Mitrowska et al. (2025) Modeling Solar Radiation Data for Reference Evapotranspiration Estimation at a Daily Time Step for Poland

Identification

• Journal: Water
• Year: 2025
• Date: 2025-11-19
• Authors: Dorota Mitrowska, Małgorzata Kleniewska, Leszek Kuchar
• DOI: 10.3390/w17223304

Research Groups

• Institute of Environmental Engineering, Warsaw University of Life Sciences, Warszawa, Poland
• Department of Mathematics, Wrocław University of Environmental and Life Sciences, Wrocław, Poland

Short Summary

This study calibrated Angström–Prescott (A-P) and Hargreaves–Sammani (H-S) solar radiation models for Poland and assessed their impact on Penman–Monteith (P-M) reference evapotranspiration (ET0) estimations, finding that local calibration generally improved accuracy, but the method of radiation determination had a significant and sometimes unexpected impact on ET0 values.

Objective

• Calibrate and evaluate the accuracy of solar radiation (H) models (Angström–Prescott and Hargreaves–Sammani) based on sunshine duration and air temperature across Poland.
• Assess how these solar radiation models affect reference evapotranspiration (ET0) estimation results using the Penman–Monteith formula.

Study Configuration

• Spatial Scale: 10 sites across Poland (Central Europe), covering coastal, inland, submontane, and lake regions.
• Temporal Scale: 16 years (2000–2015, with one station having 14 years) of daily meteorological data. ET0 calculations were performed for the warm half of the year (1 April to 30 September).

Methodology and Data

• Models used:
  • Penman–Monteith (P-M) formula (FAO-56 method) for reference evapotranspiration (ET0).
  • Angström–Prescott (A-P) equation for global solar radiation (H).
  • Hargreaves–Sammani (H-S) equation for global solar radiation (H).
  • Michalsky’s method for calculating daily total extraterrestrial solar radiation (H0) and daily maximum sunshine duration (S0).
• Data sources:
  • Daily registers of global solar radiation (H), sunshine duration (S), and air temperature (t) from 10 stations in Poland.
  • Data obtained from the Institute of Meteorology and Water Management—Polish Research Institute (IMGW-PIB).
  • Daily minimum (tdmin) and maximum (tdmax) air temperature, and daily sums of solar radiation (H) derived from hourly data.
  • Data were divided into an 11-year calibration set and a 5-year validation set.

Main Results

• Site-calibrated coefficients for both A-P and H-S models yielded the best accuracy for global solar radiation (H) estimation, with generalized coefficients for Poland performing better than global ones.
• The Angström–Prescott (A-P) model demonstrated significantly better predictive power for solar radiation in the Polish climate (R² above 0.9 for 9/10 stations, RMSE ranging from 1.30 MJ·m⁻² to 2.67 MJ·m⁻²) compared to the Hargreaves–Sammani (H-S) model (R² above 0.8 for 8/10 stations, RMSE ranging from 3.20 MJ·m⁻² to 4.65 MJ·m⁻²).
• The method of determining the radiation component had a significant, though sometimes unexpected, impact on Penman–Monteith (P-M) reference evapotranspiration (ET0) values.
• Generally, a better predictive power of the solar radiation model (higher R², lower RMSE) correlated with better accuracy in ET0 estimation, but exceptions were observed.
• Applying A-P and H-S models with global coefficients consistently led to an overestimation of ET0 sums.
• Local calibration of the H-S model within the P-M formula improved ET0 estimations at all 10 analyzed stations, with the highest RMSE reduction of 0.0000883 m (0.0883 mm) observed at Toruń.
• Mean ET0 sums for the warm half-year (2000–2015) varied spatially across Poland, from 0.484 m (Kołobrzeg) to 0.585 m (Wieluń).

Contributions

• First determination and publication of generalized Hargreaves–Sammani coefficients for Poland, distinguishing between inland and coastal areas.
• Local calibration of Angström–Prescott and Hargreaves–Sammani models for 10 distinct sites across Poland, demonstrating improved accuracy over existing global coefficients for solar radiation estimation.
• Comprehensive assessment of the impact of various solar radiation modeling approaches (local, generalized Polish, and global coefficients) on Penman–Monteith reference evapotranspiration estimations within a temperate climate zone.
• Highlighting that improved solar radiation model accuracy does not always directly translate to improved ET0 accuracy, underscoring the necessity for cautious application of indirect methods.

Funding

• Polish Ministry of Science and Higher Education for statutory activities of the Institute of Environmental Engineering, Warsaw University of Life Sciences—SGGW.

Citation

@article{Mitrowska2025Modeling,
  author = {Mitrowska, Dorota and Kleniewska, Małgorzata and Kuchar, Leszek},
  title = {Modeling Solar Radiation Data for Reference Evapotranspiration Estimation at a Daily Time Step for Poland},
  journal = {Water},
  year = {2025},
  doi = {10.3390/w17223304},
  url = {https://doi.org/10.3390/w17223304}
}