PNNL

Abstract

The timescales associated with precipitation moving through watersheds reveal hydrological processes that are critical to understanding many hydrologic systems. Measurements of environmental stable water isotope ratios (d2H and d18O) have been used as tracers to study hydrologic timescales by examining the time scale at which 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 d18O and d2H stable water isotope in precipitation (dP) and surface water (dQ) at 26 co-located sites. With an average 54 precipitation samples and 139 surface water samples per site, assessment of local meteoric water lines (LMWL) and local surface water line (LSWL) showed geographic variation across North America. Taking the ratio of estimated seasonal amplitudes of dP and dQ to calculate young water fractions (Fyw), showed a range from 1% to 93% with a most sites having Fyw below 20%. Calculated mean transit times (MTT) based on a gamma convolution model showed a range from 0.10 to 13.2 years, with half of the sites having MTT estimates lower than 2 years. Correlations were found between the Fyw and watershed area, longest flow length, and the longest flow length/slope, whereas the only correlation observed for MTT was with site latitude. The estimated Fyw and MTT provide information describing hydrologic processes at NEON sites, however limited correlations of Fyw and MTT with the environmental characteristics we analyzed demonstrate 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.