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Data Set ID:
NSIDC-0187

SMEX02 Watershed Vegetation Sampling Data, Walnut Creek, Iowa, Version 1

This data set contains vegetation parameters as part of the Soil Moisture Experiment 2002 (SMEX02).

Geographic Coverage

Parameter(s):
  • Agricultural Plant Science
  • Vegetation > Biomass
  • Vegetation > Leaf Characteristics
  • Vegetation > Plant Characteristics
  • Vegetation
  • Vegetation > Vegetation Cover
  • Vegetation > Vegetation Index
Spatial Coverage:
  • N: 42.7, S: 41.7, E: -93.2, W: -93.8

Spatial Resolution: Not Specified
Temporal Coverage:
  • 15 June 2002 to 9 July 2002
Temporal Resolution: Not specified
Data Format(s):
  • Microsoft Excel
  • ASCII Text
Platform(s) FIELD SURVEYS
Sensor(s): LICOR PLANT CANOPY ANALYZER
Version: V1
Data Contributor(s): Martha Anderson
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.

Anderson, M. 2003. SMEX02 Watershed Vegetation Sampling Data, Walnut Creek, Iowa, Version 1. [Indicate subset used]. Boulder, Colorado USA. NASA National Snow and Ice Data Center Distributed Active Archive Center. doi: http://dx.doi.org/10.5067/XCGVUPGKER17. [Date Accessed].

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Detailed Data Description

This part of the SMEX02 experiment focused on modeling soil moisture and surface fluxes during periods of rapid vegetation growth. It was important to fully and accurately quantify the temporal development and spatial variability of primary canopy characteristics. Correction for water contained in surface biomass is essential for deriving soil moisture estimates from remote microwave observations. Canopy height and cover information is also important for estimating surface roughness, soil-plant flux partitioning, and other parameters required for modeling land-atmosphere interactions. Four rounds of intensive sampling were conducted at the 31 soil moisture sampling sites within the Walnut Creek Watershed.

Land cover in the Walnut Creek Watershed consists of corn and soybean fields, with some forested areas in localized riparian zones. The vegetation sampling scheme for SMEX02 was designed to characterize variability in surface vegetation conditions within the watershed, given the range of landcover and soil types, crop planting dates, and cultivation techniques encountered in the Iowa landscape.

Format

Data are provided as ASCII text files and as a Microsoft Excel file.

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File Naming Convention

"smex02_wc_veg_site.txt" contains descriptions and coordinates of sampling locations.
"smex02_wc_veg_loc.txt" contains vegetation data averaged over rows at each sampling location.
"smex02_wc_veg_row.txt" contains vegetation data collected in each row, at each sampling location.
"smex02_wc_veg.xls" is an Excel file that contains the location-averaged data, row data, and data broken out by station.

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File Size

File sizes range from 270 KB to 2.06 MB.

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Volume

Total volume of the data files is 2.45 MB.

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Spatial Coverage

Southernmost Latitude: 41.7° N
Northernmost Latitude: 42.7° N
Westernmost Longitude: 93.8° W
Easternmost Longitude: 93.2° W

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Temporal Coverage

The study was conducted between 15 June and 9 July 2002. Actual sampling dates were 16, 17, 18, 19, 27, 28, 29, and 30 June, and 2, 3, 5, 6, 7, and 8 July 2002.

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Parameter or Variable

Parameter Description

The parameters are stand density, plant height, phenological stage, ground cover, green and dry biomass, row spacing, stem and leaf water content, and leaf area index (LAI).

The following table describes each column in the "smex02_wc_site.txt" data file.

Column heading Description
Site Watershed site location number (WC01, WC02, etc.)
Loc Sampling point for each site location
1=high vegetation cover, 2=low cover, 3=average cover
Easting UTM Easting in meters (Zone 15)
Northing UTM Northing in meters (Zone 15)
Target_corner The corner of the sampling location where the aerial target was installed
Row_sp Row spacing - the distance in meters between the center of one plant row to the center of the next plant row
Row_dir Direction of the row
Crop The type of crop growing - corn, soy

The following table describes each column in the "smex02_wc_loc.txt" data file. Units of measure are given for parameters. (The flexcoil technique refers to a planting method that has no clear row structure.)

Column heading Description Unit of Measure
Site Watershed site location number (WC01, WC02, etc.)
Crop C=corn
S=soy
Loc Sampling point for each site location
1=high vegetation cover, 2=low cover, 3=average cover
Date CST DD/MM/YYYY Central Standard Time
DOY CST Numerical day of the year (Julian date)
Time CST HHMM Central Standard Time
Row_sp m Space between rows (Flex=flexcoil planting) meters
Row_dir Row direction (NA=not applicable, flexcoil planting) East, West, North, South
AvgDensity pl/m2 Average stand density plants/m2
AvgHeight m Average plant height over five plants meters
AvgGr_cover Average ground cover percentage
AvgGBM_st g Average stem green biomass grams
AvgGBM_lf g Average leaf green biomass grams
AvgGBM_pl g Average plant green biomass grams
AvgAGBM_st kg/m2 Average stem green biomass kg/m2
AvgAGBM_lf kg/m2 Average leaf green biomass kg/m2
Avg_AGBM_pl kg/m2 Average plant green biomass kg/m2
AvgWC_st g Average stem water content grams
AvgWC_lf g Average leaf water content grams
AvgWC_pl g Average plant water content grams
AvgAWC_st kg/m2 Average stem water content kg/m2
AvgAWC_lf kg/m2 Average leaf water content kg/m2
AvgAWC_pl kg/m2 Average plant water content kg/m2
AvgLAI Average LAI

The following table describes each column in the "smex02_wc_row.txt" data file. Units of measure are given for parameters.

Column heading Description Unit of Measure
Site Watershed site location number (WC01, WC02, etc.)
Crop C=corn
S=soy
Loc Sampling point for each site location
1=high vegetation cover, 2=low cover, 3=average cover
Row Row number
Date CST DD/MM/YYYY Central Standard Time
DOY CST Numerical day of the year (Julian date)
Time CST HHMM Central Standard Time
Row_sp m Row spacing - distance in meters between the center of one plant row to the center of the next plant row (Flex=flexcoil planting) meters
Row_dir Row direction (NA=not applicable, flexcoil planting) East, West, North, South
Density pl/m2 Stand density - number of plants in a meter length within a row plants/m2
Height m Plant height over five plants meters
Pheno Average phenological stage of plants within the sampling rows
(See the Iowa State corn phenological guide and the Iowa State soy phenological guide for stage designations)
Gr_cover Ground cover percentage
GBM_st g Stem green biomass grams
GBM_lf g Leaf green biomass grams
AvgGBM_pl g Plant green biomass grams
AGBM_st kg/m2 Areal stem green biomass kg/m2
AGBM_lf kg/m2 Areal leaf green biomass kg/m2
AGBM_pl kg/m2 Areal plant green biomass kg/m2
WC_st g Stem water content grams
WC_lf g Leaf water content grams
WC_pl g Plant water content grams
AWC_st kg/m2 Areal stem water content kg/m2
AWC_lf kg/m2 Areal leaf water content kg/m2
AWC_pl kg/m2 Areal plant water content kg/m2
LAI LAI
Sky Weather conditions sun or shade

Parameter Source

LAI is measured by a plant canopy analyzer. Other parameters were measured manually.

Sample Data Record

The following sample is taken from the "smex02_wc_site.txt" data file.

Site    Loc   Easting    Northing    Target_corner  Row_sp  Row_dir  Crop
 -      -        m          m          -             m       -        -
WC01    1     437265.98  4646549.05    NE           0.76    E-W      corn
WC01    2     437256.94  4646703.58    NE           0.76    E-W      corn
WC01    3     437220.73  4646847.81    NE           0.76    E-W      corn
WC03    1     437728.68  4647902.96    SE           0.38    N-S      soy

The next sample is taken from the "smex02_wc_row.txt" data file. Only the first 19 columns are represented in the sample.

Site Crop Loc  Row      Date     DOY     Time   Row_sp  Row_dir  Density  Height   Pheno Gr_cover GBM_st  GBM_lf  GBM_pl  AGBM_st  AGBM_lf AGBM_pl ...
 -     -    -            CST     CST      CST      m        -      pl/m2     m       -         %     g       g       g      kg/m2    kg/     kg/m2  ...   
WC01   C    1   2  6/29/2002     180      820     0.76     E-W      6.56    1.52     v8       80  461.31  195.73  657.04     3.03     1.28    4.31  ...
WC01   C    1   4  6/29/2002     180      820     0.76     E-W      7.87    1.55     v9       80  406.32  174.76  581.08     3.20     1.38    4.58  ...
WC01   C    1   6  6/29/2002     180      820     0.76     E-W      7.22    1.46     v9       80  324.18  149.64  473.82     2.34     1.08    3.42  ...
WC01   C    1   8  6/29/2002     180      820     0.76     E-W      7.22    1.49     v8       80  392.21  169.69  561.90     2.83     1.22    4.06  ...
WC01   C    1  10  6/29/2002     180      820     0.76     E-W      5.91    1.47     v8       80  369.57  175.63  545.20     2.18     1.04    3.22  ...

The next sample is taken from the "smex02_wc_loc.txt" data file. Only the first 16 columns are represented in the sample.

Site Crop Loc      Date     DOY     Time   Row_sp  Row_dir  AvgDensity AvgHeight AvgGr_cover AvgGBM_st  AvgGBM_lf  AvgGBM_pl  AvgAGBM_st        ...
 -     -    -       CST     CST      CST      m        -          pl/m2       m            %       g          g          g          kg/m2       ...
WC01   C    1 6/29/2002     180      820     0.76     E-W         6.96      1.50          80    390.72     173.09     563.81        2.72        ...
WC01   C    2 6/29/2002     180      820     0.76     E-W         6.69      1.00          55    218.02     114.37     332.39        1.42        ...
WC01   C    3 6/29/2002     180      820     0.76     E-W         8.53      1.32          70    290.52     172.51     463.03        2.47        ... 
WC01   C    1  7/6/2002     187     1100     0.76     E-W         8.40      2.09          90    481.76     220.00     701.75        4.04        ...
WC01   C    2  7/6/2002     187     1100     0.76     E-W         8.53      1.53          70    394.28     164.13     558.42        3.38        ...


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Software and Tools

Quality Assessment

In some data files, a misalignment between the sensor mask and the sun caused direct sunlight to enter the detector. In these cases, the LAI was recomputed excluding the offending sensor ring/location combination. All data were assessed and corrected for these errors. The data were also examined for evidence of variable sky conditions during the measurement sequence.

Phenology observations in later sampling rounds were corrected slightly for low leaves that had likely matured and fallen off and may have been disregarded in the phenological stage assessment. In corn, 3 leaves were added in Round 3 to plants in V stages. In Round 4, 5 leaves were added to plants in V stages, and capped at stage V18.

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Data Acquisition and Processing

Vegetation data were collected in watershed soil moisture sampling sites during four rounds of sampling: the 12 watershed sites containing flux towers were sampled 4 times during the experiment; the remaining 19 sites were sampled twice. This sampling schedule allowed characterization of the rapid evolution in biomass and surface cover that occurred during the course of this study.

Three vegetation sampling locations were identified in each site using aerial imagery. One location with high vegetation cover, one with low cover, and a third with average cover (see Figure 1).

Schematic of vegetation sampling strategy

Figure 1. Schematic of vegetation sampling strategy

At the corner of each sampling location, a large white aerial target (1 m by 1 m) was stapled to the ground (see Figure 2). The locations of each target were recorded on the ground with handheld Global Positioning System (GPS) units.

Photograph of aerial target

Figure 2. Photograph of aerial target

Each sampling location was 10 rows across and 12 m long. Biomass samples, LAI, plant height, and stand density measurements were collected from every second row, yielding 5 sets of observations per location times 3 locations per field, which equals 15 data sets per field per sampling round. In the first sampling round, vegetation samples were collected in the first 3 m segment closest to the aerial target; in the second round, from the second 3 m segment, etc.

Theory of Measurements

The purpose of vegetative sampling is to provide an estimate of the variation in the vegetative components in the corn and soybean fields across the SMEX02 study sites.

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Data Acquisition Methods

Stand Density

Stand density was measured by placing a meter stick along the row in each of the five rows sampled. The meter stick was placed at the center of a plant stem, with that stem counting as the first plant. All plants within the 1 m length were counted. If a plant was at the end of the meter stick and more than half of the stalk extended beyond the end of the meter stick, it was not counted.

For soybean fields planted with the flexcoil technique, which has no clear row structure (WC10 and WC14), a wire frame with dimensions 1 m by 0.5 m was used to measure stand density. The frame was placed on the soil surface and all plants within the frame were counted.

Vegetation Height

Samplers used a meter stick, to estimate the average height of the canopy within a 3 m segment in each sampling row. In corn, this nominal height estimate excluded the extreme tips of vertical leaves.

Ground Cover

Samplers assessed percentage ground cover as the fraction of the spacing between rows that the crop canopy occupied. For example, if a soybean row were visualized as a cylinder, ground cover would be the ratio between the diameter of the cylinder and the row spacing.

Green and Dry Biomass

Biomass samples were collected in each sampling row, yielding five plants sampled per location per sampling round. One plant of average height was cut at the ground surface from each sampling row and transported back to the lab for weighing.

Corn: Leaves and stalks were removed from each corn plant in the field and placed in separate paper bags. The samples were dropped off at the lab as quickly as possible.

Soybean: In the field, each whole plant was placed in a paper bag. Lab workers separated leaf and stem components at the lab.

At the lab, each paper bag was weighed and the green biomass and tare weight recorded. (Several empty bags were weighed to assess tare weights.) The filled bags were taken to drying ovens operated by the Iowa State University Agronomy Department and dried for 4-5 days at 110° F. Weights of representative corn samples were obtained periodically during the drying process to ascertain that weight loss had ceased before the samples were removed from the ovens. After drying, the dry biomass and tare weights were recorded.

Leaf Area Index (LAI)

Leaf Area Index was measured using an LAI-2000 (LiCor Inc.) canopy analyzer, which compares above- and below-canopy light levels to compute LAI and canopy architecture. The canopy analyzers were set to average four locations into a single value, using one observation taken above the canopy and four beneath the canopy: in the row, ¼ of the way across the row, ½ of the way across the row and ¾ of the way across the row. This gives a good spatial average for row crops of partial cover.

The manufacturer recommends using the LAI-2000 units only in diffuse light, in overcast conditions, or near dawn or dusk. Direct-beam radiation reflected into the sensor from upper leaves in the canopy can be confused with open sky, causing LAI to be underestimated. Because of restrictions in time and manpower, LAI observations in SMEX02 were taken in both sunlit and shaded/diffuse conditions. Observers were instructed to note local sky conditions that existed during each measurement. If no shadows could be seen during the measurement, then the measurement was marked "shade." If shadows could be seen during the measurement then the measurement was marked "sun." If sky conditions changed during the course of a measurement, the measurement was not used. A series of observations were taken in several fields to test the effect of sunlit vs. shaded conditions on derived LAI, and a simple analytical routine is being developed to correct observations that were acquired in sun. The effect appears to be a negative bias on the order of <0.5 units of LAI.

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Derivation Techniques and Algorithms

Biomass water content (BWC; g/plant) in plant stems and leaves was computed as:

BWC = Bg' - Bd'

where Bg' is the green biomass + tare weight and Bd' is the dry biomass plus tare weight. This assumes that water loss from the tares (paper bags) was negligible in comparison with that from the plant samples.

In row crops, stand density (ASD; plants/m2) was estimated from the row plant density (RD; plants/m) using:

ASD = RD/RS (row crops)

where RS is the row spacing. In flexcoil plots, it was computed as

ASD = AD * 2 (flexcoil plots)

where AD is the plant density measured using the 1m by 0.5 m wire frame. Biomass water content (kg/m2) was then computed as:

ABWC = 10-3 * BWC * ASD
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References and Related Publications

Contacts and Acknowledgments

Martha Anderson
Assistant Scientist
Space Science and Engineering Center
University of Wisconsin, Madison, WI, USA.

Acknowledgments: 

Many graduate students and volunteers collected the field samples.

Document Information

DOCUMENT CREATION DATE

September 2003

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