This data set is part of the Soil Moisture Experiment 2002 (SMEX02) project, which is associated with the Advanced Microwave Scanning Radiometer - Earth Observing System (AMSR-E) validation project. It consists of Vegetation Water Content (VWC) data. VWC was derived from the Normalized Difference Water Index (NDWI), which was derived from Landsat 5 Thematic Mapper (TM) and Landsat 7 Enhanced Thematic Mapper plus (ETM+) imagery. The TM satellite data were acquired during select dates in June and July 2002. These data cover the SMEX02 Walnut Creek watershed and the regional Iowa study areas in Iowa, USA.
Data are provided as flat binary files and are available via FTP. Individual data values must be divided by 25.5 to obtain the actual vegetation water content in kg/m3.
These data were collected as part of a validation study for the Advanced Microwave Scanning Radiometer - Earth Observing System (AMSR-E). AMSR-E is a mission instrument launched aboard NASA's Aqua Satellite on 04 May 2002. AMSR-E validation studies linked to SMEX are designed to evaluate the accuracy of AMSR-E soil moisture data. Specific validation objectives include assessing and refining soil moisture algorithm performance; verifying soil moisture estimation accuracy; investigating the effects of vegetation, surface temperature, topography, and soil texture on soil moisture accuracy; and determining the regions that are useful for AMSR-E soil moisture measurements.
Jackson, T. and M. Cosh. 2003. SMEX02 Vegetation Water Content, Iowa Regional and Walnut Creek Watershed. [indicate subset used]. Boulder, Colorado USA: NASA DAAC at the National Snow and Ice Data Center.
|Data format||Files are in flat binary format with no header.
Regional files for VWC are 1851 columns by 3831 rows.
Watershed files for VWC are 1216 columns by 611 rows.
Data are in PC byte order (little endian). SGI and Sun users will need to byte-swap these data before using them.
|Spatial coverage||42° N, 93° W|
|Temporal coverage and resolution||Daily coverage for the dates 25 June through 18 July 2002.|
|Tools for accessing data||Open these files in an appropriate image processing or image viewing application.|
|File naming convention||File names begin with the Julian date, followed by the data type, VWC. Walnut Creek watershed files include "WC" before the data type.|
|File size||Files in the regional directory are 6.76 MB each. Files in the watershed directory are 725 KB each.|
|Parameter(s)||Vegetation Water Content (VWC) derived from Normalized Difference Water Index (NDWI).|
|Procedures for obtaining data||Data are available via FTP.|
Thomas J. Jackson, Hydrologist, and Michael H. Cosh, General Physical Scientist, USDA ARS Hydrology Lab.
NSIDC User Services
National Snow and Ice Data Center
CIRES, 449 UCB
University of Colorado
Boulder, CO 80309-0449 USA
phone: +1 303.492.6199
fax: +1 303.492.2468
form: Contact NSIDC User Services
Daoyi Chen, University of Manchester, Martha Anderson, University of Wisconsin, Fuquin Li, Hydrology and Remote Sensing Lab, USDA-ARS
Files are in flat binary format with no header. Data are in PC byte order (little endian). SGI and Sun users will need to byte-swap these data before using them.
Regional files for VWC are 1851 columns by 3831 rows.
Watershed files for VWC are 1216 columns by 611 rows.
There are two main directories, regional and watershed.
Individual data values must be divided by 25.5 to obtain the actual VWC in kg/m3.
File names begin with the Julian date, followed by the data type, VWC. Regional files have no other designation. For example, "198_VWC.bil." Walnut Creek watershed files include "WC" before the data type. For example, "198_WC_VWC.bil."
The following table shows the day, month, and year for the Julian dates used in the file names.
|Regional Files||Watershed Files||Date|
Files in the regional directory are 6.76 MB each. Files in the watershed directory are 725 KB each.
The coordinates for both areas given below are in latitude and longitude and in Universe Transverse Mercator (UTM) Zone 15. The coordinates represent the pixel centers of each corner pixel.
|Upper Left||42.729 N||93.841 W||431100.000 E||4731100.000 N|
|Upper Right||42.732 N||93.163 W||486600.000 E||4731100.000 N|
|Lower Left||41.694 N||93.827 W||431100.000 E||4616200.000 N|
|Lower Right||41.697 N||93.161 W||486600.000 E||4616200.000 N|
|Upper Left||42.037 N||93.832 W||431100.000 E||4654300.000 N|
|Upper Right||42.040 N||93.392 W||467550.000 E||4654300.000 N|
|Lower Left||41.872 N||93.830 W||431100.000 E||4636000.000 N|
|Lower Right||41.875 N||93.391 W||467550.000 E||4636000.000 N|
The Landsat TM and ETM+ data were used to produce high-resolution (30 m) NDWI and VWC data.
Universal Transverse Mercator (UTM) Zone 15.
Daily coverage for the dates 25 June through 18 July 2002.
The parameter measured in this data set is VWC, derived from NDWI. NDWI is the difference between the visible (red) and near-infrared (NIR) bands over their sum. NDWI divides the difference between reflected green-light and reflected near-infrared by the sum of those two bands. NDWI gives an estimate of the soil moisture.
TM scenes from Landsat 5 and Landsat 7 were acquired during the primary study period. These data were used to produce high-resolution (30 m) NDVI and NDWI data sets. The following table details the Landsat coverage for the dates of the study.
The following image is a screen shot of a portion of the image file "198_VWC.bin."
The next image is a screen shot of a portion of the image file "198_WC_VWC.bil."
Data are available via FTP.
Total volume of all images is 186 MB.
Open these files in an appropriate image processing or image viewing application.
See Soil Moisture Experiment (SMEX) for more information.
TM is a multispectral scanning radiometer carried on Landsat 4 and 5. The TM has seven spectral bands, with a spatial resolution of 30 m for most bands.
ETM+, an improved version of TM, is carried on Landsat 7. The ETM+ has eight spectral bands with a spatial resolution of 30 m for most bands. ETM+ calibration is good to within five percent.
Radiance from a satellite platform is strongly affected by the presence of the atmosphere. Atmospheric correction for NDWI was made to convert satellite-based radiance to an estimate of ground reflectance. Please see the SMEX Iowa Satellite Vegetation and Water Index (NDVI and NDWI) Data documentation for information about the atmospheric correction made for NDWI.
VWC was derived from the NDWI, which is a ratio of bands available from the Landsat 5 TM and Landsat 7 ETM+. The ratio is defined as:
Five scenes were used to estimate VWC for the entire watershed. The first step was to determine the trend of VWC versus day of year (DOY) and compare it to the trend of NDWI versus DOY. Vegetation sampling was conducted before and during the SMEX02 to support this endeavor. 31 watershed fields were sampled between 2 and 4 times during June and July 2002 to provide an estimate of VWC per field. Within each field, locations were selected to represent a high, a low, and an average vegetation cover. The corners of each sampling site are the basis for data retrieval for the proceeding analysis.
Average VWC values were calculated for each field for each day of sampling. Some days were combined so that the VWC estimate is the average of five samples at three sampling sites for a total of 15 samples. An initial value of 0.05 kg/m2 for VWC was set on June 10, 2002 which is characteristic of the region. From these data points, VWC values were interpolated for the remaining dates for each field. Functions were then developed for both corn and soybean fields.
Average NDWI values were retrieved from the five scenes for each of the WC fields sampled. Field averages were calculated by sampling the four corners of each sampling site for a total of 12 pixels per field. Some pixels were contaminated by nearby roadways or streams and these pixels were eliminated from the analysis. These field averages were then linearly extrapolated between the five days of record to provide an interpolated NDWI product for each field. A relationship was established for NDWI as a function of DOY for the two dominate crop types, corn and soybean.
The DOY functions for both VWC and NDWI were combined to produce new functions where VWC is predicted by NDWI. These functions are as follows:
Corn: VWC = 9.8166 * NDWI + 0.0522
Soybean: VWC= 1.468 * NDWI + 1.3615 * NDWI + 0.3394
In addition, areas determined to be forested were given a VWC of 10 kg/m2 and grassland were given a VWC of 0.5 kg/m2.
These functions were used to generate VWC maps for the five original TM/ETM+ scenes available. From the five available dates, VWC was determined for the length of the experiment by interpolating on a pixel-by-pixel basis from the five dates of VWC maps. Using a simple piecewise cubic Hermite interpolation for the five days, a time series was established from 6 June through 17 July 2002. This interpolation maintains the pattern for each day of record and interpolates between them for missing days. It was noted that for both NDVI and NDWI, the satellite sensors tended to saturate on or around 8 July 2002 for the very green vegetation. To compensate for this, the values for 17 July 2002 were adjusted in the case of VWC values equal to 5 kg/m3. If a pixel had a value of 5 kg/m2 (near saturation) the VWC was increased to 7 kg/m3. This value was used for interpolation.
To verify the accuracy of this method, the pixels relating to the vegetation ground sampling were retrieved for statistical analysis.
Corn: Bias: -0.0097 rmse: 0.5759
Soy: Bias: -0.0153 rmse: 0.1713
Total: Bias: -0.0118 rmse: 0.4695 R2: 0.9302
Preliminary regional VWC maps were generated similarly to the watershed, with some exceptions. Three sets of scenes were available for the Iowa region during the experiment, 23 June, 1 July, and 17 July. The two scenes for each day were mosaicked and regional maps of NDWI were generated. From the NDWI maps, VWC maps were generated using the same relationships that were developed in the Walnut Creek (WC) watershed region. From these three scenes, a linear interpolation was developed for the entire study period. To verify the accuracy of the preliminary interpolation method, the WC sampling sites were revisited and statistics were recalculated.
Corn: Bias: -0.2619 rmse: 0.6501
Soy: Bias: -0.0497 rmse: 0.1965
Total: Bias: -0.1883 rmse: 0.5381 R2: 0.9150
The high corn bias can be explained by the inability to interpolate during the later stages of the study period, when there was a precipitation event and rapid growth. This fact coupled with the tendency for NDWI to saturation at approximately 5 kg/m2 results in the poor fit of this data.
A primary reason for the preliminary nature of this data is the presence of clouds on 17 July 2002 which potentially influenced VWC readings for that area. Future datasets will provide a better remedy for this issue.
Gao, B.C., 1996: NDWI - A normalized difference water index for remote sensing of vegetation liquid water from space. Remote Sensing of Environment, Vol. 58: 257-266
Landsat Project information
The following acronyms and abbreviations are used in this document.
|AMSR-E||Advanced Microwave Scanning Radiometer - Earth Observing System|
|NDVI||Normalized Difference Vegetation Index|
|NDWI||Normalized Difference Water Index|
|SMEX||Soil Moisture Experiment|
|UTM||Universal Transverse Mercator|
|VWC||Vegetation Water Content|