Altitudinal Control of Isotopic Composition and Application in Understanding Hydrologic Processes in the mid Merced River Catchment, Sierra Nevada, California, USA

Abstract. Mountain snowpack has been declining and more precipitation has fallen as rainfall than snowfall, particularly in the US West. Isotopic composition in stream water, springs, groundwater, and precipitation was examined to understand the impact of declining snowpack on hydrologic processes in the mid Merced River catchment (1,873 km²), Sierra Nevada, California, USA. Mean isotopic values in small tributaries (catchment area < 122 km²), rock glacier outflows and groundwater from 2005–2008 were strongly correlated with mean catchment elevation (R² = 0.96 for δ²H, n = 16, p < 0.001), with an average isotopic lapse rate of -1.9 ‰/100 m for δ²H and -0.22 ‰/100 m for δ¹⁸O in meteoric water. The lapse rate did not change much over seasons and was not strongly affected by isotopic fractionation. A catchment-characteristic isotopic value was thus established for each sub-catchment based on the relation between isotopic composition and the mean catchment elevation to elucidate hydrometeorologic and hydrologic processes. Compared to Tenaya Creek without water falls, flow and flow duration of Yosemite Creek are much more sensitive to temperature increase due to a strong evaporation effect caused by waterfalls, suggesting possible prolonged dry-up period of Yosemite Falls in the future. Groundwater in the Yosemite Valley (~900–1,200 m) was recharged primarily from the upper snow-rain transition zone (2,000–2,500 m), suggesting its strong vulnerability to shift in snow-rain ratio. The information gained from this study helps advance our understanding of hydrologic responses to climate change in snowmelt-fed river systems.