Stable isotope variations of precipitation and streamflow reveal the young water fraction of a permafrost watershed

Abstract

The streamflow age is an essential descriptor of catchment functioning that controls runoff generation, biogeochemical cycling, and contaminant transport. The young water fraction (F yw ) of streamflow, which can be accurately estimated with tracer data, is effective at characterizing the water age proportions of heterogeneous catchments. However, the F yw values of permafrost catchments are not known. We selected a watershed in the permafrost region of the Qinghai-Tibet Plateau (QTP) as our study area. Daily interval stable isotopes (deuterium and oxygen-18) of precipitation and streamflow were studied during the 2009 thawing season. The results show that the stable isotope compositions of precipitation and stream water have significant spatial and temporal variations. HYSPLIT backwards trajectory results demonstrate that the moisture in the study area mainly derived from the westerlies and southern monsoons. Thawing processes in the active layer of the permafrost significantly altered the stable isotope compositions of the stream water. The soil temperature, soil moisture, and air temperature are the main drivers of the stable isotope variations in the stream water. We estimated the young water fractions of the five catchments in the study area, which were the first estimates of the F yw in permafrost catchments in the QTP. The results show that an average of 15% of the streamflow is younger than 43 days. Additional analyses show that the vegetation cover significantly controls the young water fraction of the streamflow. These results will improve our understanding of permafrost hydrological processes and water resource utilization and protection. KEYWORDS permafrost hydrology, Qinghai-Tibet Plateau, stable isotopes, watershed hydrology, young water fraction

Sensitivity of young water fractions to hydro-climatic forcing and landscape properties across 22 Swiss catchments

“…DeWalle et al, 1997;Soulsby et al, 2000), however, the inverse correlation of F * yw and F yw with catchment area only becomes significant (ρ = −0.49, p < 0.05) when the five high-elevation, snowdominated sites are omitted from the analysis (Fig. 6).…”

“…Because the total catchment storage in the first case (including snowpack storage) is larger than the subsurface storage alone, the resulting young water fractions are expected to be smaller. Previous studies that estimated young water fractions in snow-dominated watersheds (Jasechko et al, 2016;Song et al, 2017) did not differentiate between these two concepts of catchment storage and simply used incoming precipitation in the young water fraction calculations, thus implicitly considering snowpack storage as part of catchment storage (as in the first case outlined above). This approach is practical in view of the challenges of measuring or modelling snowmelt and its isotopic composition.…”

“…Mean transit times have been variably found to be correlated with (for example) flow path lengths and gradients (McGuire et al, 2005), drainage density (Soulsby et al, 2010), the areal fraction of hydrologically responsive soils , bedrock permeability (Hale and McDonnell, 2016), or combinations of multiple factors (Hrachowitz et al, 2009;Seeger and Weiler, 2014). So far, only a few catchment inter-comparison studies have used young water fractions (Song et al, 2017;Stockinger et al, 2016;Jasechko et al, 2016). A global analysis of 254 watersheds, revealed large spatial variability in young streamflow, which correlated inversely with average topographic gradients and water table depths (Jasechko et al, 2016).…”

Sensitivity of young water fractions to hydro-climatic forcing and landscape properties across 22 Swiss catchments

“…Because the total catchment storage in the first case (including snowpack storage) is larger than the subsurface storage alone, the resulting young water fractions are expected to be smaller. Previous studies that estimated young water fractions in snow-dominated watersheds (Jasechko et al, 2016;Song et al, 2017) did not differentiate 15 between these two concepts of catchment storage and simply used incoming precipitation as one endmember in the young water fraction calculations, thus implicitly considering snowpack storage as part of catchment storage (as in the first case outlined above). This approach is practical in view of the challenges of measuring or modeling snowmelt and its isotopic composition.…”

The impact of land surface temperatures on suprapermafrost groundwater on the central Qinghai‐Tibet Plateau

“…For the last 10 years, the annual average air temperature of the study area is −5.2°C (Wang et al, ), the maximum temperature is 24.7°C, and the minimum temperature is −38.5°C. June to September precipitation accounts for 83% of the total annual precipitation, and the annual average precipitation is 328.9 mm (Song et al, ). The average land surface temperature is −0.5°C, the active layer thickness is approximately 0.8–2.5 m, and the permafrost thickness is 50–120 m (Wang, Li, Wu, & Wang, ).…”