Comparison of neutron probe and time domain reflectometry techniques of soil moisture analysis

The Environmental Science Group of Los Alamos National Laboratory collected soil water content data using Time Domain Reflectometry (TDR) and neutron probe in order to correlate the results from the two techniques in a study performed at Los Alamos, NM. Two well-characterized field plots (3m {times} 10m) were instrumented with TDR waveguides and neutron probe access tubes at four locations. At each location, TDR rod pairs were placed horizontally at 5-,15-,30-,45- and 60-cm depths and vertical sets of rod pairs with lengths of 15-,30-,45- and 60-cm were installed. Neutron probe measurements were taken at 15-,30-,45- and 60-cm depths. Because the neutron probe measures a spherical volume, soil water content values were compared with horizontal TDR readings averaged over the same zone. Additionally, vertical TDR readings were compared to neutron probe readings averaged over the depths corresponding to each TDR length. Measurements for each technique were taken weekly since Fall of 1992. A time series comparison of the results is presented.