Characterizing spatial and temporal variation in 18O and 2H content of New Zealand river water for better understanding of hydrologic processes

Time series of hydrogen and oxygen stable isotope ratios (delta H-2 and delta O-18) in rivers can be used to quantify groundwater contributions to streamflow, and timescales of catchment storage. However, these isotope hydrology techniques rely on distinct spatial or temporal patterns of delta H-2 and delta O-18 within the hydrologic cycle. In New Zealand, lack of understanding of spatial and temporal patterns of delta H-2 and delta O-18 of river water hinders development of regional and national-scale hydrological models. We measured delta H-2 and delta O-18 monthly, together with river flow rates at 58 locations across New Zealand over a two-year period. Results show: (a) general patterns of decreasing delta H-2 and delta O-18 with increasing latitude were altered by New Zealand's major mountain ranges; delta H-2 and delta O-18 were distinctly lower in rivers fed from higher elevation catchments, and in eastern rain-shadow areas of both islands; (b) river water delta H-2 and delta O-18 values were partly controlled by local catchment characteristics (catchment slope, PET, catchment elevation, and upstream lake area) that influence evaporation processes; (c) regional differences in evaporation caused the slope of the river water line (i.e., the relationship between delta H-2 and delta O-18 in river water) for the (warmer) North Island to be lower than that of the (cooler, mountain-dominated) South Island; (d) delta H-2 seasonal offsets (i.e., the difference between seasonal peak and mean values) for individual sites ranged from 0.50 parts per thousand to 5.07 parts per thousand. Peak values of delta O-18 and delta H-2 were in late summer, but values peaked 1 month later at the South Island sites, likely due to greater snow-melt contributions to streamflow. Strong spatial differences in river water delta H-2 and delta O-18 caused by orographic rainfall effects and evaporation may inform studies of water mixing across landscapes. Generally distinct seasonal isotope cycles, despite the large catchment sizes of rivers studied, are encouraging for transit time analysis applications.