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United States Department of Agriculture

Agricultural Research Service

Title: Potential use of aquarius scatterometer observations to estimate vegetation water content

item Zhao, T
item Jackson, Thomas
item Bindlish, R
item O'neill, P

Submitted to: BARC Poster Day
Publication Type: Abstract Only
Publication Acceptance Date: 3/27/2012
Publication Date: 4/1/2012
Citation: Zhao, T., Jackson, T.J., Bindlish, R., O'Neill, P.E. 2012. Potential use of aquarius scatterometer observations to estimate vegetation water content. BARC Poster Day. 2012 CDROM.

Interpretive Summary:

Technical Abstract: Information about vegetation water content (VWC) is useful in agriculture, forestry and hydrology. It will be also employed in several of the soil moisture retrieval algorithms. All of these algorithms utilize variations of the same radiative transfer equation that accounts for vegetation attenuation with a vegetation optical depth parameter (tau). Earlier work has established a strong relationship between tau and VWC of the canopy, which is vegetation type dependent. Furthermore, vegetation indices such as the Normalized Difference Vegetation Index (NDVI) derived from visible-near infrared satellite sensors has been widely used to estimate VWC. Aquarius/SAC-D is a new combined passive/active L-band microwave instrument that was launched on 10 June 2011. It provides brightness temperature and backscattering measurements for three adjacent footprints at different incidence angles (28.7°, 37.8°, 45.6°) at a resolution of about 100 km. Aquarius is the first sensor that provides concurrent active/passive L-band observations globally. We are exploring the use of Aquarius for soil moisture mapping, although it is not a primary mission product. In addition to the use of optical data, a potentially better way of estimating VWC is to use Aquarius scatterometer data based on L-band active microwave technology, which has a greater penetration depth and is more sensitive to vegetation variation. Backscatter observations over vegetated areas are a function of vegetation biomass, surface roughness and soil moisture. There have been numerous studies that have indicated the sensitivity of backscatter to vegetation parameters. What has been lacking is a robust relationship that relates backscatter measurements to vegetation parameters. In this investigation we will focus on an index called the Radar Vegetation Index (RVI). RVI has been shown to be predominantly a function of vegetation biomass. Previous studies have found a good relationship between VWC and RVI for agricultural areas. The validity of the relationship will be explored at regional and global scales based on Aquarius observations. The VWC data was derived using satellite estimates of NDVI, which was developed as one of the ancillary datasets of Soil Moisture Active Passive (SMAP) mission. The global patterns of RVI and VWC are compared and seasonal phenological relationships are developed for different biomes. Prior studies have examined this relationship using observations made at a constant incidence angle. This work will consider the incidence angle effects and will attempt incorporate this into the relationship. This study would provide us with the basis for a combined active and passive microwave soil moisture estimation model for Aquarius and SMAP missions.

Last Modified: 06/22/2017
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