The soil water matric potential data described herein are contributed by the Agricultural Research Service, Grazinglands Research Laboratory, El Reno, OK. The data for SGP97 period (June and July) is provided for the ARS SHAWMS stations in the Little Washita River Watershed. The data consists of matric potential values at 5, 10, 15, 20, 25, and 60 cm below ground surface, and is provided in 1-hour increments. A documentation file provides the necessary information of data units and a disclaimer.

The data is located at the following FTP address:

ftp://164.58.150.49/pub/sgp97/shawms/

The LAT.-LONG. coordinates of each station are given in a table and the approximate location is shown on a map of the Little Washita River Watershed. The contents and organization of each file is described in the data documentation file.

DOCUMENTATION

SHAWMS is an acronym for Soil Heat And Water Measurement System. The soil water sensor incorporated into the SHAWMS is Campbell Scientific’s Model 229-L soil heat dissipation sensor. This sensor is designed to provide measurements of soil matric potential (a measure of how tightly the soil holds water); therefore, knowledge of a particular soil’s soil water retention curve is required for conversion of matric potential into volumetric water content. The sensor was evaluated by Reece (1996) and found to give reliable estimates of matric potential between -10 J/kg and -1200 J/kg (0.1 bar to 12 bar), inclusive. Briefly, the 229-L consists of a small, hollow, stainless steel tube inserted into a porous ceramic matrix. The stainless steel tube contains a miniature heater and thermocouple. An initial soil temperature is taken with the sensor, then the sensor is heated for 21 seconds and an ending soil temperature reading is made. A del T value is calculated from the initial and ending soil temperature readings. del T is inversely related to soil water matric potential, and the del T soil matric potential relationship is typically ascertained via sensor calibration.

SHAWMS 229-Ls were installed uncalibrated in the Little Washita. This was necessary due to the time required to calibrate an individual sensor (on the order of weeks) and the shear number of sensors to be deployed (124). Statistical analysis of data from 10 laboratory-calibrated 229-Ls indicated that a generalized equation could be applied to the sensors in the Little Washita.

 According to the generalized equation, only the del Tdry (i.e., for a dry sensor) need be known for a given sensor. These del Tdry values were obtained for each of the sensors by identifying the maximum del T value in the complete data record (typically 2 years) for a given 229-L. These in situ del Tdry values matched closely to observed laboratory values.

 Sensor deployment: There are 8 229-Ls per SHAWM site. 3 sensors are located at 5 cm below the soil surface (noted as 5a, 5b, 5c), and 1 each at 10, 15, 20, 25, and 60 cm below the surface.

Data Files:

Only a low-level quality control (i.e., obvious errors detected) was performed on the data. Therefore, use of these data should proceed with this information in mind, and the data should be treated as preliminary.

DISCLAIMER:

The ARS SHAWMS data of the Little Washita River Watershed are accepted and used by the recipient upon the express understanding that the ARS and it's employees make no warranties, expressed or implied, concerning the accuracy, completeness, reliability or suitability for any one purpose, and that ARS and it's employees shall be under no liability to any person by reason of any use made thereof.

The ARS requests that the recipient of the ARS SHAWMS data does not distribute, publish or disseminate the data under the recipient's name without full and up-front acknowledgement of ARS as the source of the data, and that the recipient acknowledges the support and role of ARS in publications that use and are based on the ARS data. For further information regarding the SHAWMS data contact Dr. Patrick Starks, Soil Scientist, USDA-ARS, Grazinglands Research Laboratory, 7207 W. Cheyenne. St., El Reno, Oklahoma 73036, Tel: 405-262-5291, FAX: 405-262-0133, email: pstarks@grl.ars.usda.gov.

LITTLE WASHITA WATERSHED MAP  
 

TABLE OF STATION COORDINATES

Reference:

Reece, C.F., 1996. Evaluation of a line heat dissipation sensor for measuring soil matric potential, Soil Science Society of America Journal 60:1022-1028.