Microwave remote sensing of soil moisture: evaluation of the TRMM microwave imager (TMI) satellite for the Little River Watershed Tifton, Georgia

Soil moisture plays a critical role in many hydrological processes including infiltration, evaporation, and runoff. Satellite-based passive microwave sensors offer an effective way to observe soil moisture conditions over vast areas. There are currently several satellite systems that can detect soil moisture. Calibration, validation, and characterization of data received from these satellite systems are an ongoing process. One approach to these requirements is to collect and compare long-term in situ (field) measurements of soil moisture with remotely sensed data. The in situ measurements for this paper were collected at the Little River Watershed (LRW) Tifton, Georgia and compared to the Tropical Rainfall Measurement Mission Microwave Imager (TMI) 10.65 GHz vertical and horizontial (V and H) sensors and vegetation density Normalized Difference Vegetation Index (NDVI) from the Moderate Resolution Imaging Spectroradiometer (MODIS) for the period from 1999 through 2002. The in situ soil moisture probes exist in conjunction with rain gauge stations throughout the sampling region. It was found that the TMI was able to observe soil moisture conditions when vegetation levels were low. However, during several months each year high vegetation levels mask the soil moisture signal from the TMI. When the monthly averaged observation from the TMI, MODIS, and in situ probes were subjected to a multivariable comparison the correlation value increased slightly, improving the accuracy of the TMI—soil moisture correlation. Our results show that the TMI estimate would not result in an adequate monitoring of large land areas but when used in conjunction with other satellite sensors and in situ networks and model output can constitute an effective means of monitoring soil moisture of the land surface.