AbstractPermafrost is earth material that has temperature at or below 0°C for at least two consecutive summers. Above the permafrost is the active layer, a zone that freezes in winter and thaws in summer. Even though the principles governing water movement in permafrost areas are the same as those in more temperate regions, interactions of extremes in climate and the land surface characteristics render permafrost hydrology different from the hydrology of temperate latitudes. Ice‐rich permafrost prevents infiltration of rainfall or snowmelt water, often maintaining a moist to saturated active layer where the permafrost table is shallow. Most hydrologic activities are confined above ground or in the thin active layer, which supplies summer moisture to plants and for evaporative flux. Limited storage capacity of the thawed zone does not support extended baseflow in a stream, though the proportion of baseflow increases as the percentage of permafrost extent decreases. In areas where permafrost is discontinuous or where it has thawed substantially near the surface, local hydrology may display a marked different character as there are stronger exchanges between the surface water and the ground water system, or water may drain laterally resulting in drier surface conditions. Runoff paths range from interhummock cracks, soil pipes, water tracks to distinct channels; and permafrost affects the ground water and contaminant migration pathways. Understanding the interdependence of permafrost, hydrology, and ecosystems is critically important to enable accurate projections of future conditions in the high latitudes.
