Gravimetric Surface Sampling Results

Introduction

The Data

Data and File Characteristics

The Science

Sampling Plan

Sampling Protocol

GSM Sampling Procedure

Full sampling Site

Profile Sampling Site

Taking a GSM Sample

GSM Sample Processing

Wet Weight Procedure

Dry Weight Procedure

Data Access and Contacts

FTP Site

Points of Contact

Introduction

The results obtained in the surface gravimetric soil moisture sampling are provided. Soil moisture is gravimetric, not volumetric. For interpretation of the codes used for areas and sites please refer to the experiment plan. Field notebooks are available for use at the USDA Hydrology Lab. Also included in this information file is the background material from the sampling plan describing the sampling design and protocol.

The Data

Data and File Characteristics

Two data files will be available, the summary (means and standard deviations of all fields and days) and the raw data for each field on each day. Files are all ASCII text. The data contained in these files is as follows

The Science

Sampling Plan

For the most part, sampling was performed on sites approximately a quarter section (0.8 km by 0.8 km) in size. These are referred to as full sampling sites. Some sites were sampled primarily for surface-profile soil moisture correlation studies and consist of the area immediately surrounding a profile location. These are referred to as profile sites.

Full sampling sites generally involved two transects separated by 400-m with a sample every 100-m resulting in 14 samples per site. There were some exceptions to this in a few fields where logistical issues were involved. Profile only sites consisted of 9 samples collected over a 20-m by 20-m grid near the profile location. A standardized tool was used to extract a sample of the 0 to 5-cm soil layer. This sample was then split in half to provide a 0 to 2.5-cm and a 2.5 to 5-cm sample at the point. Cans were used sequentially during sampling with all odd numbers being 0 to 2.5-cm samples and all even numbers 2.5 to 5-cm samples. Sample location was not critical in this approach. The grid was used only as an aid in stratifying the distribution of samples.

Sampling Protocol

The gravimetric soil moisture (GSM) sampling was intended to estimate the site average and standard deviation. Precise location within the site was not important, however, the samples should be spatially distributed to obtain meaningful statistics. For this reason a grid was used. Two types of sampling designs were employed, Full and Profile. The actual soil moisture sampling is the same but the distribution and number of samples is different.

GSM Sampling Procedure

Full sampling Site

The goal of this sampling was to characterize the mean of what we hope is a "homogeneous" field. A total of 14 points in each site were sampled. Some sites were not square and in these situations it was necessary to adapt the procedure.

• Upon arrival at site, note site id, your name(s) and time in notebook. Draw a schematic of the field (It might be a good idea to do this before you go out for the day).
• Assemble 28 sequential cans and indicate on schematic where they will be used. Odd numbered cans will be used for the 0 - 2.5 cm sample and even numbered cans will be used for the 2.5 - 5 cm sample.
• From a reference point for the site (usually a corner), measure 200 m along one side to locate the first transect.
• From this location initiate a sampling transect across the site. Take the first sample at 100 m and repeat every 100 m until you are 100 m from the edge of the site. For a standard quarter section site this will result in 7 samples on the transect. At selected points you will also collect soil temperature and surface thermal infrared temperature (TIR-if assigned). These are indicated in Figure 15.
• After completing this transect move 400 m perpendicular into the site and initiate a new transect. This will result in a total of 14 sampling points.
• As you move along the transect note any anomalous conditions on the schematic in your notebook, i.e. standing water.
• Record your stop time and place cans in box. Try to keep them cool.

Profile Sampling Site

These are locations at which the objective was solely to correlate gravimetriC surface soil moisture to the data collected by the insitu heat dissipation 5 cm sensors. Nine samples were collected on a nominal grid 10 m apart (total of 20 m by 20 m area) immediately adjacent to the sensor enclosure.

Taking a GSM Sample

1. Remove vegetation and litter.
2. Use the large spatula (6 cm) to cut a vertical face at least 5 cm deep
3. Push the GSM tool into this vertical face. The wings of the scoop should rest on the soil surface.
4. Use the large spatula to cut a vertical face on the front edge of the scoop.
5. Use the small spatula to cut the sample into a 0-2.5 and a 2.5-5 cm depth sample.
6. Place each sample depth in a separate can, small spatula aids extraction Remember that the odd numbered cans are for the top layer and the even are for the second layer.
7. A video clip showing the gravimetric sampling technique can be downloaded from an anonymous ftp site hydrolab.arsusda.gov/pub/sgp99/gsmsamp.avi.
8. At the specific sampling points where it is required, measure the soil temperature at 1, 5, and 10 cm depths using the digital thermometer provided. Record these values in degrees C to one decimal point in the field notebooks at the point location on the map.
9. If you are assigned a TIR, at the specific sampling points where it is required, measure the surface temperature. Record these values in degrees C to one decimal point in the field notebooks at the point location on the map.

GSM Sample Processing

All GSM samples were processed to obtain a wet and dry weight. It was the sampling teams responsibility to perform the wet weighing and placement of the samples in the drying ovens. A lab team performed the removal of samples from the oven, dry weighing, and can cleaning.

The Gravimetric Soil Moisture (GSM) samples were collected from all sites on a daily basis. After the retrieval of all samples they were weighed in their wet state and put in the oven. The next day, approximately 22 hours later, the oven-dry samples were weighed again. To speed up weighing and to reduce the data processing, the process was partially automated by connecting the balances with PC's.

Wet Weight Procedure

Dry Weight Procedure

Data Processing

Data Access and Contacts

FTP Site

The Gravimetric Soil Moisture Summary File from SGP99 resides on DAAC anonymous FTP. You may access it from this document,

Summary file

Raw Data files

or directly via FTP at

ftp disc.gsfc.nasa.gov

login: anonymous

password: < your internet address >

cd /data/sgp99/GravSM/

Points of Contact

The Principal Investigator for the SGP99 Gravimetric Surface Sampling data is

For information about or assistance in using SGP99 DAAC data, contact