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

==== 11.6.2.3 Soil Moisture Deficits ====

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There are limited long-term measurements of soil moisture from ground observations ( [[#Dorigo--2011|Dorigo et al., 2011]] ; [[#Qiu--2016|Qiu et al., 2016]] ; [[#Quiring--2016|Quiring et al., 2016]] ), which impedes their use in the analysis of trends. Among the few existing observational studies covering at least two decades, several studies have investigated trends in ground soil moisture in East Asia ( [[#11.9|Section 11.9]] ; [[#Chen--2015b|Chen and Sun, 2015b]] ; [[#Liu--2015|Liu et al., 2015]] ; [[#Qiu--2016|Qiu et al., 2016]] ). Alternatively, microwave-based satellite measurements of surface soil moisture have also been used to analyse trends ( [[#Dorigo--2012|Dorigo et al., 2012]] ; [[#Jia--2018|Jia et al., 2018]] ). Although there is regional evidence that microwave-based soil moisture estimates can capture well drying trends in comparison with ground soil moisture observations ( [[#Jia--2018|Jia et al., 2018]] ), there is only ''medium confidence'' in the derived trends, since satellite soil moisture data are affected by inhomogeneities ( [[#Dorigo--2015|Dorigo et al., 2015]] ; [[#Rodell--2018|Rodell et al., 2018]] ; [[#Preimesberger--2021|Preimesberger et al., 2021]] ). Furthermore, microwave-based satellites only sense surface soil moisture, which differs from root-zone soil moisture ( [[#Berg--2017a|Berg et al., 2017a]] ), although relationships can be derived between the two ( [[#Brocca--2011|Brocca et al., 2011]] ). Several studies have also analysed long-term soil moisture time series from observation-driven land-surface or hydrological models, including land-based reanalysis products ( [[#Albergel--2013|Albergel et al., 2013]] ; [[#Jia--2018|Jia et al., 2018]] ; [[#Gu--2019b|Gu et al., 2019b]] ; [[#Markonis--2021|Markonis et al., 2021]] ). Such models have also been used to assess changes in land water availability, estimated as precipitation minus ET, which is equal to the sum of soil moisture and runoff ( [[#Greve--2014|Greve et al., 2014]] ; [[#Padrón--2020|Padrón et al., 2020]] ).

Overall, evidence from global studies suggests that several land regions have been affected by increased soil moisture drying or water balance drying in past decades, despite some spread among products ( [[#Albergel--2013|Albergel et al., 2013]] ; [[#Greve--2014|Greve et al., 2014]] ; [[#Gu--2019b|Gu et al., 2019b]] ; [[#Padrón--2020|Padrón et al., 2020]] ). Drying has not only occurred in dry regions but also in humid regions ( [[#Greve--2014|Greve et al., 2014]] ). Some studies have specifically addressed changes in soil moisture at regional scale ( [[#11.9|Section 11.9]] ). For AR6 regions, several studies suggest an increase in the frequency and areal extent of soil moisture deficits, with examples in East Asia ( [[#Cheng--2015|Cheng et al., 2015]] ; Y. [[#Qin--2015|]] [[#Qin--2015|Qin et al., 2015]] ; [[#Jia--2018|Jia et al., 2018]] ), Western and Central Europe ( [[#Trnka--2015b|Trnka et al., 2015b]] ), and the Mediterranean ( [[#Hanel--2018|Hanel et al., 2018]] ; [[#Moravec--2019|Moravec et al., 2019]] ; [[#Markonis--2021|Markonis et al., 2021]] ). Nonetheless, some analyses also show no long-term trends in soil drying in some AR6 regions – for example, in Eastern North America ( [[#Park%20Williams--2017|Park Williams et al., 2017]] ) and Central North America ( [[#Seager--2019|Seager et al., 2019]] ), as well as in North Eastern Africa ( [[#Kew--2021|Kew et al., 2021]] ). The soil moisture drying trends identified in both global and regional studies are generally related to increases in ET (associated with higher AED) rather than decreases in precipitation, as identified on global land for trends in water balance in the dry season ( [[#Padrón--2020|Padrón et al., 2020]] ), as well as for some regions ( [[#Teuling--2013|Teuling et al., 2013]] ; [[#Cheng--2015|Cheng et al., 2015]] ; [[#Trnka--2015a|Trnka et al., 2015a]] ; [[#van%20Der%20Linden--2019|van Der Linden et al., 2019]] ; X. [[#Li--2020|]] [[#Li--2020|]] [[#Li--2020|Li et al., 2020]] ).

Evidence from observed or observations-derived trends in soil moisture and precipitation minus ET, are combined with evidence from SPEI and PDSI-PM studies to derive regional assessments of changes in agricultural and ecological droughts ( [[#11.9|Section 11.9]] ). This assessment is summarized in [[#11.6.2.6|Section 11.6.2.6]] .

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