Runoff Simulation in Predictions in Ungauged Basins (PUB) Using the Improved HBV-PML Model with Evapotranspiration Constraints

Actual evapotranspiration (AET) estimates are crucial for hydrological processes, especially in Predictions in Ungauged Basins (PUB). However, traditional hydrological models often struggle to provide reliable ET values, which limits their performance in regions with sparse observational data. To address this, we integrate evapotranspiration constraints derived from the PML-V2 model, calibrated using flux tower data, into the improved HBV hydrological model (HBV-PML). This approach allows HBV to replicate ET estimates from PML-V2 model, ensuring the two models fully coupled. We applied the HBV-PML model to 4641 small catchments distributed globally, using a basin attribute similarity method to transfer parameters from 10 similar basins for each target basin. Ensemble results from 10 parameter sets showed that the runoff simulations from improved HBV-PML were slightly lower than those from the original HBV model, but both models achieved a median Nash-Sutcliffe Efficiency (NSE) greater than 0.6 for monthly runoff and greater than 0.35 for daily runoff. This is remarkable for HBV-PML since its ET output was identical to that of PML-V2. Our work demonstrates the potential of incorporating more accurate ET values into hydrological models, achieving reliable runoff simulations while dramatically improving the accuracy of evapotranspiration estimates.