Snowpack Aging, Water Isotope Evolution, and Runoff Isotope Signals, Palouse Range, Idaho, USA

Abstract

A snowpack’s δ2 H and δ18 O values evolve with snowfall, sublimation, evaporation, and melt, which produces temporally variable snowpack, snowmelt, and runoff isotope signals. As a snowpack ages, the relatively depleted δ2 H and δ18 O values of snow will become less depleted with sublimation and evaporation, and the internal distribution of isotope signals is altered with melt moving through and out of the snowpack. An examination of δ2 H and δ18 O values for snowpack, snowmelt, and ephemeral creek water in the Palouse Range of northern Idaho indicated an evolution from variably depleted snowpack to enriched snowmelt and relatively consistent isotope signals in springtime ephemeral creeks. Within the primary snow band of the mountain range and during the winter–spring period of 2019–2020, the snowpack had an isotope range of −130 to −75‰ for δ2 H and −18 to −10.5‰ for δ18 O with resulting snowmelt values of −120 to −90‰ for δ2 H and −16.5 to −12.5‰ for δ18 O. With runoff of snowmelt to ephemeral creeks, the isotope values compressed to −107 to −104‰ for δ2 H and −15.5 to −14.5‰ for δ18 O. Aging of the snowpack produced increasing densities in the base, middle, and upper layers along with a corresponding enrichment of isotope values. The highest elevation site indicated the least enrichment of δ2 H and δ18 O in the snowpack base layer, and the lowest elevation site indicated the strongest enrichment of δ2 H and δ18 O in the snowpack base layer. Deuterium excess decreased with snowpack aging processes of accumulation and melt release, along with the migration of water vapor and snowmelt within the snowpack. It is likely that winter melt (early depleted signal) is a primary contributor to creeks and groundwater along the Palouse Range, but the strong variability of snowpack isotope signals provides a wide range of possible isotope signals to surface-water and groundwater systems at the mountain front.