A method for predicting hydrogen and oxygen isotope distributions across a region’s river network using reach-scale environmental attributes

Abstract

<p><span>Long-term isotope values of river water provide information on hydrological flow pathways and atmospheric exchange. However, current development of isotope maps for rivers is limited by methodology that spatially interpolates point measures to entire river networks. Catchment environmental characteristics and structures that affect river water isotope values also affect downstream reaches via flow, but many (such as man-made dams) are no more likely to affect nearby unconnected catchments than distant ones. Hence, geospatial and statistical interpolation methods used to develop isoscapes for precipitation and terrestrial systems may be less appropriate for river networks. We developed a modified ‘water balance’ river isotope mapping method to consider the effects of reach-scale catchment environmental characteristics and applied it across the entire stream network of New Zealand. This network comprises over 600,000 reaches and over 400,000 kilometres of rivers. The method uses national rainfall precipitation isoscapes, a digital elevation layer, a national river water isotope monitoring dataset (currently over 3 years of monthly sampling at 58 sites) and reach scale river environmental databases across the New Zealand river network. δ<sup>2</sup>H and δ<sup>18</sup>O isoscapes produced using our method showed improved fit to validation data, compared to a model for which residuals were applied as a correction factor across the river network using ordinary kriging. Hence, we show that a water balance modelling approach can provide a good representation of long-term river water δ<sup>2</sup>H and δ<sup>18</sup>O values when combined with a regression-based correction using catchment environmental data. </span></p>