Development of a model agnostic isotope tracer simulator

Isotope tracers can benefit hydrologic modeling, by adding observational data relating to evaporation and water ages as a supplement to flow data. A few hydrologic models have had tracers embedded in their software, resulting in numerous studies and identified benefits from isotope tracer simulation. A key barrier to more wide-spread application of linked flow and isotope simulation 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 expand the utilization of isotope tracers, a model agnostic isotope tracer simulator (MAITsim) has been developed, which can currently simulate two common stable isotope tracers (deuterium and oxygen-18) in association with a wide range of hydrologic models. 

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 in order to simulate both flow and isotope tracers. The 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 is designed to be numerically stable in wet and desiccating conditions. 

The MAITsim model results are compared to an existing embedded isotope tracer model (isoWATFLOOD). The next phase in development is to test MAITsim functionality and performance in multiple existing hydrologic models.