Bozorgi et al. (2025) Assessing the performance of multi-timescale drought indices for monitoring agricultural drought impacts on wheat yield

Identification

• Journal: Agricultural Water Management
• Year: 2025
• Date: 2025-12-18
• Authors: Mahsa Bozorgi, Jordi Cristóbal, Jaume Casadesús
• DOI: 10.1016/j.agwat.2025.110092

Research Groups

• Efficient Use of Water in Agriculture Program, Institute of AgriFood, Research and Technology (IRTA), Lleida, Spain.
• Department of Geography, Universitat Autònoma de Barcelona (UAB), Bellaterra, Spain.

Short Summary

This study evaluates the performance of three multi-timescale drought indices (SPEI, SPET, and SEDI) in monitoring rainfed wheat yield variability across four Spanish regions. The findings identify the Standardized Evapotranspiration Deficit Index (SEDI) as the most robust indicator for agricultural drought impact assessment.

Objective

• To determine which drought index (meteorological vs. remote sensing-based) best explains wheat yield response to drought and to identify the specific accumulation timescales that most effectively capture yield variability in different agroclimatic regions of Spain.

Study Configuration

• Spatial Scale: Regional scale covering four autonomous communities in Spain: La Rioja, Castilla y León, Castilla-La Mancha, and Andalucía.
• Temporal Scale: 19 years, spanning from 2003 to 2021.

Methodology and Data

• Models used: Two-Source Energy Balance (TSEB) model for estimating actual crop evapotranspiration ($ET{c\ act}$); FAO-56 Penman-Monteith equation for reference evapotranspiration ($ETo$).
• Data sources: MODIS Aqua satellite observations (1 km resolution) for biophysical variables; ERA5 reanalysis ($0.25^\circ$ resolution) for meteorological forcing; AEMET ROCÍO gridded precipitation (5 km resolution); ESYRCE farm-level survey for wheat yield data.
• Drought Indices:
  • Standardized Precipitation-Evapotranspiration Index (SPEI) based on $P - ETo$.
  • Standardized Precipitation-Actual Evapotranspiration Index (SPET) based on $P - ET{c\ act}$.
  • Standardized Evapotranspiration Deficit Index (SEDI) based on $ET{c\ act} - ETo$.
• Analysis: Yield data were detrended using quadratic polynomials; Spearman’s rank correlation ($\rho$) was used to assess the relationship between indices (at 1–12 month timescales) and standardized yield residuals.

Main Results

• Index Performance: SEDI exhibited the strongest and most consistent correlation with wheat yield variability across the study regions.
• Regional Correlations: The strongest yield–drought correlations were found in La Rioja ($r = 0.81$ for SEDI at a 6-month timescale in July), while the weakest were in Andalucía ($r \approx 0.33–0.35$).
• Timescale Sensitivity:
  • Mediterranean regions (Andalucía and Castilla-La Mancha) showed peak sensitivity at short timescales (1–3 months) during late spring.
  • Temperate continental regions (Castilla y León and La Rioja) responded more strongly to longer timescales (3–6 months) in early summer.
• Quantitative Validation: Validation of the underlying $ET_{c\ act}$ model against eddy covariance towers showed a mean bias of 1.16 mm/day and an RMSE of 1.76 mm/day.

Contributions

• Provides a systematic, long-term evaluation of both meteorological and remote sensing-based drought indices against farm-level yield data in a Mediterranean context.
• Demonstrates that integrating remotely sensed actual evapotranspiration ($ET_{c\ act}$) significantly improves the characterization of crop water stress compared to indices relying solely on precipitation or potential evapotranspiration.
• Identifies region-specific drought accumulation timescales, offering a framework for more precise operational agricultural water management and yield forecasting.

Funding

• Ministry of Science and Innovation (MICIN-AEI), Spain: Project ET4DROUGHT (No. PID2021–127345OR-C31).
• Ministry of Science and Innovation (MICIN-AEI), Spain: Project DigiSPAC (No. TED2021–131237B-C21).

Citation

@article{Bozorgi2025Assessing,
  author = {Bozorgi, Mahsa and Cristóbal, Jordi and Casadesús, Jaume},
  title = {Assessing the performance of multi-timescale drought indices for monitoring agricultural drought impacts on wheat yield},
  journal = {Agricultural Water Management},
  year = {2025},
  doi = {10.1016/j.agwat.2025.110092},
  url = {https://doi.org/10.1016/j.agwat.2025.110092}
}

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