KBS000: MCSE: Soil Moisture by Time Domain Reflectometry
In use from 1989-01-01 to 2003-01-01
Sampling Frequency: Precipitation-Event Basis (1989 to 1994); hourly 1996 to 2003
Soil moisture analysis is measured at soil depths of 15 cm and 50 cm using Time Domain Reflectometry (TDR) (Topp 1987) . Three pairs of stainless steel rods either 17 cm or 52 cmin length are embedded vertically into the soil (to 15 or 50 cm) in each main site plot at sampling station 3 . One pair of stainless steel rods placed 5 cm apart comprises one sampling unit . At each sampling station a pair of rods are placed within the agricultural crop row, 1/4 of the distance between the crop rows, and midway between the crop row. Within the Native successional plots, rods are placed at 0, 7.5 and 15 inches (same distances as rods in corn/soybean plots) . Weekly measurements are made with a Tektronix Metallic Time Domain Reflectometer . Determination of soil water content over the depth of rod placement, 15 cm and 50 cm, is calculated using the following equation developed by Topp et al . (1980) calibrated for our soils:
Ov = -0.053 + 0.029 * Ka – 5.5 × 10-4 Ka2 + 4.3 * 10 × 10-6 Ka3 .
where Ov = water content and Ka = (Ct/L)2
C = propagation velocity of an electromagnetic
wave in free spaces, 3 × 10-10ms-1
t = signal travel time measured by the Tektronix TDR
L = the length of the transmission line .