Tropical SMAP Soil Moisture Validation using Triple Collocation: Analysing the error
Chan, F.C. (a) , Mohd Reba, M.N. (a*)

a) Faculty of Built Environment and Surveying, Universiti Teknologi Malaysia (UTM), 81310 UTM Johor Bahru, Johor, Malaysia
*nadzri[at]utm.my

Abstract

Accurate soil moisture (SM) estimation is critical for understanding the land-atmosphere interactions, hydrological processes, and climate dynamics. Yet, the accurate SM retrieval in tropical areas remains challenging due to the heterogeneity of land cover and precipitation dynamics. Conventionally, satellite-retrieved SM products are validated against the collocated in-situ SM measurements, which are often assumed as the ground truth. However, the spatial representation error owing to the point-scale in situ measurements and areal-scale satellite footprint can degrade the reliability of in-situ measurements as a benchmark in the validation. Triple Collocation-based validation is found to minimize the scale issue between grid-scale satellite SM and point-scale in-situ SM. This study aims to expand the validation of SMAP SM products beyond conventional in-situ comparisons by employing TCA in a tropical environment. The objectives are twofold: (1) to evaluate the error characteristics of SMAP- (2) to investigate the potential errors introduced by precipitation and vegetation density in SMAP products. The study attempts to minimize the confounding factors related to surface heterogeneity by focusing on a homogeneous land cover. This study includes the hourly in-situ measurements, SM products from Soil Moisture Active Passive (SMAP) at 9 km spatial resolution, and modelled SM products from ERA5-Land reanalysis from March 2024 to March 2025 in a homogeneous tropical agricultural site. Results show SMAP demonstrates moderate correlation (R = 0.406) but a negative bias (-0.022m3/m3), and higher ubRMSE (0.0337m3/m3), attributed to signal attenuation under dense vegetation and rapid moisture fluctuations. Precipitation intensifies SMAP errors (ubRMSE peaks at 0.065m3/m3) during heavy rainfall. This study affirms the utility of TCA for SM validation in tropical areas and provides insights into the performance of SMAP in understudied tropical regions. The findings aim to contribute to the development of SM retrieval algorithms.

Keywords: SMAP, soil moisture, passive microwave remote sensing, triple collocation analysis, tropics

Topic: Topic A: General Remote Sensing