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

==== 11.6.1.3 Soil Moisture Deficits ====

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Soil moisture shows an important correlation with precipitation variability ( [[#Khong--2015|Khong et al., 2015]] ; [[#Seager--2019|Seager et al., 2019]] ), but ET also plays a substantial role in further depleting moisture from soils, in particular in humid regions during periods of precipitation deficits ( [[#Teuling--2013|Teuling et al., 2013]] ; [[#Padrón--2020|Padrón et al., 2020]] ). In addition, soil moisture plays a role in drought self-intensification under dry conditions in which ET is decreased and leads to higher AED ( [[#Miralles--2019|Miralles et al., 2019]] ), an effect that can also contribute to triggering flash droughts ( [[#Otkin--2016|Otkin et al., 2016]] , 2018; [[#DeAngelis--2020|DeAngelis et al., 2020]] ; [[#Pendergrass--2020|Pendergrass et al., 2020]] ). If soil moisture becomes limited, ET is reduced, which may decrease the rate of soil drying, but can also lead to further atmospheric dryness through various feedback loops ( [[#Seneviratne--2010|Seneviratne et al., 2010]] ; [[#Miralles--2014a|Miralles et al., 2014a]] , 2019; [[#Teuling--2018|Teuling, 2018]] ; [[#Vogel--2018|Vogel et al., 2018]] ; S. [[#Zhou--2019|]] [[#Zhou--2019|Zhou et al., 2019]] ; [[#Liu--2020|Liu et al., 2020]] ). The process is complex since vegetation cover plays a role in modulating albedo and in providing access to deeper stores of water (both in the soil and groundwater). Also, changes in land cover and in plant phenology may alter ET ( [[#Sterling--2013|Sterling et al., 2013]] ; [[#Woodward--2014|Woodward et al., 2014]] ; [[#Frank--2015|Frank et al., 2015]] ; [[#Döll--2016|Döll et al., 2016]] ; [[#Ukkola--2016|Ukkola et al., 2016]] ; [[#Trancoso--2017|Trancoso et al., 2017]] ; [[#Hao--2019|Hao et al., 2019]] ; [[#Lian--2020|Lian et al., 2020]] ). Snow depth has strong and direct impacts on soil moisture in many systems ( [[#Gergel--2017|Gergel et al., 2017]] ; [[#Williams--2020|Williams et al., 2020]] ).

Soil moisture directly affects plant water stress and ET. Soil moisture is the primary factor that controls xylem hydraulic conductance – that is, water uptake in plants ( [[#Sperry--2016|Sperry et al., 2016]] ; [[#Hayat--2019|Hayat et al., 2019]] ; X. [[#Chen--2020|]] [[#Chen--2020|Chen et al., 2020]] ). For this reason, soil moisture deficits are the main driver of xylem embolism, the primary cause of plant mortality ( [[#Anderegg--2012|Anderegg et al., 2012]] , 2016; [[#Rowland--2015|Rowland et al., 2015]] ). Also carbon assimilation by plants strongly depends on soil moisture ( [[#Hartzell--2017|Hartzell et al., 2017]] ), with implications for carbon starvation and plant dying if soil moisture deficits are prolonged ( [[#Sevanto--2014|Sevanto et al., 2014]] ). These mechanisms explain that soil moisture deficits are usually more relevant than AED excess to explain gross primary production anomalies and vegetation stress, mostly in sub-humid and semi-arid regions ( [[#Stocker--2018|Stocker et al., 2018]] ; [[#Liu--2020|Liu et al., 2020]] ). High CO <sub>2</sub> concentrations are shown to potentially decrease plant ET and increase plant water-use efficiency, affecting soil moisture levels, but this effect interacts with other CO <sub>2</sub> physiological and radiative effects ( [[#11.6.5.2|Section 11.6.5.2]] and Cross-Chapter Box 5.1), and has less relevance under low soil moisture ( [[#Morgan--2011|Morgan et al., 2011]] ; Z. [[#Xu--2016|]] [[#Xu--2016|Xu et al., 2016]] ; [[#Nackley--2018|Nackley et al., 2018]] ; [[#Dikšaitytė--2019|Dikšaitytė et al., 2019]] ). ESMs represent both surface (around 10cm) and total column soil moisture, whereby total soil moisture is of more direct relevance for root water uptake, in particular by trees. There is evidence that surface soil moisture projections are substantially drier than total soil moisture projections, and may overestimate drying of relevance for most vegetation ( [[#Berg--2017a|Berg et al., 2017a]] ).

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