December
11, 2006 They examined data from the TOPEX/Poseidon satellite, taken
from four selected sites around TOPEX/Poseidon measures the altitude of objects with radar. Since 1992, the joint mission of NASA and the French Space Agency has used this technology, called altimetry, to measure global sea height. In the Graduate student Hyongki Lee developed the technique to chart how the wetland water level changed over the ten years. He presented the results in a poster session Tuesday, December 12, 2006 at the American Geophysical Union annual meeting in San Francisco. This preliminary study doesn't say much about the general
state of the These kinds of observations could one day help quantify the depletion or rise of water level in wetlands. In this first study, we were mainly just trying to show that the technique works," Lee said. Wetlands pose two challenges to radar altimeter studies, he explained. Wetland plants diffuse the radar signal, while open patches of water are often so calm that their surface reflects the signal very strongly. Strong signals confuse the satellite's onboard receiver, so the software that converts the radar signal to height measurements doesn't work properly. Scientists believe wetlands are important ecologically,
because they filter
wastes out of the water. They also provide a barrier between the land
and large
bodies of water, such as the But gauging the health of wetlands can pose a challenge. The interior regions are hard to reach, which makes it difficult for scientists to measure water levels manually. Most in-ground gauges are placed in easily accessible areas along the edges. Case in point: two of the four locations imaged by radar in this study were deep within the wetlands, with the closest in-ground water gauge miles away. That posed a difficulty for Shum's team as they tried to determine whether their satellite data was accurate. "Some of the gauges were quite far away, but at least we were able to show a qualitative agreement with seasonal changes," Shum said. In future studies, they want to examine data from more locations, and couple the data with more ground measurements. "Because of the ground track layout for TOPEX/Poseidon, most times one has to be lucky that the tracks fall on the wetlands. We may try using other radar altimeter satellites to improve the spatial resolution of the wetland study. However, the primary limitation for TOPEX/Poseidon is its narrow measurement swath on the ground," Shum said. The satellite gathers data in a strip, up to only 3 miles wide. Shum pointed to the proposed WATER HM (short for Water And
Terrestrial
Elevation Recovery Hydrosphere Mapper) satellite mission, led by Doug
Alsdorf, assistant professor of earth sciences at In the meantime, Ohio State hydrologist Motomu Ibaraki plans to use the TOPEX/Poseidon radar data to model how water moves through the wetlands; other scientists could use those models in ecological studies. Lee did this work as part of his Ph.D. dissertation. Yuchan Yi, the lead author of the study, performed the detailed analysis of the satellite altimeter data. This work was funded by NASA through the Earth Science Enterprise Observing System Interdisciplinary Science Program and the Jason Project. ##
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