SMAP Soil Moisture Measurement in Hungary over Agricultural Land

Abstract

Monitoring spatial and temporal dynamics of soil moisture is essential for managing hydrological extremes such as droughts and inland excess water. This study assesses the accuracy of NASA's Soil Moisture Active Passive (SMAP) Level 3 soil moisture product using long-term in situ measurements (2016–2024) from the national drought monitoring network in Hungary. Two distinct 36 km satellite footprint cells were extensively analyzed, representing different land use and soil type conditions.
Daily averaged in situ soil moisture data measured in the upper 10 cm layer were compared to SMAP observations for both AM and PM overpasses. Statistical metrics used to evaluate performance include the Pearson correlation coefficient (R), bias, root mean square error (RMSE), and unbiased RMSE (ubRMSE). Results indicate that PM values generally exhibit stronger agreement with in situ measurements.
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. However, variability in soil types and topography in more heterogeneous regions highlights the need for additional ground-based measurements and higher-resolution satellite data to enhance local-scale validation.