Leveraging flux tower data to systematically evaluate evapotranspiration formulas in conceptual hydrological models

Accurately representing actual evapotranspiration (AET) is crucial for hydrological modelling, as it is a major component of the catchment water balance. However, AET is often neglected when calibrating conceptual rainfall-runoff models to reproduce observed streamflow. This oversight can lead to models with accurate streamflow predictions but flawed internal fluxes, which could become problematic under changing environmental conditions. To address this gap, we systematically evaluated 15 evapotranspiration equations by substituting them into three widely used conceptual hydrological models (GR4J, Simhyd, and VIC). These equations represent diverse process assumptions found across common conceptual rainfall-runoff models by variously converting potential evapotranspiration (PET) and soil moisture into AET. The performance of each model-equation combination was assessed using a multi-objective calibration approach, accounting for simulated streamflow and flux tower-derived AET. We applied this method across seven Australian catchments spanning diverse climatic conditions.

Our analysis reveals that the choice of evapotranspiration equation significantly influences both internal flux accuracy and streamflow predictions. The performance among the tested equations varied extensively. It was found some widely used evapotranspiration equations struggled to replicate observed AET, underscoring potential limitations in their assumptions. Conversely, the better performing equations captured observed evapotranspiration signatures and achieved higher objective function values for both AET and streamflow, suggesting they better represent underlying hydrological processes. One equation consistently performed best across model structures and catchments. This equation incorporated a non-linear relationship between soil moisture and AET, while limiting AET to below potential evapotranspiration (PET).

Our findings underscore the need to improve the realism of evapotranspiration processes in conceptual hydrological models, particularly in relation to vegetation dynamics and their interactions with soil and atmosphere. By incorporating flux tower observations into model calibration and evaluation, our study bridges the gap between experimental data and catchment-scale modelling. We recommend that similar systematic reviews be undertaken on other continents to assess global patterns and differences. Enhancing the representation of these processes could improve model reliability across temporal and spatial scales, especially under changing climatic and environmental conditions.