Evidence of Hydrological Intensification and Regime Change From Northern Alaskan Watershed Runoff

Snowmelt‐dominated runoff regimes have defined northern Alaskan rivers. Discharge records from three watersheds within the National Petroleum Reserve in Alaska (NPR‐A) span 19 years and capture three notable periods of changing runoff. In the first, 2001–2008, mean annual runoff (MAR) averaged 90 mm, characterized by sharp snowmelt runoff and summer drought. Over the next 7 years, larger MAR averaged 120 mm driven by high and early snowmelt runoff. The most recent 4 years, 2016–2019, had even higher MAR of 163 mm with high and sustained late summer flows. Hydrograph analysis suggests a shift toward rainfall‐dominated runoff in the most recent period compared to snowmelt‐dominated hydrographs in the previous two. Declining sea ice appears closely linked to increasing late summer precipitation and a shift toward rainfall runoff. Future development in the NPR‐A will require continued hydrological monitoring and planning to mitigate flood and erosion hazards, permafrost degradation, and ecosystem impairment. Plain Language Summary Water is expected to cycle more rapidly as the Arctic climate warms, yet it is uncertain whether conditions will get wetter or dryer. Watershed runoff, measured in the form of river flow, captures the balance of precipitation and evaporation over wide land areas to detect how the water cycle is changing. Historically, arctic river flows are supplied mainly by spring snowmelt and become quite low during summers with limited rainfall. River flow records from three watersheds in northern Alaska over the last 19 years provide evidence that total water inputs from snow and rain are increasing relative to water losses from evaporation. Most recently, contributions from rainfall have increased greatly, and this may be related to enhanced moisture supply from more ice‐free conditions of the Arctic Ocean. Higher river flows that come later in the summer may affect erosion and sedimentation, frozen soils, fish habitat, and human infrastructure. Key Points Arctic Coastal Plain watershed runoff regimes are traditionally snowmelt‐dominated Discharge records show increasing annual runoff over the past 19 years and much higher rainfall runoff contributions over the last 4 years Increasing ice‐free ocean conditions correspond to higher rainfall runoff, yet attribution of hydrologic responses remain uncertain