Pegas et al. (2026) Extending the Thornthwaite–Mather Water Budget Model for Surface Runoff Estimation: A Nationwide Assessment in Mainland Portugal

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Identification

• Journal: Water
• Year: 2026
• Date: 2026-09-20
• Authors: João Pedro Pegas, María Manuela Portela, João Filipe Santos, Miguel Potes
• DOI: 10.3390/w18182343

Research Groups

Not specified in the provided text.

Short Summary

This study evaluates the Thornthwaite–Mather Water Budget (TM-WB) model for estimating water availability across mainland Portugal, concluding that the Penman–Monteith formulation outperforms the Thornthwaite model and that Available Water Capacity (AWC) is a robust substitute for soil storage parameters.

Objective

• To assess the performance of the TM-WB model in quantifying national-scale water availability using different potential evapotranspiration (PET) formulations and soil water storage parameters.

Study Configuration

• Spatial Scale: National scale (Mainland Portugal), covering twelve watersheds across a north-south climatic gradient.
• Temporal Scale: 43 hydrologic years (1 October 1980 to 30 September 2023).

Methodology and Data

• Models used: Thornthwaite–Mather Water Budget (TM-WB) model, utilizing both Penman–Monteith and Thornthwaite PET formulations; Monte Carlo synthetic generation for uncertainty analysis.
• Data sources: ERA5-Land reanalysis database (precipitation and potential evapotranspiration) and globally available soil databases (for AWC).

Main Results

• Both PET formulations successfully captured spatial and temporal runoff dynamics, but the Penman–Monteith model demonstrated superior overall performance.
• The Thornthwaite model generally overestimated runoff compared to the Penman–Monteith model.
• Mean annual runoff estimates were 317 mm/year (Penman–Monteith) and 371 mm/year (Thornthwaite), aligning closely with the Portuguese Environment Agency's (APA) estimate of 358 mm/year.
• Grid-based Available Water Capacity (AWC) proved to be a practical substitute for the conventional $S_{max}$ parameter, with runoff estimates showing low sensitivity to AWC uncertainty.

Contributions

• Demonstrates the efficacy of simple water budget models for national-scale assessments in regions with scarce ground data.
• Validates the use of globally available soil databases (AWC) to implement the TM-WB model consistently across large areas.
• Provides a simplified methodology suitable for application under future climate change scenarios.

Funding

Not specified in the provided text.

Citation

@article{Pegas2026Extending,
  author = {Pegas, João Pedro and Portela, María Manuela and Santos, João Filipe and Potes, Miguel},
  title = {Extending the Thornthwaite–Mather Water Budget Model for Surface Runoff Estimation: A Nationwide Assessment in Mainland Portugal},
  journal = {Water},
  year = {2026},
  doi = {10.3390/w18182343},
  url = {https://doi.org/10.3390/w18182343}
}