Rising temperatures around the globe are rapidly thawing permafrost, and a fifth of frozen soil underlying tundra around the world could thaw by 2040—even if we take drastic steps to mitigate climate change. As permafrost thaws, landscapes change.
In the Tasiapik Valley of northern Quebec, Canada, which lies within the discontinuous permafrost zone, vegetation cover has been increasing since the 1950s. Where open tundra, lichen, and herbs once dominated, shrubs and black spruce forests have expanded as the average temperature has climbed. Previous research showed that the land cover in the region evolves over an estimated 90-year-long period from an “immature” landscape of lichens and herbs underlain by permafrost to a “mature” one of trees and shrubs, without permafrost. In a new study, Young et al. sought to determine how this progression affects groundwater recharge in the region’s catchments.
They used the water table fluctuation method to evaluate groundwater recharge at study sites in the valley that included five types of land cover: tundra (or permafrost), lichens and herbs, low shrubs and lichens, medium shrubs, and trees. During field campaigns in 2012 and 2014, the team installed piezometers and water and temperature probes to estimate groundwater recharge rates at each site over the course of four hydrological years (in North America, the hydrological year runs from 1 October to 30 September).
The team found that as vegetation height increases, the depth of freezing decreases and the date of the first belowground freeze occurs later. All the sites thawed around the same time, however, when temperatures climbed above freezing. Across all sites and in each hydrological year, the authors saw a shift in the timing of the first water level rise that coincided with the first spring melt. In areas with the tallest vegetation, the water level rise occurred up to a month later than in regions with the shortest vegetation. The delay, the authors note, likely results from trees and taller shrubs shading the local snowpack and slowing down melt. As a result, instead of quickly flowing toward rivers and streams, meltwater can slowly soak into the soil, which allows for more groundwater recharge.
Ultimately, the impact of greater groundwater recharge in forested areas also depends on other factors, such as the depth of the snowpack and the location of underground aquifers. Future studies incorporating models of unsaturated zone dynamics could further refine our understanding of the effect that climate change will have on Arctic groundwater recharge. (Geophysical Research Letters, https://doi.org/10.1029/2020GL087695, 2020)
—Kate Wheeling, Science Writer