Hall, 2019]] ). Projections of high wind speed and low humidity (even for just a portion of the day) can also inform studies examining fruit desiccation and rice cracking ( [[#Grotjahn--2021|Grotjahn, 2021]] ). Drought also raises disease infection rates for West Nile virus ( [[#Paull--2017|Paull et al., 2017]] ), and the alternation of dry and wet spells induces swelling and shrinkage of clay soils that can lead to sinkholes and destabilize buildings ( [[#Hadji--2014|Hadji et al., 2014]] ).

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==== 12.3.2.8 Fire Weather ====

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Complex fire weather indices shed light on conditions that increase the likelihood of wildfire and shifts in the fire season ( [[#Flannigan--2013|Flannigan et al., 2013]] ; [[#Bedia--2015|Bedia et al., 2015]] ; [[#Jolly--2015|Jolly et al., 2015]] ; [[#Harvey--2016|Harvey, 2016]] ; [[#Littell--2016|Littell et al., 2016]] ; [[#Westerling--2016|Westerling, 2016]] ; [[#Abatzoglou--2019|Abatzoglou et al., 2019]] ), which pose particularly acute challenges for indigenous communities ( [[#Christianson--2019|Christianson and McGee, 2019]] ). Projection of future lightning frequency provides information on an important natural triggering mechanism, particularly when coupled with long-term warming and drying trends ( [[#Romps--2014|Romps et al., 2014]] ; [[#Jin--2015|Jin et al., 2015]] ; [[#Veraverbeke--2017|Veraverbeke et al., 2017]] ). Fuel aridity metrics also help determine vegetative fuel desiccation and therefore the ignitability, flammability and spread of fires when they occur ( [[#Abatzoglou--2016|Abatzoglou and Williams, 2016]] ). The presence of snow cover can influence the length of the fire season and the penetration of fire danger into new portions of the Arctic tundra ( [[#Young--2017|Young et al., 2017]] ; [[#Abatzoglou--2019|Abatzoglou et al., 2019]] ). Data on the changing characteristics of local wind circulations like the Santa Ana in California shed light on future intensity and spread patterns for fires ( [[#Jin--2015|Jin et al., 2015]] ). Fires also produce smoke plumes that reduce air and water quality (via deposition), adversely affecting health, visibility and water resources both near and far downwind ( [[#Dennekamp--2011|Dennekamp and Abramson, 2011]] ; [[#McKenzie--2014|McKenzie et al., 2014]] ; [[#Dreessen--2016|Dreessen et al., 2016]] ; [[#Liu--2016|Liu et al., 2016]] ; [[#Martin--2016|Martin, 2016]] ).

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