Abstract:
Soil moisture is a critical parameter for predicting and detecting floods and
droughts, as well as indicating crop and vegetation health. Current indicators utilize
surrogate or modeled measures of soil moisture. Actual observed soil moisture
measurements have the potential to improve understanding of floods, droughts, and crop
health.
In this study, ground soil moisture daily average values were compared to
estimates obtained from two microwave sensors, the EOS Aqua Advanced Microwave
Scanning Radiometer (AMSR-E) and the Tropical Rainfall Measurement Mission
Microwave Scanning Radiometer (TMI), as well as one optical sensor, the EOS Aqua
Moderate Resolution Imaging Spectroradiometer (MODIS). The study areas were the
Little Washita River Experimental Watershed in Oklahoma and the Little River
Experimental Watershed in Georgia. This research compared AMSR-E, TMI, and
MODIS data to ground data from the Little Washita Berg station and also compared
AMSR-E and TMI data to ground data from the Little River Soil Climate Analysis
Network station.
AMSR-E and TMI performed better in Little Washita than in Little River during
the crop-covered season. This may be due to the vegetation type, distribution, and
density at Little River. AMSR-E exhibited a smaller range of variability than the TMI or
in-situ measurements at both study sites for all time periods. In the crop-covered season
of June, July, and August of 2003, MODIS soil moisture retrieval at the Little Washita
site correlated better (R^2 = 0.772) with the in-situ measurements than AMSR-E or TMI
soil moisture retrievals. The spatial resolution of MODIS (1 km) is finer than the spatial
resolution of AMSR-E (~25 km) or TMI. Spatial resolution is an important factor
because topography, soil properties, and vegetation cover may vary significantly over
satellite footprints. Both microwave sensors are limited by their coarse spatial resolution.
However, optical measurements are limited to cloud-free conditions. Future work
includes research on algorithms which combine optical and microwave measurements to
provide the advantages of each.
