Stream water sourcing from high elevation snowpack inferred from stable isotopes of water: A novel application of d-excess values

Abstract. About 80 % of the precipitation in the Colorado River’s headwaters is snow, and the resulting snowmelt-driven hydrograph is a crucial water source for about 40 million people. Snowmelt from alpine and subalpine snowpack contributes substantially to groundwater recharge and river flow. However, the dynamics of snowmelt progression are not well understood because observations of the high elevation snowpack are difficult due to challenging access in complex mountainous terrain as well as the cost- and labor-intensity of methods. We present a novel approach to infer the processes and dynamics of high elevation snowmelt contributions predicated upon stable hydrogen and oxygen isotope ratios observed in stream discharge. We show that d-excess values of stream water can serve as a comparatively cost-effective proxy for a catchment integrated signal of high elevation snow melt contributions to catchment runoff.

We sampled stable hydrogen and oxygen isotope ratios of the precipitation, snowpack, and stream water in the East River, a headwater catchment of the Colorado River and the stream water of larger catchments at sites on the Gunnison River and Colorado River.

The d-excess of snowpack increased with elevation; the upper subalpine and alpine snowpack (>3200 m) and had a substantially higher d-excess compared to lower elevations (<3200 m) in the study area. The d-excess values of stream water reflected this because d-excess values increased as the higher elevation snowpack contributed more to stream water generation later in the snowmelt/runoff season. Endmember mixing analyses based on the d-excess data showed that the share of high elevation snowmelt contributions within the snowmelt hydrograph was on average 44 % and generally increased during melt period progression, up to 70 %. The observed pattern was consistent during six years for the East River, and a similar relation was found for the larger catchments on the Gunnison and Colorado Rivers. High elevation snowpack contributions were found to be higher for years with lower snowpack and warmer spring temperatures. Thus, we conclude that the d-excess of stream water is a viable proxy to observe changes in high elevation snowmelt contributions in catchments at various scales. Inter-catchment comparisons and temporal trends of the d-excess of stream water could therefore serve as a catchment-integrated measure to monitor if mountain systems increasingly rely on high elevation water inputs during snow drought.