Ecohydrological processes in heavily managed catchments are often inadequately represented in models. We applied a tracer-aided model STARR (Spatially distributed Tracer-Aided Rainfall-Runoff) in an ET (evapotranspiration)-dominated region (the Middle Spree, NE Germany) with major management impacts. Water isotopes were useful in identifying runoff contributions and partitioning ET even at sparse resolution. Trade-offs between discharge- and isotope-based calibrations could be partially mitigated by integrating more process-based conceptualizations into the model.