Data Set ID:
AQ3_WKSM

Aquarius L3 Gridded 1-Degree Weekly Soil Moisture, Version 5

This data set contains Level-3 gridded weekly global soil moisture estimates derived from the NASA Aquarius passive microwave radiometer on the Satélite de Aplicaciones Científicas (SAC-D).

Version Summary:

Changes to this version include:

  • Updated Version 5 Aquarius brightness temperature data were used as input
  • File-level metadata were modified to more closely align with Climate and Forecast (CF) metadata
    conventions

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Geographic Coverage

Parameter(s):
  • Soils > Soil Moisture/Water Content
Spatial Coverage:
  • N: 90, S: -90, E: 180, W: -180

Spatial Resolution:
  • 1 Degree x 1 Degree
Temporal Coverage:
  • 25 August 2011 to 7 June 2015
Temporal Resolution: 7 day
Data Format(s):
  • XML
  • HDF
Platform(s) SAC-D
Sensor(s): AQUARIUS_RADIOMETER
Version: V5
Data Contributor(s): Rajat Bindlish, Thomas Jackson

Data Citation

As a condition of using these data, you must cite the use of this data set using the following citation. For more information, see our Use and Copyright Web page.

Bindlish, R. and T. Jackson. 2018. Aquarius L3 Gridded 1-Degree Weekly Soil Moisture, Version 5. [Indicate subset used]. Boulder, Colorado USA. NASA National Snow and Ice Data Center Distributed Active Archive Center. doi: https://doi.org/10.5067/1VZM9975ARHP. [Date Accessed].

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The Aquarius L3 Gridded 1-Degree Weekly Soil Moisture Data are produced by NASA Goddard Space Flight Center's Aquarius Data Processing Segment (ADPS).

Detailed Data Description

Format

Data are provided in Hierarchical Data Format 5 (HDF5) files. Data values are stored as bytes, 2-byte integers, and 4-byte floats. Soil moisture data are 32-bit float and color palette values are 8-bit unsigned integer. Each data file is paired with an associated XML file. XML files contain file level metadata and location, platform, and campaign information.

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File and Directory Structure

Data are available at: 

https://n5eil01u.ecs.nsidc.org/AQUARIUS/AQ3_WKSM.005/

Data files are organized in directories by date in YYYY.MM.DD format, for example: 

/2011.08.25/*
/2011.08.27/
/2011.09.03/
/2012.09.10/

* Note: The weekly file for 25 August 2011 coincides with the beginning of the Aquarius mission and therefore only contains data from 25 August 2011 through 26 August 2011.

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File Naming Convention

All files are named according to the convention shown in the following example. File name variables are defined in Table 1. 

Example
Q20142442014273.L3m_WK_SOILM_V5.0_rad_sm_1deg

Naming Convention
QYYYYDDDyyyyddd.L3m_TT_SOILM_V5.0_rad_sm_1deg

Table 1. File Naming Convention
Variable Description
Q Indicates Aquarius instrument
YYYYDDD Start date (4-digit year, 3-digit day)
yyyyddd End date (4-digit year, 3-digit day)
L3m Processing level (Level-3 mapped)
TT Observation period: 
WK: Weekly
SOILM Parameter (SOILM: soil moisture)
V5.0 Data version number (V5.0)
rad_sm_1deg 1-degree radiometer soil moisture

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File Volume

The total volume for all Aquarius L3 weekly soil moisture files is approximately 70 MB.

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Spatial Coverage

The spatial coverage for this data set is global.

Spatial Resolution

Data are 1-degree spatial resolution.

Projection and Grid Description

The main HDF5 data group, called l3m_data, is a two-dimensional array (180 rows, 360 columns) of an Equidistant Cylindrical projection (also known as Plate Carrée) of the globe. 

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Temporal Coverage

The temporal coverage for this data set spans from 25 August 2011 through 07 June 2015.

Due to a power failure on the Satélite de Aplicaciones Científicas (SAC)-D spacecraft on 08 June 2015, data from the NASA Aquarius instrument are no longer being produced. For more information on this event, refer to the official NASA announcement, International Spacecraft Carrying NASA’s Aquarius Instrument Ends Operations. The NASA National Snow and Ice Data Center Distributed Active Archive Center (NSIDC DAAC) will continue to distribute Aquarius soil moisture and polar-gridded data sets for the full duration of the mission, 25 August 2011 through 07 June 2015.

Temporal Resolution

The temporal resolution for this data set is weekly.

Note: Weekly data are provided in 7-day intervals beginning 27 August 2011 until the end of the mission in 2015. File names specify start and end dates for Aquarius coverage. For example, the following weekly file contains data from Day of Year (DOY) 239, 2014 to DOY 245, 2014: Q20142392014245.L3m_7D_SOILM_V4.0_rad_sm_1deg. 

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Parameter or Variable

This Level-3 product is a representation of a binned data product generated from Aquarius data. The HDF5 data group called l3m_data represents mean soil moisture at each grid point. 

Parameter Description

Each Level-3 soil moisture file contains the group l3m_data with the attributes described in the following table.

Table 2. Attributes of the l3m_data Group
Name Value
Scaling linear
Scaling Equation1 (Slope*l3m_data) + Intercept = Parameter value
Slope 1.0
Intercept 0
_FillValue -32767.0
add_offset 0.0
scale_factor 1.0
Scaling equations are typically provided to rescale the data for plotting purposes; however, in this case the scaling equation results in no change to the data values, and therefore can be disregarded.

The group called palette in each data file is the color palette used in the graphics generated from the mapped files. This, or any other palette, can be used with these data.

Metadata are included as global attributes within each data file and have been adapted for Version 5 to more closely align with Climate and Forecast (CF) metadata conventions. A total of 68 metadata fields are provided, such as minimum and maximum data values, units, and platform and projection information. Values for some data fields may vary from granule to granule. 

Figure 1 shows the average soil moisture estimates for the first week of September 2011.

Sample Data Image
Figure 1. This plot shows Aquarius soil moisture estimates acquired using all three beams for the first week of September 2011.


For additional soil moisture map images, see the NASA Aquarius Gallery: Soil Moisture.

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Software and Tools

The following resources provide access to software for reading and viewing HDF5 data files: 

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Data Acquisition and Processing

Theory of Measurements

The Aquarius Single Channel Algorithm (SCA) uses the L-band horizontally polarized (h-pol) brightness temperature observations due to the higher sensitivity of this channel to soil moisture. The Aquarius SCA approach is based on the simplified radiative transfer model developed under the assumption that the canopy and soil temperatures are the same (Jackson 1993). The SCA is applied to the individual Aquarius footprint Level-2 brightness temperature observations to produce a swath-based time-ordered product. (Bindlish and Jackson 2013; Bindlish et al. 2013). Details on these steps are provided in the Aquarius L2 Soil Moisture documentation.

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Data Acquisition Methods

This Version 5 Aquarius L3 soil moisture data set is generated from brightness temperature measurements included in the NASA Aquarius Level-2 Sea Surface Salinity & Wind Speed Data V5.0 product. The best quality data are selected for each orbit during Level-0 (L0) to Level-1A (L1A) data processing and are then used to create the Level-2 (L2) file that is input to the L3 science file.

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Derivation Techniques and Algorithms

The Aquarius L3 gridding algorithm uses local polynomial fitting to grid the L2 soil moisture retrievals on a 1-degree grid (Fan and Gijbels 1996; Lilly and Lagerloef 2008). The L3 processing of Aquarius satellite data takes measurements at the boresight locations of the three radiometer beams, which have already been converted into physical units of soil moisture, and maps these onto a 1-degree grid.

This method fits a Pth-order polynomial at each grid point xm. For data values gn observed at locations xn, n=1, 2,…N, this corresponds to minimizing

Equation 1   (Equation 1)

at every grid point x=xm, where

decaying weighting function   (Equation 2)

is a decaying weighting function which depends upon the bandwidth h, with K(x) being a probability distribution function.

The regression coefficients

regression coefficients   (Equation 3)

p=1, 2,…P vary with spatial location, and are estimated at all grid point locations.

The function g(x) is estimated by the lowest order coefficient,

lowest order co-efficient   (Equation 4)

while higher-order regression coefficients estimate the derivatives of the field through

higher order regression co-efficients   (Equation 5)

The above discussion focuses on a 1-dimensional application, but can be extended to a 2-dimensional application. A complete description for the 2-dimensional problem is available in Fan and Gijbels (1996) and Lilly and Lagerloef (2008).

Processing Steps

Each product represents data binned over the period covered by the original Aquarius product. The mean for the observation period is used to obtain the values for the grid points from the binned data products. Each product contains one soil moisture image and is stored in one physical HDF5 file. The data are not filtered during the gridding process. The user is advised to refer to the flags in the Aquarius L2 Swath Single Orbit Soil Moisture Data.

Version History

The following table outlines the version history for this product.

Table 3. Version History Details
Version  Description
V5

Changes to this version include: 

  • Updated Version 5 Aquarius brightness temperature data were used as input
  • File-level metadata were modified to more closely align with Climate and Forecast (CF) metadata conventions
V4 For Version 4 Aquarius L3 Gridded 1-Degree Soil Moisture Data, the data set was updated to use the most recent version (Version 4) of Aquarius Brightness Temperatures as input.
V3

The Aquarius L3 Gridded 1-Degree Soil Moisture Data, Version 3 were processed from the Aquarius L2 Swath Single Orbit Soil Moisture Data, Version 3. Changes to this version include:

  • Use of the most recent version (Version 3) of Aquarius Brightness Temperatures as input
  • Aquarius Brightness Temperatures are no longer re-calibrated before soil moisture retrievals as was done for Version 2 data
  • Soil moisture observations are valid over a wider range of brightness temperatures compared to Version 2 data
  • Updates were made to the soil moisture model parameters (b and ω)
V2 First public data release

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Sensor or Instrument Description

Aquarius/SAC-D is a collaboration between NASA and the Argentinean space agency, Comisión Nacional de Actividades Espaciales (CONAE), with participation from Brazil, Canada, France, and Italy. The Aquarius instrument was built jointly by the NASA Jet Propulsion Laboratory and NASA Goddard Space Flight Center.

The Aquarius instrument includes three radiometers and one scatterometer. The soil moisture data are collected by the radiometers. The radiometers measure brightness temperature at 1.414 GHz in the horizontal and vertical polarizations (TH and TV). The scatterometer is a microwave radar sensor that measures backscatter for surface roughness corrections. The following table summarizes instrument characteristics.

Table 4. Aquarius Instrument Characteristics
Instrument Characteristics
3 radiometers in push-broom alignment
  • Frequency: 1.413 GHz
  • Band width: less than or equal to 26 MHz
  • Swath width: 390 km
  • Science data block period: 1.44 sec
  • Footprints for beams: 74 km along track x 94 km cross track, 84 x 120 km, and 96 x 156 km, yielding a total cross track of 390 km.
  • Beam incidence angles of 29.36, 38.49, and 46.29 degrees incident to the surface; beams point away from the sun
Scatterometer
  • Frequency: 1.26 GHz
  • Band width: 4 MHz
  • Swath width: 390 km
  • Science data block period: 1.44 sec

SAC-D spacecraft Orbit Parameters:

  • 98 minute sun-synchronous
  • 6 p.m. ascending orbit, 6 a.m. descending orbit
  • 657 km equatorial altitude (655 km minimum, 685 km maximum over the orbit)
  • Ground-track repeat interval: Weekly, 103 orbits
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References and Related Publications

Contacts and Acknowledgments

Rajat Bindlish
NASA Goddard Space Flight Center
Hydrological Sciences Laboratory
Code 617, Bldg 33, G216
Greenbelt, MD 20771 USA

Thomas Jackson
United States Department of Agriculture
Agricultural Research Service
Hydrology and Remote Sensing Laboratory
Beltsville, MD 20705 USA

Acknowledgments: 

This work was funded by NASA under the interagency agreement NNH10AN10I. Tianjie Zhao helped with development of the soil moisture algorithm. The support provided by Michael Cosh, Peggy O'Neill, Thomas Holmes, and Wade Crow is acknowledged. The investigators would like to acknowledge the support provided by Gary Lagerloef, David Le Vine, Gene Feldman, and the ADPS group in the implementation of the Aquarius Soil moisture algorithm.

Document Information

DOCUMENT CREATION DATE

December 2013

DOCUMENT REVISION DATE

September 2018

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