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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 Jackson, Thomas
item Bindlish, Rajat
item Zhao, Tianjie
item O'neill, Peggy

Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: 2/1/2012
Publication Date: 3/5/2012
Citation: Jackson, T.J., Bindlish, R., Zhao, T., O'Neill, P. 2012. Potential use of aquarius scatterometer observations to estimate vegetation water content[abstract]. 12th Specialist Meeting on Microwave Radiometry and Remote Sensing of Environment. 2012 CDROM.

Interpretive Summary:

Technical Abstract: Aquarius is a combined passive/active L-band microwave satellite 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, and 45.6 degrees) 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. There are several options available for performing soil moisture retrieval, all of which utilize variations of the same radiative transfer equation that accounts for vegetation attenuation with a vegetation optical depth parameter (tau). Tau can be estimated using ancillary data or retrieved if the appropriate number of independent microwave channels is available. Earlier work established a strong relationship between tau and the vegetation water content (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 sensors has been widely used to estimate VWC. While useful and readily available on a global basis, optical sensors have a number of limitations: observations can only be made during the day; observations are affected by the presence of atmosphere and cannot see through clouds; and NDVI observations saturate at high vegetation levels limiting their ability to estimate higher VWC, which are needed in soil moisture retrieval algorithms. In this study, we will investigate the use of Aquarius scatterometer to derive vegetation information. 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. A previous studies found a good relationship between VWC and RVI for agricultural areas. The validity of the relationship will be explored at regional and global scales using Aquarius observations. The VWC data was derived using satellite estimates from 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 to incorporate this into the relationship. This study will contribute to establishing a basis for a combined active and passive microwave soil moisture estimation model for the Aquarius and SMAP missions. USDA is an Equal Opportunity Employer.

Last Modified: 06/23/2017
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