Impacts of permafrost degradation on streamflow in the northern Himalayas

The Himalayan water tower provides crucial water resources for Asia.Permafrost degradation is deemed to exert important impacts on streamflow in the Himalayan rivers.Yet,the magnitudes of such impacts remain poorly quantified.Here,we established a robust hydrological model that incorporated active layer deepening and ground ice melt for the drainage basin of the largest river in the northern Himalayas-the Yarlung Zangbo River(YZR).We estimated that permafrost degradation led to~0.65 km^(3)/yr decrease in surface runoff and~0.35 km^(3)/yr increase in baseflow and ground ice melt contributed~0.25% to the annual streamflow in the YZR for the period 2001-2022.The“fill-and-spill”mechanism helps explain the seeming contradiction of observed increasing versus decreasing baseflow in different permafrost regions worldwide.We propose that the dilution of riverine dissolved organic carbon(DOC)concentrations by baseflow may lead to the riverine DOC hysteresis patterns.This study not only lays solid scientific basis for water resources management in the Himalayas,but also yields new insights into how to interpret measured river discharge and nutrient flux in permafrost regions over the globe.

Chlorophyll-a concentrations in 82 large alpine lakes on the Tibetan Plateau during 2003–2017: temporal–spatial variations and influencing factors

As essential parts of the unique ecosystem of Tibetan Plateau(TP),the sizes and associated physical properties of alpine lakes have long been investigated.However,little is known about one of the most critical biogeochemical properties,i.e.the Chlorophyll-a(Chl-a)concentrations.Here,for the first time,we presented a comprehensive investigation of the temporal–spatial variations in Chl-a in 82 lakes(>50 km^(2))across the entire TP region,based on MODIS observations in the period of 2003–2017.The results showed that the 82 lakes exhibited an average longterm mean Chl-a of 3.3±4.3 mg m^(−3),with high Chl-a lakes concentrated in the eastern and southern inner TP basin and northeastern parts of the TP.An interannual trend analysis revealed that lakes exhibiting(significantly)decreasing Chl-a trends and(significantly)increasing Chl-a trends were comparable in numbers but differed in distribution patterns.A correlation analysis indicated that at least 70%of the interannual variability in Chl-a values of lakes was significantly correlated with one of the four environmental factors(wind speed,ice cover duration,lake water surface temperature and surface runoff)and lake size.In addition,glacier meltwater tended to reduce lake Chl-a while salinity levels showed minor influences.