Evaluating land surface moisture conditions from the remotely sensed temperature/vegetation index measurements: An exploration with the simplified simple biosphere model

Abstract

Land soil moisture conditions play a critical role in evaluating terrestrial environmental conditions related to ecological, hydrological, and atmospheric processes. Extensive efforts to exploit the potential of remotely sensed observations to help quantify this complex variable are still underway. Among the various methods, several investigators have explored a combination of surface temperatures and spectral vegetation index (SVI) measurements, the TVX method, as a means to account for the variable influence of vegetation cover in soil moisture assessment. Although considerable empirical evidence has been presented exploring the potential of TVX methods to assess regional moisture conditions, less attention has been given to assessing the underlying biophysics of the observed TVX patterns. In this study, the Simplified Simple Biosphere (SSiB) model is exploited to examine the factors that lead to the observed TVX relation. For a range of typical, midlatitude, growing season conditions, the SSiB model produces the expected TVX relationship, surface temperature decreases with increasing SVI values. The most critical factors that cause the TVX relation to vary include near-surface soil moisture (2 cm), incident radiation (IR), and, to a lesser degree, wind speed. Whereas many empirical studies have suggested that the slope of the TVX relation may provide an important diagnostic of soil moisture conditions, in this analysis, the impact of plant stomatal function is shown to confuse this interpretation of the TVX slope. However, other aspects of the TVX metrics, specifically bare soil temperature and canopy temperature, do provide diagnostic near-surface soil moisture information. Growing season variations in TVX metrics were examined for the conditions recorded at the Hydrological and Atmospheric Pilot Experiment—Modelisation du Bilan Hydrique (HAPEX-Mobilhy) study site. The results from this analysis indicate that soil and canopy temperatures vary as a function of soil moisture conditions and, to a lesser degree, as a result of varying solar insolation and wind speed. The results also show that the TVX metrics are able to provide daily soil moisture variation up to 2 cm of soil depth and seasonal trend up to 10 cm. Using the satellite-derived surface temperatures and a SSiB-derived retrieval equation, the retrieved soil moistures at the HAPEX-Mobilhy site generally closely approximate the conditions recorded on the ground.