|ARS SHAWMS Data for SGP97|
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:
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.
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.
Two files per site (one per month of SGP’97 experiment period)
where LW signifies Little Washita, ?? is the SGP site number, jun is the month of June, and jul is the month of July.
Col 2 = SGP’97 site ID number
Col 3 = Date (YYMMDD)
Col 4 = Day of year 1997 (DOY)
Col 5 = Time of day (HHMM) [Note: data are only taken hourly, and are given in Central Standard Time]
Col 6 = Sensor 5a reading
Col 7 = Sensor 5b reading
Col 8 = Sensor 5c reading
Col 9 = Sensor reading at 10 cm
Col 10 = Sensor reading at 15 cm
Col 11 = Sensor reading at 20 cm
Col 12 = Sensor reading at 25 cm
Col 13 = Sensor reading at 60 cm
Measurement unit of sensor readings is -J/kg, but only the absolute value is shown and the minus sign is to be understood. Additionally, it should be noted that the work of Reece indicates that the performance limits of these sensors is between -10 J/kg and -1200 J/kg, but the values calculated above and below these limits are reported in the data files.
Close inspection of the data reveals diurnal oscillations in matric potential at a given depth. At present it is suspected that these oscillations are due to data logger/sensor electronics. Further, these oscillations become more pronounced as the soil dries.
In some cases, it may be apparent that sensors at the deeper depths immediately "wet up" after a rainfall event that logically should not have penetrated to that depth. This may be due to preferential flow paths that have developed along the sensor cabling.
Data entries of -666 indicate missing data.
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.
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: firstname.lastname@example.org.
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.