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United States Department of Agriculture

Agricultural Research Service

Title: Upscaling Ground Observations of Vegetation Water Content, Canopy Height, and Leaf Area Index During Smex 02 Using Aircraft and Landsate Imagery

Authors
item Anderson, Martha - UNIV OF WI
item Neile, Christopher - UTAH STATE UNIV
item Li, Fuqin
item Norman, John - UNIV OF WI
item Kustas, William
item Jayanthi, H. - UTAH STATE UNIV
item Chavez, Jose - UTAH STATE UNIV

Submitted to: Remote Sensing of Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 3, 2004
Publication Date: September 30, 2004
Citation: Anderson, M.C., Neale, C.C.M., Li, F., Norman, J., Kustas, W.P., Jayanthi, H., Chavez, J.L. 2004. Upscaling ground observations of vegetation water content, canopy height, and leaf area index during SMEX02 using aircraft and Landsat imagery. Remote Sensing of Environment. 92:447-464.

Interpretive Summary: Microwave-based remote sensing algorithms for mapping soil moisture are sensitive to water contained in surface vegetation at moderate levels of canopy cover. Correction schemes require spatially distributed estimates of vegetation water content at scales comparable to that of the microwave sensor pixel resolution of 1 to 10 km. This study compares the relative utility of high-resolution (1.5 m) aircraft and coarser-resolution (30 m) Landsat imagery in upscaling an extensive set of ground-based measurements of canopy biophysical properties collected during the Soil Moisture Experiment of 2002 (SMEX02) within the Walnut Creek Watershed. The upscaling was accomplished using nonlinear relationships developed between spectral vegetation indices and measurements of leaf area index, canopy height and vegetation water content. Of the various indices examined, the Normalized Difference Water Index (NDWI), derived from near- and shortwave-infrared reflectances, was found to be least susceptible to saturation at high levels of leaf area index. With the aircraft dataset, which did not include a short-wave infrared band, the Optimized Soil Adjusted Vegetation Index (OSAVI) yielded best correlation with observations and highest saturation levels. Both indices were used to estimate field-scale mean canopy properties and variability for each of the intensive soil-moisture-sampling sites within the watershed study area. While the aircraft data provide good, detailed maps of in-field variability, retrievals of canopy properties based on the OSAVI index underestimate properties in corn near the end of the experiment due to saturation of the VI as the corn entered the tasseling stage. The NDWI did not show signs of saturation during SMEX02; retrievals based on Landsat NDWI data will therefore be useful for mapping canopy properties at scales coarser than 30 m.

Technical Abstract: Microwave-based remote sensing algorithms for mapping soil moisture are sensitive to water contained in surface vegetation at moderate levels of canopy cover. Correction schemes require spatially distributed estimates of vegetation water content at scales comparable to that of the microwave sensor footprint (101 to 104 m). This study compares the relative utility of high-resolution (1.5 m) aircraft and coarser-resolution (30 m) Landsat imagery in upscaling an extensive set of ground-based measurements of canopy biophysical properties collected during the Soil Moisture Experiment of 2002 (SMEX02) within the Walnut Creek Watershed. The upscaling was accomplished using expolinear relationships developed between spectral vegetation indices and measurements of leaf area index, canopy height and vegetation water content. Of the various indices examined, the Normalized Difference Water Index (NDWI), derived from near- and shortwave-infrared reflectances, was found to be least susceptible to saturation at high levels of leaf area index. With the aircraft dataset, which did not include a short-wave infrared band, the Optimized Soil Adjusted Vegetation Index (OSAVI) yielded best correlation with observations and highest saturation levels. At the observation scale (10 m), LAI was retrieved from both NDWI and OSAVI imagery with an accuracy of 0.6, vegetation water content at 0.7 kg m-2, and canopy height to within 0.2 m. Both indices were used to estimate field-scale mean canopy properties and variability for each of the intensive soil-moisture-sampling sites within the watershed study area. While the aircraft data provide good, detailed maps of in-field variability, retrievals of canopy properties based on the OSAVI index underestimate properties in corn near the end of the experiment due to saturation of the VI as the corn entered the tasseling stage. The NDWI did not show signs of saturation during SMEX02; retrievals based on Landsat NDWI data will therefore be useful for mapping canopy properties at scales coarser than 30 m. Results regarding scale invariance over the SMEX02 study area in transformations from band reflectance and vegetation indices to canopy biophysical properties are also presented.

Last Modified: 9/2/2014
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