Relationship between isotope ratios in precipitation and stream water across watersheds of the National Ecological Observation Network

The timescales associated with precipitation moving through watersheds reveal processes that are critical to understanding many hydrologic systems. Measurements of environmental stable water isotope ratios (delta 2H and delta 18O) have been used as tracers to study hydrologic timescales by examining how long it takes for incoming precipitation tracers become stream discharge, yet limited measurements both spatially and temporally have bounded macroscale evaluations so far. In this observation driven study across North American biomes within the National Ecological Observation Network (NEON), we examined delta 18O and delta 2H stable water isotope in precipitation (delta P) and stream water (delta Q) at 26 co-located sites. With an average 54 precipitation samples and 139 stream water samples per site collected over 2014-2022, assessment of local meteoric water lines and local stream water lines showed geographic variation across North America. Taking the ratio of estimated seasonal amplitudes of delta P and delta Q to calculate young water fractions (Fyw), showed a Fyw range from 1% to 93% with most sites having Fyw below 20%. Calculated mean transit times (MTT) based on a gamma convolution model showed a MTT range from 0.10 to 13.2 years, with half of the sites having MTT estimates lower than 2 years. Significant correlations were found between the Fyw and watershed area, longest flow length, and the longest flow length/slope. Significant correlations were found between MTT and site latitude, longitude, slope, clay fraction, temperature, precipitation magnitude, and precipitation frequency. The significant correlations between water timescale metrics and the environmental characteristics we report share some similarities with those reported in prior studies, demonstrating that these quantities are primarily driven by site or area specific factors. The analysis of isotope data presented here provides important constraints on isotope variation in North American biomes and the timescales of water movement through NEON study sites. This figure describes the workflow to obtain the correlations between isotope processes (delta P and delta Q), water age metrics (Fyw and MTT), and the environmental characteristics of the 26 National Ecological Observatory Network (NEON) sites studied. The key findings report the strong and documented correlations between isotope processes and environmental characteristics. The correlations between the water age metrics and the environmental characteristics. Finally, the correlations between the isotope processes and the water age metrics, as well water age metrics and themselves.image