Chapter 6: Terrestrial hydrology

• Precipitation has increased over recent decades, especially incold seasons, and is associated with an increase in rainfallin all seasons and a decrease in snowfall in summer, withspatially varying trends in winter. This century, precipitationevents presently regarded as extremes are expected to becomeroutine. Snow mass has decreased across northern NorthAmerica, but in Eurasia the trend has been negligible andsnow depth has increased in parts of Eurasia. • Permafrost thaw is likely to drive changes in the waterbalance in Arctic areas, but the relevant subsurface processesare difficult to observe directly at the catchment scale.However, observed changes in streamflow dynamics andwater chemistry indicate that permafrost thaw is influencinghydrological connectivity by creating deeper and longerwaterflow pathways through catchments across the Arctic. • Increasing trends in annual river discharge to the Arctic Oceanfrom both continents have continued, providing compellingevidence of intensification of the Arctic water cycle. Asignificant increase in base streamflow during the cold seasonis observed across most regions of the pan-Arctic drainagebasin. The magnitude of maximum river discharge has notchanged significantly; however, the timing of snowmelt freshethas become earlier almost everywhere across the pan-Arctic. • Lake area is declining across the discontinuous permafrostzone. In the continuous permafrost zone, however, the numberof sites with decreasing lake area is similar to the numberwith increasing lake area. Stronger lake area declines in thediscontinuous permafrost zone is consistent with permafrostthaw being further advanced there than in the continuouspermafrost zone. • Ice-cover duration on rivers has declined significantly in coldregions over the past several decades due to later freeze-upand earlier breakup. The observed decline in river ice is likelyto continue in the future due to the projected increase in airtemperature. Maximum river-ice thickness has decreasedsignificantly on most pan-Arctic rivers over the last 50 to60 years, with the greatest decrease observed before 2000. • Lakes are rapidly losing ice across the Northern Hemisphere,with later ice-on dates, earlier ice-off dates, and in some years,some lakes not freezing at all. • Freshwater delivery from Arctic land ice is roughly equivalentto that from North American rivers. Eurasian river dischargeis roughly three times higher. However, the increase in Arcticriver discharge was 1.6 times smaller than the increase infreshwater flux from Arctic land ice. Most of the increasedland-ice freshwater discharge originated from Greenland andArctic Canada. A further increase in freshwater flux fromland ice reduction is likely to continue with the projectedfuture increase in Arctic warming. • Changes in the terrestrial hydrological system have importantimpacts on ecosystems and Arctic livelihoods. Declining snowcover, permafrost, lake area, and lake ice have implications forecosystems, as well as for hunting, fishing, reindeer herding,transportation, and drinking water availability. Impactsalso include feedbacks to the climate and ocean circulationthrough increased freshwater fluxes to the Arctic Ocean andchanges in lake area and ice cover.

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