Hydrogen and oxygen stable isotope mass balance evaluation of the National Water Model (v2.1) streamflow, runoff and groundwater flows

This data release contains the results of an isotopic mass balance approach to provide an estimate of the long-term average isotope ratios of NWM streamflow for the summer season (JJA) between 2000 and 2019 in the Western United States. The NWM-estimated long-term average isotope ratios are compared directly to 6426 stream stable isotope observations in 995 unique catchments. Quantified similarities and differences, in the form of p-values, provide useful information about important hydrologic processes. Significant p-values mean that the observed isotope ratio differs from the long-term average mass balance calculated isotope ratios and indicates that flows may be influenced by processes that are not accounted for by the NWM. For δ18O, 1821 observations were significantly different from the NWM estimate, whereas for δ2H, 4281 observations were significantly different from the NWM estimate.