Quantitative Separation of Precipitation and Permafrost Waters Used for Evapotranspiration in a Boreal Forest: A Numerical Study Using Tracer Model

Abstract

Arctic precipitation (PG) that occurs as rainfall (Prain) or snowfall (Psnow) depending on the prevailing climatic conditions results in seasonally specific hydrological events. Climate change can affect the PG‐ and permafrost‐originated water (Pice) regimes, resulting in change to ecohydrological processes. However, the relative influences of source waters (i.e., Prain, Psnow, and Pice) on terrestrial hydrological processes have not yet been fully established. Here, we report the development and implementation of a numerical water tracer model designed to quantify changes in the storages and fluxes of the source waters and the hydrogen and oxygen isotopic tracers associated with hydrometeorological events. The presented tracer model was used to illustrate the spatiotemporal variability of the tracers in the surface–subsurface system of a deciduous needleleaf boreal forest and to separate the contribution rates of the tracer waters to evapotranspiration (ET). Although Psnow accounted for 22%–57% of ET and the subcomponents, the contribution rates to soil evaporation and transpiration were significant only during spring. The major source water for soil moisture was Prain, which accounted for 69.2% of ET and showed an increasing trend during 1980–2016. Additionally, Prain also accounted for 77.2% of transpiration. Under the present conditions of warming permafrost, Pice demonstrated negligibly low impact on ET. The tracer model was shown capable of quantifying the contribution rates of tracer waters to ET, highlighting the advantages of the tracer model for a similar quantitative separation regarding future climate change.

Contribution of water rejuvenation induced by climate warming to evapotranspiration in a Siberian boreal forest

“…Moreover, CHANGE includes a water isotope tracer module designed to characterize the spatiotemporal variability of isotopes and water sources, originating from precipitation and ground ice, in the hydrological system (Park et al, 2021). With this module, it is possible to quantify the contribution of ground ice meltwater induced by permafrost thawing to soil water storages and to the subsequent subsurface flow and evapotranspiration, especially the water footprint of permafrost degradation caused by climate warming.…”

“…In a hydrological system, water input/output variations directly affect the water fluxes, thereby influencing the water age. In the new CHANGE module, water ages in storages (i.e., canopy, snowpack, and soil) are estimated along the onedimensional water flow direction, with a scheme (Figure 1) similar to that used for tracer flux calculation (Park et al, 2021). The volumetric water age (A i ) variation in the lower storage (i), following an input of water (W i ) from the upper storage (i−1) at time step (t), is expressed as:…”

“…Active-layer soil moisture depends primarily on inputs from snow meltwater in spring and precipitation in summer , and on outputs from evapotranspiration. In addition, the water content of the active layer before soil thaws in spring is representative of moisture conditions from the preceding autumn (Park et al, 2021). Similarly, active-layer soil moisture conditions reflect climatic and ecohydrological events.…”

“…We have developed a water age calculation module, which uses five water isotopic tracers as modeling targets, and coupled it to our process-based land surface model (Park et al, 2021). The main objective of this study was to quantify, using the modified land surface model, the influence of climate-driven permafrost variations on model-simulated water storages, fluxes, and ages, particularly evapotranspiration water ages, of a boreal forest in 1980-2016.…”

Late Summer Water Sources in Rivers and Lakes of the Upper Yana River Basin, Northern Eurasia, Inferred from Hydrological Tracer Data

“…In various settings, basic water chemistry, stable water isotopes, dissolved organic carbon, iron and rare earth elements, and organic compounds were monitored to study water transfer through complex permafrost terrain in a single-or multi-proxy analysis. Stable water isotopes are widely used to infer water origin, travel times and dominant pathways in surface runoff and evapotranspiration across permafrost regions [37][38][39][40][41].…”