The potential of multitemporal Aqua and Terra MODIS apparent thermal inertia as a soil moisture indicator

Variations in surface thermal inertia-the resistance to temperature variations-can be indicative for variations in soil moisture. In this paper, we present a flexible multitemporal approach to derive an approximation of thermal inertia, called apparent thermal inertia (ATI), from daily Aqua and Terra MODIS observations. In a first step, a varying number of land surface temperature measurements were, together with the time of observation, fit to a sinusoidal function to obtain diurnal surface temperature amplitudes. These were subsequently combined with surface albedo to derive ATI. This was done for the southern part of the African continent for the year 2009. Apparent thermal inertia was compared both spatially and temporally to AMSR-E soil moisture, generated by the algorithm developed by the Vrije Universiteit Amsterdam and NASA. The temporal behaviour of apparent thermal inertia, derived using MODIS data only, showed a strong correspondence to that of AMSR-E soil moisture, especially in arid and semi-arid environments. The approach showed some limitations for vegetated terrains. Further post-processing is required to filter meteorologically induced noise and to transform ATI to actual soil moisture content.