Isotope tracers can benefit hydrologic modeling, by adding observational data relating to groundwater-surface water interactions, evaporation and water ages as a supplement to flow data. However, to realize these benefits, an isotope tracer model must be linked to the hydrologic model. A key barrier to more wide-spread application isotope tracers in hydrologic modeling is the considerable effort required to add an isotope tracer simulation to an existing model, which requires an uncommon overlapping expertise in both hydrologic model development and isotope tracer science. To reduce the barriers to entry, a model agnostic isotope tracer simulator (MAITsim) has been developed, which can simulate two stable isotope tracers (deuterium and oxygen-18) in association with a wide range of hydrologic models. _x000D_ _x000D_ MAITsim runs as a post-processing model using outputs from a hydrologic model as inputs, such that only the model specific linkage needs to be set up to simulate both flow and isotope tracers. This numerically stable tracer simulator is compatible with any flux-state model with unidirectional flow paths, as it uses no pre-determined spatial sub-divisions (any combination of soil layers, sub-catchments and hydrologic response units can be linked to MAITsim). The model includes both mixing and evaporative fractionation and its equivalence to a previously published embedded isotope tracer model has been verified. This new open-source model can be used to improve modeling of surface-groundwater interactions or as a template to embed stable isotope tracer simulations in more hydrologic models.
