Estimation of Root Zone Soil Moisture Using Apparent Thermal Inertia With MODIS Imagery Over a Tropical Catchment in Northern Thailand

Understanding the variability of soil moisture content (SMC) is important for studying ecohydrological processes because it provides insights into surface water and energy balances. To comprehend the dynamics of SMC under different land use/cover types in tropical environments, this study proposes an apparent thermal inertia (ATI) approach with the Moderate Resolution Imaging Spectroradiometer (MODIS) imagery for estimating root zone SMC. Root zone SMC at depths of 10 cm, 100 cm, and 200 cm were estimated for seven sites over a northern Thailand catchment from 2005 to 2008, and compared with in situ observations. Pearson correlation coefficient and the Nash-Sutcliffe efficiency coefficient between the ATI-MODIS retrieved SMC and the measurements were respectively 0.80, 0.84, and 0.84, and 0.57, 0.537, and 0.492 for the 10 cm, 100 cm, and 200 cm depths. Root Mean Square Errors were 0.055, 0.025 and 0.029 (m³ m^-3) for the three respective depths. Potential issues of the mixed land cover types within the 1 km MODIS pixel were examined; the mixed agricultural land cover types at two of the seven sites with agricultural activities such as irrigation water use might have affected the SMC estimation. Overall, the ATI-MODIS approach performed well, particularly for the 100 cm depth.