Advancing isotope‐enabled hydrological modelling for ungauged calibration of data‐scarce humid tropical catchments

Realistic projections of the future climate and how this translates to water availabilityis crucial for sustainable water resource management. However, data availability con-strains the capacity to simulate streamflow and corresponding hydrological pro-cesses. Developing more robust hydrological models and methods that cancircumvent the need for large amounts of hydro-climatic data is crucial to supportwater-related decisions, particularly in developing countries. In this study, we usenatural isotope tracers in addition to hydro-climate data within a newly developedversion of the spatially-distributed J2000iso as an isotope-enabled rainfall-runoffmodel simulating both water and stable isotope (δ2H) fluxes. We pilot the model forthe humid tropical San Carlos catchment (2500 km2) in northeastern Costa Rica,which has limited time series, but spatially distributed data. The added benefit of sim-ulating stable isotopes was assessed by comparing different amounts of observationdata using three model calibration strategies (i) three streamflow gauges, (ii) threegauges with stream isotopes and (iii) isotopes only. The J2000iso achieved a stream-flow Kling–Gupta efficiency (KGE) of 0.55–0.70 across all the models and gauges,but differences in hydrological process simulations emerged when including stablewater isotopes in the rainfall-runoff calibration. ... mehrHydrological process simulation var-ied between the standard J2000 rainfall-runoff model with a high simulated surfacerunoff proportion of 37% as opposed to the isotope version with 84%–89%simulated baseflow or interflow. The model solutions that used only isotope data forcalibration exhibited differences in simulated interflow, baseflow and model perfor-mance but captured bulk water balances with a reasonable match between the simu-lated and observed hydrographs. We conclude that J2000iso has shown thepotential to support water balance modelling for ungauged catchments using stableisotope, satellite and global reanalysis data sets.