Time domain reflectometry soil moisture data for Niwot Ridge, 1992 - 2002.

Soil moisture was estimated at several locations on Niwot Ridge using a portable PC in conjunction with time-domain reflectometry (TDR) technology. The system used consisted of a Tektronics 1502B reflectometer, a Tektronics SP232 interface module, a Compaq LTE/232 PC, coaxial cable, and a probe that was left semi-permanently in the soil. The reflectometer generated an electrical step voltage pulse which was propagated down the cable to the probe. The reflected voltage waveform was measured, stored, and displayed by the reflectometer and the Tektronics SP232 interface module. The waveform is represented as a 2-dimensional data array, with the x-axis representing time (which is converted to distance using the dielectric constant of the cable) and the y-axis representing voltage for the given time: in this system this value is represented in millirho units (Tektronics 1991). The length of the waveform is determined by measuring the difference between the soil air interface (SAI) and the end of the voltage step reflection (RE). The SAI value is equivalent to the apex of the first peak in the wave and the RE value is equivalent to the intersection of tangents "drawn" at (a) the bottom of the wave and (b) the point of the steepest rise (see Taylor and Seastedt 1992). The waveform length is then used to estimate the volumetric and gravimetric moisture of the surrounding soil.