Ssembajwe et al. (2025) Assessment and Validation of FAPAR, a Satellite-Based Plant Health and Water Stress Indicator, over Uganda

Identification

• Journal: Remote Sensing
• Year: 2025
• Date: 2025-10-21
• Authors: Ronald Ssembajwe, Amina Twah, Godfrey H. Kagezi, Tuula Löytty, Judith Kobusinge, Anthony Gidudu, Geoffrey Arinaitwe, Qingyun Du, Mihai Voda
• DOI: 10.3390/rs17203501

Research Groups

• College of Agricultural and Environmental Sciences, Makerere University, Uganda
• National Coffee Research Institute, National Agricultural Research Organization, Uganda
• Smart & Lean Hub Oy (S&L), Finland
• College of Engineering, Design Art and Technology, Makerere University, Uganda
• School of Resource and Environmental Sciences, Wuhan University, China
• Faculty of Geography, Dimitrie Cantemir University, Romania

Short Summary

This study assessed and validated satellite-based Fraction of Absorbed Photosynthetically Active Radiation (FAPAR) as a plant health and water stress indicator over Uganda, finding it to be a robust proxy with strong correlations to established drought and water stress indices. The research revealed increasing photosynthetic activity and FAPAR-centered stress across significant portions of the country, influenced by climatic and land use factors.

Objective

• To assess, compare, and validate satellite-based Fraction of Absorbed Photosynthetically Active Radiation (FAPAR) as a plant health and water stress indicator over Uganda.
• To explore the spatiotemporal variability of FAPAR across Uganda.
• To determine trends in FAPAR and FAPAR-centered stress indices (Standardized FAPAR Index (SFI) < −1).
• To compare and indirectly validate FAPAR using the Standardized Precipitation and Evapotranspiration Index at scale 3 (SPEI-03) and the crop water stress index (CWSI) over Uganda at high spatial resolutions.

Study Configuration

• Spatial Scale: Uganda, with FAPAR data at approximately 5 km resolution and precipitation/evapotranspiration data at approximately 4 km resolution.
• Temporal Scale: 1983–2013 (31 years), using daily and monthly temporal resolutions.

Methodology and Data

• Models used:
  • Standardized FAPAR Index (SFI)
  • Standardized Precipitation and Evapotranspiration Index at scale 3 (SPEI-03)
  • Crop Water Stress Index (CWSI)
  • k-Means clustering for spatial variability analysis
  • Modified Mann–Kendall trend test and Sen’s slope estimator for trend analysis
  • Pearson correlation coefficient for association analysis
• Data sources:
  • Satellite: Daily FAPAR (AVH15C1) from NOAA-CDR.
  • Reanalysis: Monthly precipitation and actual/potential evapotranspiration data from TerraClimate.

Main Results

• Uganda's photosynthetic activity (FAPAR) (1983–2013) is characterized by six classes, with Class 4 (0.36–0.45 units) dominating 47% of the total land area, primarily spanning cropland.
• Significant increases in monthly FAPAR were observed over 85% of the total land area, with the strongest trend magnitudes reaching 1.8 × 10⁻⁵ per month.
• FAPAR-centered stress (SFI < −1) showed increasing trends over 60% of the total land area, with notable magnitudes up to 7 × 10⁻⁵ per month.
• The Standardized FAPAR Index (SFI) exhibited moderate to strong positive correlations (r = 0.5–0.9) with SPEI-03 over 80% of the total land area, particularly in cropland, grassland, and forest regions.
• SFI showed moderate to strong negative correlations (r = −0.6 to −0.9) with CWSI over less than 80% of the total land area, especially in cropland, grassland, and forest areas.
• The observed distribution and patterns of photosynthetic activity are jointly governed by precipitation, evapotranspiration, land use/land cover (LULC), and teleconnections.

Contributions

• Provides the first comprehensive assessment and validation of satellite-based FAPAR as a plant health and water stress indicator specifically for Uganda.
• Offers detailed insights into the spatiotemporal variability and long-term trends of FAPAR and FAPAR-centered stress indices across Uganda.
• Indirectly validates FAPAR using established agricultural drought (SPEI-03) and plant water stress (CWSI) indicators at high spatial resolutions, enhancing its applicability in the region.
• Recommends NOAA-AVHRR FAPAR (AVH15C1) as a robust proxy for vegetation health and water stress monitoring over Uganda, with potential applications in crop yield prediction and irrigation management.
• Generates critical information for accurate crop condition monitoring, effective agricultural planning, and drought management for resilience building in Uganda.

Funding

• Dimitrie Cantemir University IR-BE-200465 project
• European Commission through the Erasmus+ student exchange program
• RCMRD-GMES Africa Project

Citation

@article{Ssembajwe2025Assessment,
  author = {Ssembajwe, Ronald and Twah, Amina and Kagezi, Godfrey H. and Löytty, Tuula and Kobusinge, Judith and Gidudu, Anthony and Arinaitwe, Geoffrey and Du, Qingyun and Voda, Mihai},
  title = {Assessment and Validation of FAPAR, a Satellite-Based Plant Health and Water Stress Indicator, over Uganda},
  journal = {Remote Sensing},
  year = {2025},
  doi = {10.3390/rs17203501},
  url = {https://doi.org/10.3390/rs17203501}
}