A systematic evaluation of 15 actual evapotranspiration formulations within conceptual hydrological models

Abstract. Actual evapotranspiration (AET) is a major component of the water balance, yet it is rarely assessed for accuracy in conceptual rainfall-runoff models that are often calibrated to match streamflow only. Inaccurate representation of underlying AET processes may cause models to incorrectly simulate long-term changes in partitioning between AET and streamflow, even if this partitioning was relatively accurate during calibration. To investigate AET representation within conceptual hydrological models, we systematically tested 15 evapotranspiration (ET) equations that convert potential evapotranspiration (PET) and soil moisture to AET. The 15 equations represent common practice, having been sourced from a published comprehensive review of conceptual hydrological models. Each of these 15 formulations were trialled within three conceptual hydrological models (GR4J, Simhyd and Vic). Following multi-objective calibration, we evaluated performance across both streamflow and flux tower AET measurements at seven catchments from a range of Australian climates. A small number of AET equations outperformed the rest, with one equation standing out, which uses a non-linear relationship with soil moisture storage and can scale down AET such that it cannot equal PET. This equation achieved a higher objective function value for both AET and streamflow and accurately captured evapotranspiration signatures. However, even this equation showed limitations in reproducing observed AET, suggesting persistent issues across commonly used formulations. These shortcomings may reflect missing vegetation-related dynamics and other simplifications. Our findings highlight the importance of ET equation selection in modelling AET and streamflow, and we recommend the identified equation as a promising option for future Australian studies. Further work is needed to test equations for consistency with known processes to improve the physical realism of conceptual hydrological models.