Editing IPCC:AR6/SROCC/Chapter-3 (section)

===== 3.4.1.3.2 Runoff and surface water =====

A general trend of increasing discharge has been observed for large Siberian (Troy et al., 2012 <sup>[[#fn:r1470|1470]]</sup> ; Walvoord and Kurylyk, 2016 <sup>[[#fn:r1471|1471]]</sup> ) and Canadian (Ge et al., 2013 <sup>[[#fn:r1472|1472]]</sup> ; Déry et al., 2016 <sup>[[#fn:r1473|1473]]</sup> ) rivers that drain to the Arctic Ocean ( ''medium confidence'' ). Between 1976 and 2017, trends are 3.3 ± 1.6% for Eurasian rivers and 2.0 ± 1.8% for North American rivers (Holmes et al., 2018 <sup>[[#fn:r1474|1474]]</sup> ) (Figure 3.10). Extreme regional runoff events have also been identified (Stuefer et al., 2017 <sup>[[#fn:r1475|1475]]</sup> ). An observed increase in baseflow in the North American (Walvoord and Striegl, 2007 <sup>[[#fn:r1476|1476]]</sup> ; St. Jacques and Sauchyn, 2009) and Eurasian Arctic (Smith et al., 2007 <sup>[[#fn:r1477|1477]]</sup> ; Duan et al., 2017 <sup>[[#fn:r1478|1478]]</sup> ) over the last several decades is attributable to permafrost thaw and concomitant enhancement in groundwater discharge. The timing of spring season peak flow is generally earlier (Ge et al., 2013 <sup>[[#fn:r1479|1479]]</sup> ; Holmes et al., 2015 <sup>[[#fn:r1480|1480]]</sup> ). There is consistent evidence of decreasing summer season discharge for the Yenisei, Lena, and Ob watersheds in Siberia (Ye et al., 2003 <sup>[[#fn:r1481|1481]]</sup> ; Yang et al., 2004a <sup>[[#fn:r1482|1482]]</sup> ; Yang et al., 2004b <sup>[[#fn:r1483|1483]]</sup> ) and the majority of northern Canadian rivers (Déry et al., 2016 <sup>[[#fn:r1484|1484]]</sup> ). Long-term records indicate water temperature increases (Webb et al., 2008 <sup>[[#fn:r1485|1485]]</sup> ; Yang and Peterson, 2017 <sup>[[#fn:r1486|1486]]</sup> ); attribution to rising air temperatures is complicated by the influence of reservoir regulation over Siberian regions (Liu et al., 2005 <sup>[[#fn:r1487|1487]]</sup> ; Lammers et al., 2007 <sup>[[#fn:r1488|1488]]</sup> ). Increases in discharge and water temperature in the spring season represent notable freshwater and heat fluxes to the Arctic Ocean (Yang et al., 2014 <sup>[[#fn:r1489|1489]]</sup> ).

A large proportion of low-lying Arctic land areas are covered by lakes because permafrost limits surface water drainage and supports ponding even across areas with high moisture deficits (Grosse et al., 2013 <sup>[[#fn:r1490|1490]]</sup> ). While thaw in continuous permafrost is linked to intensified thermokarst activity and subsequent ponding (resulting in lake/wetland expansion), observations of change in surface water coverage across the Arctic are regionally variable (Nitze et al., 2017 <sup>[[#fn:r1491|1491]]</sup> ; Ulrich et al., 2017 <sup>[[#fn:r1492|1492]]</sup> ; Pastick et al., 2019 <sup>[[#fn:r1493|1493]]</sup> ). In landscapes with degrading ice-wedge polygons, subsidence can reduce inundation, increase runoff, and decrease surface water (Liljedahl et al., 2016 <sup>[[#fn:r1494|1494]]</sup> ; Perreault et al., 2017 <sup>[[#fn:r1495|1495]]</sup> ). In discontinuous permafrost, thaw opens up pathways of subsurface flow, improving the connection among inland water systems which supports the drainage of lakes and overall reduction in surface water cover (Jepsen et al., 2013 <sup>[[#fn:r1496|1496]]</sup> ). Enhanced subsurface connectivity from thaw in discontinuous permafrost serves tempers short-term lake fluctuations (Rey et al., 2019 <sup>[[#fn:r1497|1497]]</sup> ).

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