Tornay et al. (2026) SMAP Soil Moisture Measurement in Hungary over Agricultural Land

Identification

• Journal: Periodica Polytechnica Civil Engineering
• Year: 2026
• Date: 2026-02-13
• Authors: Enikő Tornay, Zsófia Kugler
• DOI: 10.3311/ppci.41463

Research Groups

• Department of Photogrammetry and Geoinformatics, Faculty of Civil Engineering, Budapest University of Technology and Economics, Budapest, Hungary

Short Summary

This study assessed the accuracy of the NASA SMAP Level 3 soil moisture product over agricultural land in Hungary using long-term in situ measurements. It found that PM (ascending) overpasses generally showed stronger agreement with ground data, meeting the <0.04 m³/m³ unbiased Root Mean Square Error (ubRMSE) validation requirement in homogeneous areas, and that spatial averaging of in situ data significantly improved correlations.

Objective

• To assess the accuracy and suitability of NASA's Soil Moisture Active Passive (SMAP) Level 3 soil moisture product for estimating the moisture status of the upper soil layer and filling data gaps over agricultural areas of Hungary, considering varied soil and extreme precipitation conditions.

Study Configuration

• Spatial Scale: Two distinct 36 km x 36 km SMAP satellite footprint cells in Hungary, each containing up to five in situ monitoring stations. The study area covers agricultural land across Hungary.
• Temporal Scale: Long-term analysis from 1 January 2016 to 28 February 2025 (nearly ten years) of daily averaged data.

Methodology and Data

• Models used: NASA SMAP L3 Radiometer Global Daily Soil Moisture, Version 9 (L3SMP) dataset.
• Data sources:
  • Satellite: NASA Soil Moisture Active Passive (SMAP) Level 3 (L3SMP) soil moisture product, providing daily composites at 36 km spatial resolution for both descending (AM) and ascending (PM) overpasses.
  • Observation: Long-term in situ soil moisture measurements (2016–2024) from the national drought monitoring network in Hungary (General Directorate of Water Management, OVF). Data collected at 10 cm depth using Decagon 5TM and later TEROS 12 sensors.

Main Results

• PM (ascending) overpasses generally exhibited stronger agreement with in situ measurements compared to AM (descending) overpasses.
• The SMAP product met the <0.04 m³/m³ ubRMSE validation requirement threshold over several years and locations, particularly in areas with homogeneous soil conditions.
• Satellite data generally underestimated in situ measurements, indicating a negative bias.
• Averaging spatial measurements from multiple in situ stations within a SMAP raster cell significantly improved the correlation coefficient (R) by almost 30% in the more homogeneous Raster1 area.
• Higher correlation values (R) were observed for PM passes, especially in spring (0.61) and autumn (0.52), attributed to more favorable vegetation density, absence of snow/frost, and reduced atmospheric attenuation.
• Performance was lower in more heterogeneous areas (e.g., Raster2), highlighting the need for additional ground-based measurements and higher-resolution satellite data for local-scale validation.

Contributions

• Provides a comprehensive, long-term (nearly a decade) validation of the SMAP L3 soil moisture product over diverse agricultural land in Hungary, a region prone to hydrological extremes.
• Quantifies the performance differences between AM and PM SMAP overpasses and demonstrates the benefits of spatial averaging of in situ data to mitigate scale mismatch.
• Confirms the reliability of SMAP data for monitoring soil moisture in agricultural regions under varying hydro-meteorological conditions, while also identifying limitations in heterogeneous areas.
• Offers valuable insights for improving local-scale soil moisture monitoring and management of hydrological extremes in Central Europe.

Funding

• Hungarian Ministry of Culture and Innovation through the National Research, Development and Innovation Fund.

Citation

@article{Tornay2026SMAP,
  author = {Tornay, Enikő and Kugler, Zsófia},
  title = {SMAP Soil Moisture Measurement in Hungary over Agricultural Land},
  journal = {Periodica Polytechnica Civil Engineering},
  year = {2026},
  doi = {10.3311/ppci.41463},
  url = {https://doi.org/10.3311/ppci.41463}
}