Where present, permafrost exerts a primary control on water fluxes, flowpaths, and distribution. Climate warming and related drivers of soil thermal change are expected to modify the distribution of permafrost, leading to changing hydrologic conditions, including alterations in soil moisture, connectivity of inland waters, streamflow seasonality, and the partitioning of water stored above and below ground. The field of permafrost hydrology is undergoing rapid advancement with respect to multiscale observations, subsurface characterization, modeling, and integration with other disciplines. However, gaining predictive capability of the many interrelated consequences of climate change is a persistent challenge due to several factors. Observations of hydrologic change have been causally linked to permafrost thaw, but applications of process-based models needed to support and enhance the transferability of empirical linkages have often been restricted to generalized representations. Limitations stem from inadequate baseline permafrost and unfrozen hydrogeologic characterization, lack of historical data, and simplifications in structure and process representation needed to counter the high computational demands of cryohydrogeologic simulations. Further, due in part to the large degree of subsurface heterogeneity of permafrost landscapes and the nonuniformity in thaw patterns and rates, associations between various modes of permafrost thaw and hydrologic change are not readily scalable; even trajectories of change can differ. This review highlights promising advances in characterization and modeling of permafrost regions and presents ongoing research challenges toward projecting hydrologic and ecologic consequences of permafrost thaw at time and spatial scales that are useful to managers and researchers.Abbreviations: AEM, airborne electromagnetic; ALT, active layer thickness; CALM, Circumpolar Active Layer Monitoring; EMI, electromagnetic induction; ERT, electrical resistivity tomography; GPR, ground-penetrating radar; InSAR, Interferometric Synthetic Aperture Radar; NMR, nuclear magnetic resonance; SR, seismic refraction; TDEM, timedomain electromagnetics.Permafrost hydrology is a rapidly progressing research field, and a number of new discoveries and questions have emerged in recent years. Research interest in cold regions has been spurred in part by surface temperature warming rates in high latitudes (McBean et al., 2005) and high altitudes (Pepin et al., 2015) that are greater than the global average. This warming has produced changes to the cryosphere, including permafrost (ground that is £0°C year round), that impact hydrologic processes and conditions (ACIA, 2005;Hinzman et al., 2013). Despite increased attention, there are still critical limitations in hydrologic data coverage, subsurface characterization, process-level understanding, and integrated modeling approaches. Due to its low hydraulic conductivity (K), permafrost strongly affects the movement, storage, and exchange of surface and subsurface water. In...

[1] Arctic and subarctic watersheds are undergoing climate warming, permafrost thawing, and thermokarst formation resulting in quantitative shifts in surface water -groundwater interaction at the basin scale. Groundwater currently comprises almost one fourth of Yukon River water discharged to the Bering Sea and contributes 5 -10% of the dissolved organic carbon (DOC) and nitrogen (DON) and 35-45% of the dissolved inorganic carbon (DIC) and nitrogen (DIN) loads. Long-term streamflow records (>30 yrs) of the Yukon River basin indicate a general upward trend in groundwater contribution to streamflow of 0.7-0.9%/yr and no pervasive change in annual flow. We propose that the increases in groundwater contributions were caused predominately by climate warming and permafrost thawing that enhances infiltration and supports deeper flowpaths. The increased groundwater fraction may result in decreased DOC and DON and increased DIC and DIN export when annual flow remains unchanged.Citation: Walvoord, M. A., and R. G. , Increased groundwater to stream discharge from permafrost thawing in the Yukon River basin: Potential impacts on lateral export of carbon and nitrogen, Geophys. Res. Lett., 34, L12402,

A large reservoir of bioavailable nitrogen (up to approximately 10(4) kilograms of nitrogen per hectare, as nitrate) has been previously overlooked in studies of global nitrogen distribution. The reservoir has been accumulating in subsoil zones of arid regions throughout the Holocene. Consideration of the subsoil reservoir raises estimates of vadose-zone nitrogen inventories by 14 to 71% for warm deserts and arid shrublands worldwide and by 3 to 16% globally. Subsoil nitrate accumulation indicates long-term leaching from desert soils, impelling further evaluation of nutrient dynamics in xeric ecosystems. Evidence that subsoil accumulations are readily mobilized raises concern about groundwater contamination after land-use or climate change.