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

==== 11.6.4.3 Hydrological Deficits ====

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It is often difficult to separate the role of climate trends from changes in land use, water management and demand for changes in hydrological deficits, especially on a regional scale. However, a global study based on a recent multi-model experiment with global hydrological models and covering several AR6 regions suggests a dominant role of anthropogenic radiative forcing for trends in low, mean and high flows, while simulated effects of water and land management do not suffice to reproduce the observed spatial pattern of trends ( [[#Gudmundsson--2021|Gudmundsson et al., 2021]] ). Regional studies also suggest that climate trends have been dominant compared to land use and human water management for explaining trends in hydrological droughts in some regions, for instance in Ethiopia ( [[#Fenta--2017|Fenta et al., 2017]] ), China ( [[#Xie--2015|Xie et al., 2015]] ), and North America for the Missouri and Colorado basins, as well as in California ( [[#Shukla--2015|Shukla et al., 2015]] ; [[#Udall--2017|Udall and Overpeck, 2017]] ; [[#Ficklin--2018|Ficklin et al., 2018]] ; K. [[#Xiao--2018|]] [[#Xiao--2018|Xiao et al., 2018]] ; [[#Glas--2019|Glas et al., 2019]] ; [[#Martin--2020|Martin et al., 2020]] ; [[#Milly--2020|Milly and Dunne, 2020]] ).

In other regions, the influence of human water uses can be more important to explain hydrological drought trends (Y. [[#Liu--2016|]] [[#Liu--2016|Liu et al., 2016]] ; [[#Mohammed--2016|Mohammed and Scholz, 2016]] ). There is ''medium confidence'' that human-induced climate change has contributed to an increase of hydrological droughts in the Mediterranean ( [[#Giuntoli--2013|Giuntoli et al., 2013]] ; [[#Vicente-Serrano--2014|Vicente-Serrano et al., 2014]] ; [[#Gudmundsson--2017|Gudmundsson et al., 2017]] ), but also ''medium confidence'' that changes in land use and terrestrial water management contributed to these trends ( [[#11.9|Section 11.9]] ; [[#Teuling--2019|Teuling et al., 2019]] ; [[#Vicente-Serrano--2019|Vicente-Serrano et al., 2019]] ). A global study with a single hydrological model estimated that human water consumption has intensified the magnitude of hydrological droughts by 20–40% over the last 50 years, and that the human water use contribution to hydrological droughts was more important than climatic factors in the Mediterranean, and central USA, as well as in parts of Brazil ( [[#Wada--2013|Wada et al., 2013]] ). However, [[#Gudmundsson--2021|Gudmundsson et al. (2021)]] concluded that the contribution of human water use is smaller than that of anthropogenic climate change to explain spatial differences in the trends of low flows based on a multi-model analysis. There is still ''limited evidence'' and thus ''low confidence'' in assessing these trends at the scale of single regions, with few exceptions ( [[#11.9|Section 11.9]] ).

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